Inês Ribeiro
Sara Aliácar2,3
Davy McCracken- 1MARETEC − Marine, Environment and Technology Centre, LARSyS, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- 2Rewilding Portugal, Guarda, Portugal
- 3Rewilding Europe, Heilig Landstichting, Netherlands
- 4Hill and Mountain Research Center, Scotland’s Rural College, Scotland, United Kingdom
Mediterranean landscapes are characterized by fine-grained land-cover mosaics of interspersed vegetation types and high wildfire vulnerability, where grazing plays a key role in regulating vegetation structure and composition. This study explores the early effects, over a three-year period, of a transition from extensive commercial cattle grazing to semi-wild horse grazing in two rewilding areas in the Côa Valley region, Portugal. Using grazing exclusion areas as control, we test whether the less intensive regime of semi-wild horse grazing can be used to manage vegetation structure and composition, to mitigate local fire hazard and promote biodiversity. The monitoring scheme followed a paired design, where each survey site of 40 m × 40 m comprises four sampling plots of 10 m × 10 m, including two fenced plots (grazing exclusion) and two plots open to grazing. Effects on vegetation structure were assessed considering grass height, shrub height, shrub cover and aboveground biomass, as well as effects on plant species richness, turnover, and forbs-to-grasses ratio (F:G ratio) and the community-level importance of grasses and forbs. Results showed that grass height had a greater increase in ungrazed plots, suggesting that semi-wild horse grazing helps limit grass height. There were no significant differences in shrub metrics between treatments (i.e. horse grazing vs. no grazing), indicating that horse grazing did not effectively control woody vegetation. While species richness remained stable, species temporal turnover was higher in ungrazed plots. Additionally, the F:G ratio and the importance value of forbs were higher under horse grazing, suggesting potential benefits for anthophilous insects. These findings indicate that semi-wild horse grazing contributes to maintaining open habitats by controlling grass dominance, thereby reducing local fire hazard and potentially fostering habitat and food resources for insects. While this demonstrates the potential of using semi-wild horse grazing in rewilding, the results also suggest that horses alone, particularly at low densities, have limited impact on woody vegetation structure.
1 Introduction
Large herbivores contribute to key ecological functions, such as nutrient cycling, seed dispersal, and the regulation of vegetation structure and composition (Mouissie, 2004; Pringle et al., 2023). Such functions result from their use of space and their feeding behavior, which includes both consumption and trampling (hereafter collectively referred to as grazing). These effects are essential for maintaining habitat and species diversity and hold significant potential for application in ecosystem management and restoration (Osem et al., 2007; Silva et al., 2019; Török et al., 2024).
In Mediterranean landscapes, characterized by fine-grained land-cover mosaics of interspersed vegetation types and high wildfire vulnerability, grazing plays an important role in regulating vegetation quantity, spatial distribution, and successional dynamics (Casasús et al., 2007; Riedel et al., 2013), ultimately influencing the landscape’s fire resilience (Kirkland et al., 2024; Lecomte et al., 2024; Lovreglio et al., 2024). For these reasons, grazing has been recognized as a valuable management tool for maintaining open habitats, preventing shrub encroachment, and controlling the spread of invasive plants (Souther et al., 2019; Oikonomou et al., 2023). However, the use of grazing for the purpose of ecosystem management may also involve trade-offs (Ribeiro et al., 2023). Specifically, while higher grazing intensities may be required to effectively regulate vegetation quantity and its vertical and horizontal distribution (Ribeiro et al., 2023, 2024), they can also lead to soil degradation, loss of vegetation cover (Schoenbaum et al., 2018; Lai and Kumar, 2020), and affect the natural regeneration of sensitive plant species, leading to reduced plant diversity (Jáuregui et al., 2009; Saiz and Alados, 2012; Calleja et al., 2019). Furthermore, the risk of trade-offs can be exacerbated in areas of low fertility and degraded soils, where overgrazing, even under extensive grazing regimes, can pose significant risks if stocking densities exceed the system’s carrying capacity. This is particularly concerning in Mediterranean ecosystems, which are characterized by shallow, nutrient-poor soils and high erosion potential (Van-camp et al., 2004; Teixeira et al., 2015). In such systems, these processes can limit vegetation productivity, triggering cascading effects on ecosystem functioning and the composition of ecological communities (Rogers et al., 2021; Psyllos et al., 2022). Consequently, unsustainable grazing practices risk pushing already degraded ecosystems beyond their resilience thresholds (van den Elsen et al., 2020; Zeng et al., 2021). This underscores the need for balanced grazing regimes that ensure the delivery of key ecosystem services, such as regulating biomass distribution and load to mitigate fire hazard, while also accounting for and minimizing the impacts on biodiversity (Maes et al., 2012; Torralba et al., 2016; Teague and Kreuter, 2020).
From a biodiversity and ecosystem management perspective, rewilding can be a viable strategy to address ecosystem degradation. Defined as an ecological restoration approach, rewilding aims to re-establish natural processes and dynamics in degraded landscapes, to ensure the maintenance of self-regulating and resilient ecosystems (Apollonio et al., 2017; Shackelford and McDougall, 2023; Mutillod et al., 2024). A key aspect of rewilding is the enhancement of ecological processes, such as trophic interactions, natural disturbance regimes, and species dispersal (Pereira and Navarro, 2015; Perino et al., 2019). In Mediterranean landscapes, the use of large herbivores in rewilding strategies aims to restore stochastic disturbance regimes associated with herbivory processes (Pereira and Navarro, 2015; Johnson et al., 2018; Van Meerbeek et al., 2019). These processes include regulating vegetation structure and species composition, which help maintain open habitat patches and prevent the dominance of woody plants in grassland and savanna ecosystems (Honrado et al., 2017; Silva et al., 2019). These roles and functions are particularly relevant in contexts where wild herbivores are absent or at densities too low to sustain ecological functions, or where ecological constraints cannot support economically viable livestock farming. Thus, by implementing more natural grazing regimes, rewilding approaches aim to reconcile landscapes’ resilience to disturbances with the restoration of ecological processes and biodiversity conservation (Carver et al., 2021; Massenberg et al., 2023).
Here, we evaluate the early effects of a rewilding approach on vegetation structure and composition in two rewilding areas, Vale Carapito (Site 1) and Ermo das Águias (Site 2) in the Côa Valley, Portugal (Figure 1). This region, characterized by low population density and high biodiversity, has been selected by Rewilding Portugal (a conservation initiative focused on ecological restoration and biodiversity conservation in Portugal) to establish core rewilding areas, as part of a vision to create a 120,000 hectare wildlife corridor. The main management goal for these rewilding sites is to restore herbivory related ecological processes by reintroducing semi-wild herbivore populations, which will enhance habitat heterogeneity and biodiversity, as well as mitigate fire hazard by managing vegetation. The ultimate goal of Rewilding Portugal is to promote the development of self-sustaining ecosystems. Specifically, this study evaluates the effects of semi-wild horse grazing (hereafter referred to as horse grazing) on vegetation, under a low-intensity “naturalistic” grazing regime that minimizes human intervention (Seddon et al., 2014), following three years after the transition from extensive commercial cattle grazing, as a first step of a long-term rewilding strategy. Additionally, no grazing areas (i.e., fenced areas) were established as control plots to assess the effect of grazing exclusion on vegetation. We aim to compare how vegetation structure and composition metrics evolve in both treatments (horse grazing vs. no grazing) and assess the potential of horse grazing to regulate vegetation, for mitigation of local fire hazard, while considering its impact on biodiversity. While cattle grazing may contribute to functions such as biomass regulation and reduction of fire hazard, overgrazing and negative impacts on biodiversity and soil can occur if stocking densities exceed the system’s carrying capacity (Ribeiro et al., 2023). Particularly, the feeding of purchased crop forages to cattle in the two study sites may reflect the need to meet livestock’s nutritional demands, indicating a mismatch between the available natural resources and livestock requirements.
Figure 1. Location of (a) case study region in Portugal, (b) study sites in the Côa Valley region. Grazing treatments and spatial arrangement of sampling plots are shown for (c) Site 1 and (d) Site 2. Each yellow circle represents a 10 × 10 m sampling plot: full circles represent ungrazed (fenced) plots and open circles represent horse grazed (open) plots.
We hypothesize that, over time, semi-wild horse grazing will exert an observable effect on vegetation structure, contributing to its regulation compared to grazing exclusion (H1: Hypothesis 1). This effect is expected as horse grazing maintains some level of disturbance and influences vegetation through selective foraging and trampling (de Villalobos and Zalba, 2010; Rosa García et al., 2013; Dvorský et al., 2022; Molle et al., 2022). Regarding vegetation composition, we expect a decline in species richness, in the absence of grazing disturbance, in ungrazed areas (H2: Hypothesis 2) (Henning et al., 2017a; Dvorský et al., 2022; Bonavent et al., 2023). We also expect the composition of the plant communities to shift under the two grazing treatments, with increased temporal turnover in ungrazed areas, due to the suppression of grazing disturbance, when compared to horse grazing areas (H3: Hypothesis 3). Finally, we expect a shift in the relative representation of forb and grasses at the community level, driven by differences in grazing pressure (H4: Hypothesis 4). In ungrazed areas, the absence of herbivory is expected to promote grass dominance and impact the conditions for forb germination and growth (Bonavent et al., 2023). In contrast, horse grazing is expected to support a higher relative representation of forbs due to the maintenance of grazing but in a regime of lower grazing intensity (Garrido et al., 2019; Schmitz and Isselstein, 2020; Rakosy et al., 2022).
2 Materials and methods
2.1 Study area
Field experiments were conducted at two study sites – Vale Carapito (Site 1) and Ermo das Águias (Site 2) – in the Greater Côa Valley region in Portugal (Figure 1). This region has a Mediterranean climate, characterized by hot and dry summers, and cold winters. Both sites are characterized by woodland-shrubland mosaic habitats, with dense shrub patches dominated by Spanish broom (Cytisus multiflorus) and woodlands dominated by holm oak (Quercus rotundifolia) and Pyrenean oak (Quercus pyrenaica). Site 1 and Site 2 were grazed by small ruminants (sheep and goats) for the 40–60 years prior to being grazed by cattle in 2010 and 2005, respectively (Supplementary Table S1). The sites were then acquisitioned by Rewilding Portugal in 2020 and 2021, respectively. Site 1 experiences a wetter climate, with an average annual temperature of 12.7°C and precipitation of 795 mm (Supplementary Figure S1), and encompasses 65 hectares of grazing area. The grazing regime at Site 1 changed in 2021 from a baseline regime of 25 cattle under rotational grazing (2–3 months, two times per year) to low intensity continuous grazing by Sorraia horses (10 horses introduced in April 2021 and later reduced to 5 to 6 horses in 2022 and onwards) (Supplementary Table S1). Site 2 has a milder and drier climate, with an average annual temperature of 13.3°C and precipitation of 544 mm (Supplementary Figure S1) and includes 330 hectares of grazing area, where the grazing regime transitioned from a baseline of 20 to 40 cattle (continuous grazing) to low intensity continuous grazing by 16 Sorraia horses in 2022 (Supplementary Table S1). The Sorraia horse is a native Portuguese breed adapted to living in wild or semi-wild conditions, making it well-suited for rewilding projects in landscapes that require high resilience. Its robustness allows it to endure harsh environmental conditions and survive on low-quality forage (Pinheiro et al., 2013).
2.2 Experimental design and survey scheme
Within each study area, six survey sites were selected prior to field surveys to cover representative habitats and dominant vegetation types. A paired design was implemented, where each survey site (approximately 40 m × 40 m) contained four sampling plots, each measuring 100 m² (10 m × 10 m) (Figure 1). Of these, two plots were fenced as control plots to exclude grazing, allowing vegetation to naturally regenerate, and two plots were open to grazing by semi-wild horses.
Baseline data, corresponding to the existing cattle grazing regime (Supplementary Table S1), were collected for all monitored variables (see next section) at Site 1 and Site 2 in 2021, before cattle removal and the introduction of horses. Subsequent monitoring of vegetation structure and aboveground biomass was conducted in 2023 and 2024, while plant composition was only reassessed in 2024.
Within each sampling plot (10 m × 10 m), vegetation structure was assessed using four perpendicular 5-meter transects originating from the plot center, spaced at 90° angles or the maximum possible angles between them. Vegetation type (grasses, forbs, and shrubs) and height class (0–0.25 m; 0.25–0.50 m; 0.5–1.3 m; 1.3–2 m; 2–4 m; >4 m) was recorded at every meter along these transects. Plant community composition was surveyed in four 1 m × 1 m quadrats, located at the end of each transect. Within each 12m quadrat, the presence and percentage cover category of understory plant species were recorded using Braun-Blanquet’s cover scale (Kent and Coker, 1994): + (<1%); 1 (1-10%); 2 (11-25%); 3 (26-50%); 4 (51-75%), 5 (76-100%). Plants were identified to the species level whenever possible, however, some taxa were only identified at the genus level for the purpose of data analysis, to harmonize species lists and enable comparison between years (Supplementary Tables S2, S3). Aboveground biomass for forbs and grasses was determined by harvesting all plant material within a 0.4 m × 0.4 m quadrat randomly placed within the sampling plot. Biomass sampling was conducted in one fenced (ungrazed) and one non-fenced (open to grazing) plot within each survey site (40 m × 40 m). Collected plant material was sorted into two functional groups (grasses and forbs), oven-dried at 60°C for 72 hours, and weighted to determine dry biomass.
Due to unforeseen land management changes in Site 1 during this study, only four of the initial six survey sites (40 m × 40 m) remained under the planned treatments. Data analysis at Site 1 was therefore based on the remaining four survey sites.
2.3 Data processing and statistical analysis
Data collected at the survey sites (Figure 1) were used to characterize vegetation structure and composition, with data aggregated at the sampling plot level (10 m × 10 m). Four vegetation structure metrics were evaluated: mean grass height (cm), mean shrub height (cm), shrub cover (%) and aboveground shrub biomass (t/ha), the last metric being computed using field data on shrub cover and height (Table 1). These metrics allowed the assessment of semi-wild horse grazing effects, or lack thereof, on vegetation structure in the two main vegetation layers affected by grazing, i.e., the herbaceous layer and the shrub layer, and can be used as an indicator of wildfire hazard (Table 1). Vegetation structure metrics were analyzed using linear mixed models (LMMs) with separate models fitted for each site. Grazing treatment (semi-wild horse grazing vs. no grazing), year, and their interaction were set as fixed factors, and the survey site (40 m × 40 m) was specified as a random factor to account for the repeated measures over time. Assumptions of residuals’ normality and homoscedasticity were graphically checked (Zuur et al., 2009). All variables satisfied the normality and homoscedasticity assumptions. LMMs were run with the lme4 package (Bates et al., 2015).
Table 1. Description of vegetation structure and composition metrics (adapted from Ribeiro et al., 2024).
Regarding plant community composition, three metrics were evaluated: species richness, the ratio of forbs to grasses based on biomass values, and the cumulative importance value (IV) of forbs and grasses based on species relative frequency and dominance in sampling plots (Table 1). Importance values were calculated at the species level (Supplementary Tables S2, S3) following Kent and Coker (1994), and then summed for forbs and grasses. Importance values were also used to identify the species with the highest IV in the baseline year and for each treatment in 2024. Forbs were defined as all species, observed at each site, excluding those from the families Poaceae, Juncaceae, Fagaceae, Dennstaedtiaceae, Aspleniaceae, and the genera Cytisus and Genista (Fabaceae). Additionally, species temporal turnover (baseline (2021) vs. last monitoring year (2024)) was assessed for each sampling plot using the codyn package (Hallett et al., 2016) to evaluate the overall compositional change in plant communities over time. Finally, the presence frequency of Stipa gigantea in 1m2 quadrats was analyzed as an indicator of its dominance under each grazing treatment. This perennial grass is particularly relevant due to its fire-related traits, which contribute to fine fuel loads and fire hazard, including tall flower spikes, a dense and persistent tussock structure, and rapid post-fire regeneration (Prober et al., 2007; Cruz et al., 2017). Differences in species richness and temporal turnover were assessed through ANOVA and paired t-tests using the stats package (R Core Team, 2022). All analyses were conducted in R software (R Core Team, 2022).
3 Results
3.1 Vegetation structure
In Site 1, the values of shrub cover, shrub height and shrub biomass do not suggest any differences between the effects of the grazing treatment (i.e., horse grazing or no-grazing), or survey year (Figures 2a–c; p > 0.05). However, mean grass height showed a significant response to the interaction between grazing treatment and survey year (Figure 2d). More specifically, in 2023, mean grass height increased in the ungrazed plots compared to horse grazed plots (p = 0.047), with this difference becoming more pronounced in 2024 (p < 0.001).
Figure 2. Shrub cover (a) Mean shrub height (b) Shrub biomass (c) and Mean grass height (d) at Site 1 under baseline conditions (i.e., rotational cattle grazing) in 2021 and in the following monitoring years, 2023 and 2024, under continuous horse grazing and no grazing. Solid line and circles represent horse grazed (natural grazing) plots, dashed line and triangles represent ungrazed plots. Solid bars represent means and standard errors. Significant p-values of the main effects in the linear mixed models are reported.
In Site 2, shrub cover was significantly different between the plots allocated to the different treatments at the baseline year (Figure 3a; p = 0.014). While not ideal, this difference was likely caused by operational constraints during the fence installation in a challenging terrain. Shrub cover showed a marginal increase between 2024 and the baseline (p = 0.051), while significant year effects were observed for shrub height and biomass, with increases in 2023 (Figure 3b; p = 0.003 and Figure 3c; p = 0.024) and 2024 (Figures 3b, c; p < 0.001) relative to baseline. No significant year-by-treatment interactions or effects of grazing treatment alone were detected for shrub metrics. Mean grass height values (Figure 3d) showed significant changes related to the survey year, with an increase in 2024 (p = 0.002) relative to the baseline. The interaction between year and treatment was also significant in 2024 (p = 0.016), suggesting a more substantial increase in grass height in ungrazed plots during this year compared to horse grazed plots.
Figure 3. Shrub cover (a) Mean shrub height (b) Shrub biomass (c) and Mean grass height (d) at Site 2 under baseline conditions (i.e., continuous cattle grazing) in 2021 and in the following monitoring years, 2023 and 2024, under continuous horse grazing and no grazing. Solid line and circles represent horse grazed (natural grazing) plots, dashed line and triangles represent ungrazed plots. Solid bars represent means and standard errors. p-values of the main effects in the linear mixed models are reported.
3.2 Vegetation composition
A total of 86 (2021) and 81 (2024) plant taxa were recorded across the four survey sites at Site 1 (Supplementary Table S2), and 130 (2021) and 129 (2024) across the six survey sites at Site 2 (Supplementary Table S3). Average richness values at the sampling plot scale were not statistically different across treatments at both Site 1 and Site 2 (Figure 4; p > 0.05).
Figure 4. Species richness at Site 1 (a) and Site 2 (b) under baseline conditions in 2021, prior to the grazing regime shift, and in 2024, the final monitoring year, under horse grazing and no grazing control plots. Baseline (group 1) includes the sampling plots that transitioned to No grazing, and Baseline (group 2) includes the sampling plots that transitioned to horse grazing. Boxplots represent species richness measured in 10 m × 10 m plots. (mean: asterisk, median: line). No significant effects were found between treatments.
In Site 1, the forb-to-grass (F:G) ratio increased under horse grazing by 2024 (Figure 5a; p = 0.009; Supplementary Figure S2). This ratio was also marginally influenced by the year × treatment interaction in 2024 (Figure 5a; p = 0.056). In contrast, the F:G ratio was not statistically different across years in ungrazed plots (Figure 5a; p > 0.05).
Figure 5. Forb to Grasses (Forb: Grass) Ratio at Site 1 (a) and Site 2 (b) under baseline conditions (i.e., cattle grazing) in 2021 and in the following monitoring years, 2023 and 2024, under horse grazing and no grazing. Solid line and circles represent horse grazed (natural grazing) plots, dashed line and triangles represent ungrazed plots. Solid bars represent means and standard errors. Y-axes are presented on a logarithmic scale to improve visualization of differences in smaller values. p-values of the main effects in the linear mixed models are reported.
In Site 2 (Figure 5b), the F:G ratio showed a consistent increase over time, with an overall significant year effect observed in 2024 (p = 0.015). No significant year-by-treatment interactions or effects of grazing treatment were detected (Figure 5b; p > 0.05; Supplementary Figure S3).
The analysis of the cumulative importance values (IV) of grasses and forbs (Table 2) indicates similar patterns of community change at both Site 1 and Site 2. Namely, a more pronounced decline in the cumulative IV of grasses and a greater increase in that of forbs from baseline to horse grazing conditions. In terms of dominant species (i.e., those with the highest IV), composition remained relatively stable across years and treatments at both sites (Table 2), although some notable shifts were observed. At Site 1, Cytisus multiflorus and Stipa gigantea increased their IVs, and Avena barbata (a tall grass species) emerged as a new dominant in ungrazed plots. At Site 2, C. multiflorus not only consistently exhibited the highest IV but also increased in dominance over time, with treatment differences appearing less influential.
Table 2. Summary of species importance values (IV) at Site 1 and Site 2.
Species turnover in Site 2 was significantly higher in the fenced plots, i.e., that shifted from the baseline to an ungrazed regime (Figure 6b; p = 0.045), whereas differences were not statistically significant at Site 1 (Figure 6a; p > 0.05).
Figure 6. Temporal turnover (2021 – 2024) of plant communities (i.e., changes in composition over time) at Site 1 (a) and Site 2 (b) in areas under horse grazing and in ungrazed control plots, following the shift from the baseline regime in 2021. Boxplots show the distribution of values in 10 m × 10 m plots (mean: asterisk, median: line). Significant differences between the two treatments (Paired t-Test; p < 0.05) are marked in bold.
The presence frequency of S. gigantea in 1 m² plots increased in the ungrazed areas at Site 1 (20 out of 32 plots in 2024 vs. 15 out of 32 in 2021) but remained stable in the horse grazed areas (13 out of 32 in 2024 vs. 12 out of 32 in 2021). In Site 2, the species was not recorded in any plot in 2021 but was observed in one fenced 10 × 10 m sampling plot in 2024.
4 Discussion
This study examined how vegetation structure and composition respond to changes in grazing management under a rewilding approach, focusing on the effects of semi-wild horse grazing and using grazing exclusion as a control treatment. Specifically, we aimed to evaluate the effectiveness of horse grazing in reducing local fire hazard through its effects on vegetation structure while promoting positive impacts on biodiversity, particularly with regard to the composition and richness of the plant community. Finally, we aimed to provide evidence to support adaptive management in a rewilding initiative in the Côa Valley region, Portugal. The introduction of semi-wild horses, evaluated in this study, marks the first step in a long-term strategy to restore meso-herbivore diversity in the landscape.
4.1 Vegetation structural changes
The findings for vegetation structure partially align with our first hypothesis (H1), with horse grazing showing a clear effect on herbaceous vegetation but not on shrubs. The lack of significant treatment effects (i.e., grazed vs. ungrazed plots) on shrub cover and height at both sites (Figures 2, 3) suggests that horse grazing may not exert sufficient pressure to regulate woody vegetation. This is consistent with previous studies indicating that low density grazing by domestic herbivores, particularly horses, may not suppress shrub expansion in abandoned areas of the Mediterranean region (Moinardeau et al., 2016; Fagúndez et al., 2022; Ribeiro et al., 2023, 2024). Moreover, the steeper increase in shrub metrics observed over the years at Site 2 may be partially driven by post‐fire successional processes, as this site recently experienced a severe wildfire in 2017. Fire can reduce competing vegetation, create open space, and release nutrients, thereby promoting rapid regrowth of fire‐adapted shrub species (Cruz et al., 2020; Magaña Ugarte et al., 2021; Alegria, 2022). These favorable post‐disturbance conditions may enable shrub expansion under low grazing pressure (Bates and Davies, 2014; Smit and Coetsee, 2019; Siegel et al., 2022).
The similar trends observed for grass height at both sites, with the significant interaction between year and treatment, indicate that even in the short term, horse grazing can effectively limit grass height, compared to grazing exclusion. These findings reinforce the role of horses as herbaceous grazers, preferentially consuming grasses over woody species (Vulink et al., 2001), whereas cattle tend to include a higher proportion of woody plants in their diet (Cosyns et al., 2001; Menard et al., 2002; Lamoot et al., 2005). The increased presence of the fire-prone Stipa gigantea in ungrazed (control) plots in Site 1 suggests that the lack of grazing pressure may favor its establishment and aligns with previous research indicating that grazing exclusion can promote grass dominance (de Villalobos and Zalba, 2010; Schneider and Hering, 2024), potentially elevating fire hazard in these areas (Cardoso et al., 2022; Davies et al., 2022; Orr et al., 2023). Conversely, the stability of Stipa gigantea’s occurrence in open areas indicates that horse grazing may help contain its spread.
4.2 Vegetation compositional changes
In terms of vegetation composition, although we expected a decline in species richness in ungrazed (control) plots (H2) (Papanikolaou et al., 2011; Henning et al., 2017a; Dvorský et al., 2022; Bonavent et al., 2023), no significant differences were observed in species richness between grazing treatments (horses vs. no grazing) at either site. Despite the lack of significant effects, Figure 4 shows a trend towards lower species richness in ungrazed plots, with decreasing median and mean, while plots that remained grazed maintained similar richness levels. One potential explanation for the absence of statistically significant effects may be the early monitoring stage, as the short duration of grazing exclusion in fenced plots may not yet be sufficient to produce detectable effects (Song et al., 2020). Additionally, at Site 2, species temporal turnover was significantly higher in ungrazed (control) plots when compared to horse-grazed (open) plots, while no significant differences were detected at Site 1. These results provide partial support for our third hypothesis (H3), indicating a greater shift in species composition over time in the absence of grazing. It also suggests that the removal of grazing may lead to shifts in the recruitment success among plant species, such as grazing adapted species that may decline in response to exclusion (Song et al., 2020) or competitively dominant species that were previously suppressed by grazing (Zhang et al., 2023). This result aligns with other studies showing that even when species richness remains unchanged, the cessation of grazing can lead to significant changes in plant community composition over time, as certain species may outcompete others (Schultz et al., 2011; Bar-Massada and Hadar, 2017; Kaufmann et al., 2021).
Furthermore, as expected in our fourth hypothesis (H4), which anticipated a shift in the relative representation of forbs and grasses under different grazing pressure, there was an observed increase in forb-to-grass (F:G) ratio under horse grazing at Site 1, providing additional insights into how this grazing regime can interact with plant functional groups. In contrast, ungrazed plots showed no significant changes in the F:G ratio (Figure 5), suggesting that forb recruitment may be limited without grazing disturbances and may benefit from reduced competition for space with grasses (Fleurance et al., 2012; Tuomi et al., 2019). These results might also help explain the results observed in species temporal turnover, as changes in turnover can be influenced by changes in abundance (Peper et al., 2011; Hillebrand et al., 2018) and grasses are outcompeting forbs in ungrazed plots, potentially leading to lower structural and functional diversity (Yan and Liu, 2021; Cardoso et al., 2022). Additionally, these functional responses align with the results obtained by the analysis of species importance values (IV), which revealed parallel trends in community structure. At both sites, the sum of IV for grasses declined more sharply in horse-grazed plots compared to ungrazed areas, while forbs increased their cumulative IV under grazing. Both metrics suggest that natural horse grazing promotes a shift toward more forb dominated communities (with flowering plants), which enhance food and shelter resources for anthophilous insects (Lázaro et al., 2016; Henning et al., 2017b; Shapira et al., 2020; Dvorský et al., 2022; Garrido et al., 2022; Bonavent et al., 2023), while grazing exclusion favors grass dominance (Supplementary Figures S2, S3). Notably, Stipa gigantea increased its importance in ungrazed plots, reinforcing the previously discussed concerns about fine fuel accumulation (Menning and Stephens, 2007; Cardoso et al., 2022; Davies et al., 2022; Orr et al., 2023). Grass dominance in the absence of grazing could have adverse implications for wildfire hazard and biodiversity conservation, particularly in fire-prone Mediterranean landscapes (Kirkland et al., 2024; Lovreglio et al., 2024), such as the Côa Valley region.
4.3 Management insights
This study provides relevant evidence on the use of semi-wild horse grazing as a tool for vegetation management. On the one hand, horse grazing demonstrated an ability to reduce local fire hazard by effectively controlling the height of grasses (Figures 2, 3), while also promoting a higher representation of forbs in the plant community, as reflected by increases in the forb-to-grass ratio and cumulative importance values (Figure 5 and Table 2). On the other hand, the limited effects on shrub metrics at both sites, with increased shrub cover, height and biomass (Figures 2, 3), suggest that horse grazing alone may be insufficient to regulate woody vegetation. Furthermore, preliminary results obtained from soil and tree recruitment analysis developed in a management action for the study sites showed that shifting the grazing regimes improved soil organic matter (SOM) and oak recruitment (Supplementary Table S4 and Supplementary Figure S4), with greater increases in SOM and higher number of oak seedlings in horse grazing plots, compared to ungrazed plots. Overall, these findings suggest that while semi-wild horse grazing can contribute to multiple management goals, its effectiveness varies across vegetation and habitat types and would benefit from complementary interventions tailored to local vegetation dynamics and disturbance history.
In particular, semi-wild horse grazing alone may not be sufficient to achieve the desired management outcomes for shrub control in the Côa Valley region, which has been affected by recurrent and severe wildfires over the last two decades (Kirkland et al., 2024). The unchecked proliferation of tall grasses can also significantly heighten fire hazard (Menning and Stephens, 2007; Cardoso et al., 2022), which is particularly concerning under the hot and dry conditions typical of Mediterranean summers. However, in grassland-like habitats, where the accumulation of fine fuels can enable rapid fire spread (Turco et al., 2017; El Garroussi et al., 2024), low-intensity semi-wild horse grazing can be effective in preventing grass dominance and reducing wildfire risk, without imposing the grazing pressure that is needed to suppress shrub expansion. Notably, this finding signals the important ecological role of diverse and complementary herbivore communities (Orr et al., 2022; Pringle et al., 2023; Ribeiro et al., 2023). Management strategies should incorporate a diverse assemblage of herbivores, as this is likely to create a mosaic of grazing effects, fostering ecosystem function and resilience in Mediterranean landscapes (Liu et al., 2015; Orr et al., 2022). For example, while horses may be well-suited to certain contexts due to their lighter impact and broad grazing patterns, locally adapted cattle breeds and other rustic breeds, such as Tauros, can complement these effects by targeting different vegetation types or structures (Fleurance et al., 2012; Moinardeau et al., 2016; Schmitz and Isselstein, 2020). This diversified approach is particularly important in habitats prone to shrub encroachment, where it can help break fuel continuity and maintain landscape heterogeneity (Pausas, 2004; Nunes, 2023). Additionally, rewilding initiatives, where herds are semi-wild, social, and not reliant on supplementary feeding (except under extreme conditions), grazing patterns tend to be more heterogeneous than those in livestock production systems (Menard et al., 2002; López et al., 2019). This heterogeneity can promote a more dynamic and resilient vegetation structure by preventing the dominance of single plant functional groups (Adler et al., 2001; Pringle et al., 2023). An important consideration in implementing such management strategies is the definition of appropriate stocking densities for different herbivore assemblages. However, determining adequate densities is challenging, as grazing pressure is not defined by stocking density alone, and its ecological effects are highly context-dependent (Ribeiro et al., 2023). Moreover, in addition to promoting a diverse assemblage of herbivore, complementary management actions, such as initial shrub clearing and prescribed burning, might be needed to support vegetation regulation efforts. These actions can be particularly relevant in shrub encroached areas (Castro et al., 2022; Oikonomou et al., 2023) or where prescribed burning can reduce accumulated litter and biomass and foster herbaceous growth (Barbaro et al., 2001; O’Connor et al., 2020).
In the specific context of the study sites, and of ongoing rewilding efforts, Rewilding Portugal plans to introduce Taurus cattle (Goderie et al., 2013; Stokstad, 2015) at lower densities than those of the pre-existing commercial livestock production, aiming to complement the effects of horse grazing. As this rewilding plan is implemented, ongoing monitoring will be necessary to assess its impacts and provide evidence to evaluate this approach. Moreover, the significant effects of the year factor across sites highlight the need to account for temporal variability in future grazing management plans, as vegetation responses may lag behind grazing interventions (Song et al., 2020; Kaufmann et al., 2021; Ribeiro et al., 2023). Additionally, the interannual variation in weather conditions may have influenced plant growth and community dynamics, potentially interacting with the grazing treatments. Disentangling the effects of grazing and climatic variability also requires a longer monitoring interval to better understand the relative contribution of these factors to the observed changes (Kutiel et al., 2000; Kaufmann et al., 2021). Despite these limitations, early-stage assessments remain essential to detect initial vegetation responses (e.g. Balata et al., 2022; Li and Zhan, 2023), which can guide the adaptive management of rewilding initiatives, especially in fire-prone landscapes where early signs of structural change can inform hazard mitigation.
5 Conclusions
This study highlights the effects of semi-wild horse grazing, under a low-intensity regime, on vegetation structure and composition, after the transition from a more intensive commercial cattle grazing regime, and in comparison to ungrazed conditions. The findings indicate that semi-wild horse grazing can regulate grass height, reduce grass dominance and facilitate the persistence and relative prominence of forbs in the community. This is particularly relevant in fire-prone Mediterranean landscapes, where the unregulated growth of grasses in abandoned ungrazed areas may heighten wildfire hazard. However, the limited effect found on shrub structure suggests that it may not provide sufficient pressure to prevent shrub expansion. From a shrub management perspective, the results suggest the need for a multi-herbivore approach. Complementary species, such as browsers like the red deer, or cattle and other large grazers, can target and exert higher pressure on woody vegetation (Venter et al., 2019), contributing to the creation of a structurally diverse mosaic of habitats. This aligns with rewilding and natural grazing frameworks, where maintaining a diverse assemblage of wild and semi-wild herbivores fosters both temporal and spatial heterogeneity. The year effects observed in this study also emphasize that vegetation responses may exhibit a time lag, requiring long-term monitoring and adaptive management to account for temporal variability in growth cycles and weather conditions. Ultimately, this study contributes to the growing body of evidence on nature-based solutions for Mediterranean ecosystem management and provides an example of integrating scientific research with practical applications to inform management strategies.
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.
Author contributions
IR: Validation, Conceptualization, Writing – review & editing, Methodology, Investigation, Writing – original draft, Visualization, Formal analysis, Software. SA: Project administration, Conceptualization, Writing – review & editing, Investigation, Supervision. TD: Writing – review & editing, Supervision. DM: Supervision, Writing – review & editing. VP: Conceptualization, Methodology, Supervision, Writing – review & editing, Investigation, Validation, Writing – original draft, Formal analysis.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. Financial support for this research was provided by FCT/MCTES (PIDDAC) through projects UIDB/50009/2025, UIDP/50009/2025 and LA/P/0083/2020, 2020.06277.BD (I. Ribeiro) and CEECIND/04469/2017 (V. Proença); and by Fundo Europeu de Desenvolvimento Regional through project “Sheep 4.0 -Indústria 4.0 Sustentável em Ovinos de Leite em Pastagem” (POCI-01-0247-FEDER-069892) (T. Domingos and D. McCracken). This work was supported by the Endangered Landscapes and Seascapes Program (ELSP). ELSP is managed by the Cambridge Conservation Initiative and funded by Arcadia, a charitable fund of Lisbet Rausing and Peter Baldwin. The research was also supported by the EU Horizon 2020 Green Deal program “SILVANUS -Integrated Technological and Information Platform for Wildfire Management” (grant agreement no. 101037247).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declare that no Generative AI was used in the creation of this manuscript.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fevo.2025.1596560/full#supplementary-material
References
Adler P., Raff D., and Lauenroth W. (2001). The effect of grazing on the spatial heterogeneity of vegetation. Oecologia 128, 465–479. doi: 10.1007/s004420100737
Alegria C. (2022). Vegetation monitoring and post-fire recovery: A case study in the centre inland of Portugal. Sustainability 14, 12698. doi: 10.3390/su141912698
Apollonio M., Belkin V. V., Borkowski J., Borodin O. I., Borowik T., Cagnacci F., et al. (2017). Challenges and science-based implications for modern management and conservation of European ungulate populations. Mammal Res. 62, 209–217. doi: 10.1007/s13364-017-0321-5
Balata D., Gama I., Domingos T., and Proença V. (2022). Using satellite NDVI time-series to monitor grazing effects on vegetation productivity and phenology in heterogeneous mediterranean forests. Remote Sens. 14, 2322. doi: 10.3390/rs14102322
Barbaro L., Dutoit T., and Cozic P. (2001). A six-year experimental restoration of biodiversity by shrub-clearing and grazing in calcareous grasslands of the French Prealps. Biodivers. Conserv. 10, 119–135. doi: 10.1023/A:1016629507196/METRICS
Bar-Massada A. and Hadar L. (2017). Grazing and temporal turnover in herbaceous communities in a Mediterranean landscape. J. Veg. Sci. 28, 270–280. doi: 10.1111/JVS.12489
Bates J. D. and Davies K. W. (2014). Cattle grazing and vegetation succession on burned sagebrush steppe. Rangel. Ecol. Manage. 67, 412–422. doi: 10.2111/REM-D-14-00011.1
Bates D., Mächler M., Bolker B., and Walker S. (2015). Fitting linear mixed-effects models using lme4. J. Stat. Software 67, 1–48. doi: 10.18637/jss.v067.i01
Bonavent C., Olsen K., Ejrnæs R., Fløjgaard C., Hansen M. D. D., Normand S., et al. (2023). Grazing by semi-feral cattle and horses supports plant species richness and uniqueness in grasslands. Appl. Veg. Sci. 26, e12718. doi: 10.1111/avsc.12718
Calleja J. A., Escolà M., Carvalho J., Forcadell J. M., Serrano E., and Bartolomé J. (2019). Cattle grazing fails to control shrub encroachment in mediterranean landscapes. Rangel. Ecol. Manage. 72, 803–811. doi: 10.1016/j.rama.2019.04.005
Cardoso A. W., Archibald S., Bond W. J., Coetsee C., Forrest M., Govender N., et al. (2022). Quantifying the environmental limits to fire spread in grassy ecosystems. Proc. Natl. Acad. Sci. 119, e2110364119. doi: 10.1073/pnas.2110364119
Carver S., Convery I., Hawkins S., Beyers R., Eagle A., Kun Z., et al. (2021). Guiding principles for rewilding. Conserv. Biol. 35, 1882–1893. doi: 10.1111/cobi.13730
Casasús I., Bernués A., Sanz A., Villalba D., Riedel J. L., and Revilla R. (2007). Vegetation dynamics in Mediterranean forest pastures as affected by beef cattle grazing. Agric. Ecosyst. Environ. 121, 365–370. doi: 10.1016/j.agee.2006.11.012
Castro M., Castro J. P., and Castro J. (2022). Understory clearing in open grazed mediterranean oak forests: assessing the impact on vegetation. Sustain. 14, 10979. doi: 10.3390/SU141710979/S1
Cosyns E., Degezelle T., Demeulenaere E., and Hoffmann M. (2001). Feeding ecology of Konik horses and donkeys in Belgian coastal dunes and its implications for nature management. Belgian J. Zool 131, 111–118.
Cruz Ó., García-Duro J., Riveiro S. F., García-García C., Casal M., and Reyes O. (2020). Fire Severity Drives the Natural Regeneration of Cytisus scoparius L. (Link) and Salix atrocinerea Brot. Communities and the Germinative Behaviour of These Species. Forests 11, 124. doi: 10.3390/f11020124
Cruz M. G., Sullivan A. L., Hurley R., Plucinski M. P., and Gould J. S. (2017). The effect of fuel load and structure on grassland fire behaviour and fire danger. Canberra. doi: 10.13140/RG.2.2.28866.68804
Cutter J., Hovick T., McGranahan D., Harmon J., Limb R., Spiess J., et al. (2022). Cattle grazing results in greater floral resources and pollinators than sheep grazing in low-diversity grasslands. Ecol. Evol. 12, e8396. doi: 10.1002/ece3.8396
Davies K. W., Wollstein K., Dragt B., and O’connor C. (2022). Grazing management to reduce wildfire risk in invasive annual grass prone sagebrush communities. Rangelands 44, 194–199. doi: 10.1016/j.rala.2022.02.001
de Villalobos A. E. and Zalba S. M. (2010). Continuous feral horse grazing and grazing exclusion in mountain pampean grasslands in Argentina. Acta Oecologica 36, 514–519. doi: 10.1016/j.actao.2010.07.004
Dvorský M., Mudrák O., Doležal J., and Jirků M. (2022). Reintroduction of large herbivores restored plant species richness in abandoned dry temperate grassland. Plant Ecol. 223, 525–535. doi: 10.1007/s11258-022-01225-w
El Garroussi S., Di Giuseppe F., Barnard C., and Wetterhall F. (2024). Europe faces up to tenfold increase in extreme fires in a warming climate. NPJ Clim. Atmos. Sci. 2024 71 7, 1–11. doi: 10.1038/s41612-024-00575-8
Enes T., Lousada J., Fonseca T., Viana H., Calvão A., and Aranha J. (2020). Large scale shrub biomass estimates for multiple purposes. Life 10, 33. doi: 10.3390/life10040033
Fagúndez J., Lagos L., Cortés Vázquez J. A., and Canastra F. (2022). Galician wild ponies. Socio-economic context and environmental benefits: galicia area report and case study for grazeLIFE (LIFE18 PRE NL 002). Universidade da Coruña, Servizo de Publicacións. 48. doi: 10.17979/spudc.9788497498241
Fernandes P. M. (2009). Combining forest structure data and fuel modelling to classify fire hazard in Portugal. Ann. For. Sci. 66, 415–415. doi: 10.1051/forest/2009013
Fleurance G., Edouard N., Collas C., Duncan P., Farruggia A., Baumont R., et al. (2012). “How do horses graze pastures and affect the diversity of grassland ecosystems?,” in Forages and grazing in horse nutrition. Eds. Saastamoinen M., Fradinho M. J., Santos A. S., and Miraglia N. (Wageningen Academic Publishers, Wageningen), 147–161. doi: 10.3920/978-90-8686-755-4_16
Garrido P., Mårell A., Öckinger E., Skarin A., Jansson A., and Thulin C. (2019). Experimental rewilding enhances grassland functional composition and pollinator habitat use. J. Appl. Ecol. 56, 946–955. doi: 10.1111/1365-2664.13338
Garrido P., Naumov V., Söderquist L., Jansson A., and Thulin C.-G. (2022). Effects of experimental rewilding on butterflies, bumblebees and grasshoppers. J. Insect Conserv. 26, 763–771. doi: 10.1007/s10841-022-00420-4
Goderie R., Helmer W., Kerkdijk-Otten H., and Widstrand S. (2013). The Aurochs. Born to be wild. Ed. Widstrand S. (The Netherlands: Roodbont Publishers B.V).
Goosey H. B., Blanchette G. E., and Naugle D. E. (2024). Pollinator response to livestock grazing: implications for rangeland conservation in sagebrush ecosystems. J. Insect Sci. 24, 69. doi: 10.1093/jisesa/ieae069
Hallett L. M., Jones S. K., MacDonald A. A. M., Jones M. B., Flynn D. F. B., Ripplinger J., et al. (2016). codyn: An r package of community dynamics metrics. Methods Ecol. Evol. 7, 1146–1151. doi: 10.1111/2041-210X.12569
Henning K., Lorenz A., von Oheimb G., Härdtle W., and Tischew S. (2017a). Year-round cattle and horse grazing supports the restoration of abandoned, dry sandy grassland and heathland communities by supressing Calamagrostis epigejos and enhancing species richness. J. Nat. Conserv. 40, 120–130. doi: 10.1016/j.jnc.2017.10.009
Henning K., von Oheimb G., Härdtle W., Fichtner A., and Tischew S. (2017b). The reproductive potential and importance of key management aspects for successful Calluna vulgaris rejuvenation on abandoned Continental heaths. Ecol. Evol. 7, 2091–2100. doi: 10.1002/ece3.2816
Herrero-Juregui C. and Oesterheld M. (2018). Effects of grazing intensity on plant richness and diversity: a meta-analysis. Oikos 127, 757–766. doi: 10.1111/oik.04893
Hillebrand H., Blasius B., Borer E. T., Chase J. M., Downing J. A., Eriksson B. K., et al. (2018). Biodiversity change is uncoupled from species richness trends: Consequences for conservation and monitoring. J. Appl. Ecol. 55, 169–184. doi: 10.1111/1365-2664.12959
Honrado J. P., Lomba A., Alves P., Aguiar C., Monteiro-Henriques T., Cerqueira Y., et al. (2017). Conservation management of EU priority habitats after collapse of traditional pastoralism: navigating socioecological transitions in mountain rangeland. Rural Sociol. 82, 101–128. doi: 10.1111/ruso.12111
Jáuregui B. M., García U., Osoro K., and Celaya R. (2009). Sheep and goat grazing effects on three Atlantic heathland types. Rangel. Ecol. Manage. 62, 119–126. doi: 10.2111/07-120.1
Johnson C. N., Prior L. D., Archibald S., Poulos H. M., Barton A. M., Williamson G. J., et al. (2018). Can trophic rewilding reduce the impact of fire in a more flammable world? Philos. Trans. R. Soc B Biol. Sci. 373, 20170443. doi: 10.1098/rstb.2017.0443
Kaufmann R., Mayer R., Schallhart N., and Erschbamer B. (2021). Effects of climate change vs. Grazing exclusion on species diversity over 18 years along an elevation gradient in the European alps. Front. Ecol. Evol. 9. doi: 10.3389/fevo.2021.640103
Kazanis D., Xanthopoulos G., and Arianoutsou M. (2012). Understorey fuel load estimation along two post-fire chronosequences of Pinus halepensis Mill. forests in Central Greece. J. For. Res. 17, 105–109. doi: 10.1007/s10310-011-0250-0
Kent M. and Coker P. (1994). Vegetation description and analysis: a practical approach (Chichester: John Wiley and Sons).
Kirkland M., Atkinson P. W., Aliácar S., Saavedra D., De Jong M. C., Dowling T. P. F., et al. (2024). Protected areas, drought, and grazing regimes influence fire occurrence in a fire-prone Mediterranean region. Fire Ecol. 20, 88. doi: 10.1186/s42408-024-00320-9
Kutiel P., Kutiel H., and Lavee H. (2000). Vegetation response to possible scenarios of rainfall variations along a Mediterranean–extreme arid climatic transect. J. Arid Environ. 44, 277–290. doi: 10.1006/JARE.1999.0602
Lai L. and Kumar S. (2020). A global meta-analysis of livestock grazing impacts on soil properties. PloS One 15, e0236638. doi: 10.1371/journal.pone.0236638
Lamoot I., Meert C., and Hoffmann M. (2005). Habitat use of ponies and cattle foraging together in a coastal dune area. Biol. Conserv. 122, 523–536. doi: 10.1016/j.biocon.2004.09.009
Lázaro A., Tscheulin T., Devalez J., Nakas G., Stefanaki A., Hanlidou E., et al. (2016). Moderation is best: effects of grazing intensity on plant–flower visitor networks in Mediterranean communities. Ecol. Appl. 26, 796–807. doi: 10.1890/15-0202
Lecomte X., Bugalho M. N., Catry F. X., Fernandes P. M., Cera A., and Caldeira M. C. (2024). Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands. Ecol. Appl. 34, e2971. doi: 10.1002/eap.2971
Li Y. and Zhan Y. (2023). Short-term restoration effects of ecological projects detected using the turning point method in the Three River Headwater Region, China. Front. Plant Sci. 14. doi: 10.3389/fpls.2023.1239417
Liu J., Feng C., Wang D., Wang L., Wilsey B. J., and Zhong Z. (2015). Impacts of grazing by different large herbivores in grassland depend on plant species diversity. J. Appl. Ecol. 52, 1053–1062. doi: 10.1111/1365-2664.12456
López C. L., Celaya R., Ferreira L. M. M., García U., Rodrigues M. A. M., and Osoro K. (2019). Comparative foraging behaviour and performance between cattle and horses grazing in heathlands with different proportions of improved pasture area. J. Appl. Anim. Res. 47, 377–385. doi: 10.1080/09712119.2019.1649679
Lovreglio R., Lovreglio J., Satta G. G. A., Mura M., and Pulina A. (2024). Assessing the role of forest grazing in reducing fire severity: A mitigation strategy. Fire 7, 409. doi: 10.3390/FIRE7110409
Lovreglio R., Meddour-Sahar O., and Leone V. (2014). Goat grazing as a wildfire prevention tool: a basic review. iForest - Biogeosciences For. 7, 260–268. doi: 10.3832/ifor1112-007
Maes J., Paracchini M. L., Zulian G., Dunbar M. B., and Alkemade R. (2012). Synergies and trade-offs between ecosystem service supply, biodiversity, and habitat conservation status in Europe. Biol. Conserv. 155, 1–12. doi: 10.1016/j.biocon.2012.06.016
Magaña Ugarte R., Redondo M. M., and Sánchez-Mata D. (2021). Evaluating the post-fire natural regeneration of Mediterranean-type scrublands in Central Spain. Mediterr. Bot. 42, e67331. doi: 10.5209/mbot.67331
Massenberg J. R., Schiller J., and Schröter-Schlaack C. (2023). Towards a holistic approach to rewilding in cultural landscapes. People Nat. 5, 45–56. doi: 10.1002/pan3.10426
Menard C., Duncan P., Fleurance G., Georges J., and Lila M. (2002). Comparative foraging and nutrition of horses and cattle in European wetlands. J. Appl. Ecol. 39, 120–133. doi: 10.1046/j.1365-2664.2002.00693.x
Menning K. M. and Stephens S. L. (2007). Fire climbing in the forest: A semiqualitative, semiquantitative approach to assessing ladder fuel hazards. West. J. Appl. For. 22, 88–93. doi: 10.1093/wjaf/22.2.88
Moinardeau C., Mesléard F., and Dutoit T. (2016). Using different grazing practices for increasing plant biodiversity in the dykes and embankments along the rhône river (Southern France). Environ. Manage. 58, 984–997. doi: 10.1007/s00267-016-0744-9
Molle G., Cannas A., and Gregorini P. (2022). A review on the effects of part-time grazing herbaceous pastures on feeding behaviour and intake of cattle, sheep and horses. Livest. Sci. 263, 104982. doi: 10.1016/j.livsci.2022.104982
Mouissie A. M. (2004). Seed dispersal by large herbivores: Implications for the restoration of plant biodiversity (The Netherlands: University of Groningen).
Mutillod C., Buisson É., Mahy G., Jaunatre R., Bullock J. M., Tatin L., et al. (2024). Ecological restoration and rewilding: two approaches with complementary goals? Biol. Rev. 99, 820–836. doi: 10.1111/BRV.13046
Norton B. A., Bending G. D., Clark R., Corstanje R., Dunnett N., Evans K. L., et al. (2019). Urban meadows as an alternative to short mown grassland: effects of composition and height on biodiversity. Ecol. Appl. 29, 1095–1115. doi: 10.1002/eap.1946
Nunes L. J. R. (2023). Effects of climate change on temperate forests in the northwest Iberian Peninsula. Clim. 11, 173. doi: 10.3390/CLI11080173
O’Connor R. C., Taylor J. H., and Nippert J. B. (2020). Browsing and fire decreases dominance of a resprouting shrub in woody encroached grassland. Ecology 101, e02935. doi: 10.1002/ECY.2935
Oikonomou D., Vrahnakis M., Yiakoulaki M., Xanthopoulos G., and Kazoglou Y. (2023). Grazing as a management tool in mediterranean pastures: A meta-analysis based on a literature review. Land 12, 1290. doi: 10.3390/land12071290
Orr D. A., Bates J. D., and Davies K. W. (2023). Grazing intensity effects on fire ignition risk and spread in sagebrush steppe. Rangel. Ecol. Manage. 89, 51–60. doi: 10.1016/j.rama.2022.08.004
Orr D. A., Bui A., Klope M., McCullough I. M., Lee M., Motta C., et al. (2022). Context-dependent effects of shifting large herbivore assemblages on plant structure and diversity. J. Ecol. 110, 1312–1327. doi: 10.1111/1365-2745.13871
Osem Y., Perevolotsky A., and Kigel J. (2007). Interactive effects of grazing and shrubs on the annual plant community in semi-arid Mediterranean shrublands. J. Veg. Sci. 18, 869. doi: 10.1658/1100-9233(2007)18[869:ieogas]2.0.co;2
Papanikolaou A. D., Fyllas N. M., Mazaris A. D., Dimitrakopoulos P. G., Kallimanis A. S., and Pantis J. D. (2011). Grazing effects on plant functional group diversity in Mediterranean shrublands. Biodivers. Conserv. 20, 2831–2843. doi: 10.1007/s10531-011-0112-2
Pausas J. G. (2004). Changes in fire and climate in the Eastern Iberian Peninsula (Mediterranean basin). Clim. Change 63, 337–350. doi: 10.1023/B:CLIM.0000018508.94901.9c
Peper J., Jansen F., Pietzsch D., and Manthey M. (2011). Patterns of plant species turnover along grazing gradients. J. Veg. Sci. 22, 457–466. doi: 10.1111/j.1654-1103.2011.01260.x
Pereira H. M. and Navarro L. M. (2015). Rewilding European Landscapes. Eds. Pereira H. M. and Navarro L. M. (Cham: Springer International Publishing). doi: 10.1007/978-3-319-12039-3
Perino A., Pereira H. M., Navarro L. M., Fernández N., Bullock J. M., Ceaușu S., et al. (2019). Rewilding complex ecosystems. Sci. 364, eaav5570. doi: 10.1126/science.aav5570
Pinheiro M., Kjöllerström H. J., and Oom M. M. (2013). Genetic diversity and demographic structure of the endangered Sorraia horse breed assessed through pedigree analysis. Livest. Sci. 152, 1–10. doi: 10.1016/j.livsci.2012.11.017
Pringle R. M., Abraham J. O., Anderson T. M., Coverdale T. C., Davies A. B., Dutton C. L., et al. (2023). Impacts of large herbivores on terrestrial ecosystems. Curr. Biol. 33, R584–R610. doi: 10.1016/j.cub.2023.04.024
Prober S. M., Thiele K. R., and Lunt I. D. (2007). Fire frequency regulates tussock grass composition, structure and resilience in endangered temperate woodlands. Austral Ecol. 32, 808–824. doi: 10.1111/j.1442-9993.2007.01762.x
Psyllos G., Hadjigeorgiou I., Dimitrakopoulos P. G., and Kizos T. (2022). Grazing land productivity, floral diversity, and management in a semi-arid mediterranean landscape. Sustainability 14, 4623. doi: 10.3390/su14084623
Rakosy D., Motivans E., Ştefan V., Nowak A., Świerszcz S., Feldmann R., et al. (2022). Intensive grazing alters the diversity, composition and structure of plant-pollinator interaction networks in Central European grasslands. PloS One 17, e0263576. doi: 10.1371/JOURNAL.PONE.0263576
R Core Team (2022). R: A language and Environment for Statistical Computing (Vienna, Austria: R Foundation for Statistical Computing).
Ribeiro I., Domingos T., McCracken D., and Proença V. (2023). The use of domestic herbivores for ecosystem management in Mediterranean landscapes. Glob. Ecol. Conserv. 46, e02577. doi: 10.1016/j.gecco.2023.e02577
Ribeiro I., Domingos T., McCracken D., and Proença V. (2024). Evaluating domestic herbivores for vegetation structure management in transitional woodland–shrubland systems. Forests 15, 2258. doi: 10.3390/F15122258/S1
Riedel J. L., Bernués A., and Casasús I. (2013). Livestock grazing impacts on herbage and shrub dynamics in a Mediterranean Natural Park. Rangel. Ecol. Manage. 66, 224–233. doi: 10.2111/REM-D-11-00196.1
Rogers H. S., Donoso I., Traveset A., and Fricke E. C. (2021). Cascading impacts of seed disperser loss on plant communities and ecosystems. Annu. Rev. Ecol. Evol. Syst. 52, 641–666. doi: 10.1146/annurev-ecolsys-012221-111742
Rosa García R., Fraser M. D., Celaya R., Ferreira L. M. M., García U., and Osoro K. (2013). Grazing land management and biodiversity in the Atlantic European heathlands: a review. Agrofor. Syst. 87, 19–43. doi: 10.1007/s10457-012-9519-3
Saiz H. and Alados C. L. (2012). Changes in semi-arid plant species associations along a livestock grazing gradient. PloS One 7, e40551. doi: 10.1371/journal.pone.0040551
Santana V. M., Baeza M. J., Valdecantos A., and Vallejo V. R. (2018). Redirecting fire-prone Mediterranean ecosystems toward more resilient and less flammable communities. J. Environ. Manage. 215, 108–115. doi: 10.1016/j.jenvman.2018.03.063
Schmitz A. and Isselstein J. (2020). Effect of grazing system on grassland plant species richness and vegetation characteristics: Comparing horse and cattle grazing. Sustain. 12, 1–17. doi: 10.3390/SU12083300
Schneider A. R. and Hering D. (2024). Effects of extensive grazing and mowing compared to abandonment on the biodiversity of European grasslands: A meta-analysis. Appl. Veg. Sci. 27, e70003. doi: 10.1111/AVSC.70003
Schoenbaum I., Henkin Z., Yehuda Y., Voet H., and Kigel J. (2018). Cattle foraging in Mediterranean oak woodlands – Effects of management practices on the woody vegetation. For. Ecol. Manage. 419–420, 160–169. doi: 10.1016/j.foreco.2018.03.017
Schultz N. L., Morgan J. W., and Lunt I. D. (2011). Effects of grazing exclusion on plant species richness and phytomass accumulation vary across a regional productivity gradient. J. Veg. Sci. 22, 130–142. doi: 10.1111/J.1654-1103.2010.01235.X
Seddon P. J., Griffiths C. J., Soorae P. S., and Armstrong D. P. (2014). Reversing defaunation: Restoring species in a changing world. Science 345, 406–412. doi: 10.1126/science.1251818
Shackelford N. and McDougall C. (2023). Ecosystem restoration, regeneration and rewilding. BMC Ecol. Evol. 23, 1–3. doi: 10.1186/S12862-023-02165-3/METRICS
Shapira T., Henkin Z., Dag A., and Mandelik Y. (2020). Rangeland sharing by cattle and bees: moderate grazing does not impair bee communities and resource availability. Ecol. Appl. 30, 1–15. doi: 10.1002/eap.2066
Siegel K. J., Macaulay L., Shapero M., Becchetti T., Larson S., Mashiri F. E., et al. (2022). Impacts of livestock grazing on the probability of burning in wildfires vary by region and vegetation type in California. J. Environ. Manage. 322, 116092. doi: 10.1016/j.jenvman.2022.116092
Silva V., Catry F. X., Fernandes P. M., Rego F. C., Paes P., Nunes L., et al. (2019). Effects of grazing on plant composition, conservation status and ecosystem services of Natura 2000 shrub-grassland habitat types. Biodivers. Conserv. 28, 1205–1224. doi: 10.1007/s10531-019-01718-7
Smit I. P. J. and Coetsee C. (2019). “Interactions between fire and herbivory: current understanding and management implications,” in The Ecology of Browsing and Grazing II. Eds. Gordon I. J. and Prins H. H. T. (Springer International Publishing, Cham), 301–319. doi: 10.1007/978-3-030-25865-8_13
Song S., Zhu J., Zheng T., Tang Z., Zhang F., Ji C., et al. (2020). Long-term grazing exclusion reduces species diversity but increases community heterogeneity in an alpine grassland. Front. Ecol. Evol. 8. doi: 10.3389/FEVO.2020.00066/BIBTEX
Souther S., Loeser M., Crews T. E., and Sisk T. (2019). Complex response of vegetation to grazing suggests need for coordinated, landscape-level approaches to grazing management. Glob. Ecol. Conserv. 20, e00770. doi: 10.1016/j.gecco.2019.e00770
Stokstad E. (2015). Bringing back the aurochs. Science 350, 1144–1147. doi: 10.1126/science.350.6265.1144
Teague R. and Kreuter U. (2020). Managing grazing to restore soil health, ecosystem function, and ecosystem services. Front. Sustain. Food Syst. 4. doi: 10.3389/fsufs.2020.534187
Teixeira R. F. M., Proença V., Crespo D., Valada T., and Domingos T. (2015). A conceptual framework for the analysis of engineered biodiverse pastures. Ecol. Eng. 77, 85–97. doi: 10.1016/j.ecoleng.2015.01.002
Török P., Lindborg R., Eldridge D., and Pakeman R. (2024). Grazing effects on vegetation: Biodiversity, management, and restoration. Appl. Veg. Sci. 27, e12794. doi: 10.1111/AVSC.12794
Torralba M., Fagerholm N., Burgess P. J., Moreno G., and Plieninger T. (2016). Do European agroforestry systems enhance biodiversity and ecosystem services? A meta-analysis. Agric. Ecosyst. Environ. 230, 150–161. doi: 10.1016/J.AGEE.2016.06.002
Tuomi M., Stark S., Hoset K. S., Väisänen M., Oksanen L., Murguzur F. J. A., et al. (2019). Herbivore effects on ecosystem process rates in a low-productive system. Ecosystems 22, 827–843. doi: 10.1007/s10021-018-0307-4
Turco M., Von Hardenberg J., AghaKouchak A., Llasat M. C., Provenzale A., and Trigo R. M. (2017). On the key role of droughts in the dynamics of summer fires in Mediterranean Europe. Sci. Rep. 7, 1–10. doi: 10.1038/s41598-017-00116-9
Van-camp L., Bujarrabal B., Gentile A. R., Jones R. J. A., Montanarella L., Olazabal C., et al. (2004). Reports of the Technical Working Groups Established under the Thematic Strategy for Soil Protection (Luxembourg: Office for Official Publications of the European Communities).
van den Elsen E., Stringer L. C., De Ita C., Hessel R., Kéfi S., Schneider F. D., et al. (2020). Advances in understanding and managing catastrophic ecosystem shifts in mediterranean ecosystems. Front. Ecol. Evol. 8. doi: 10.3389/FEVO.2020.561101/BIBTEX
Van Meerbeek K., Muys B., Schowanek S. D., and Svenning J.-C. (2019). Reconciling conflicting paradigms of biodiversity conservation: human intervention and rewilding. Bioscience 69, 997–1007. doi: 10.1093/BIOSCI/BIZ106
Venter J. A., Vermeulen M. M., and Brooke C. F. (2019). Feeding ecology of large browsing and grazing herbivores. The Ecology of Browsing and Grazing II. 127–153. doi: 10.1007/978-3-030-25865-8_5
Vulink J. T., Cornelissen P., Drost H. J., and Prins H. (2001). “Hindgut fermentation is not an evolutionary dead end: comparitive feeding ecology of cattle and horses,” in Hungry Herds: Management of Temperate Lowland Wetlands by Grazing. Ed. Vulink J. T. (The Netherlands: Rijksuniversiteit Groningen), 87–103.
Yan H. and Liu G. (2021). Fire’s effects on grassland restoration and biodiversity conservation. Sustain. 13, 12016. doi: 10.3390/SU132112016
Zeng H., Wu B., Zhang M., Zhang N., Elnashar A., Zhu L., et al. (2021). Dryland ecosystem dynamic change and its drivers in Mediterranean region. Curr. Opin. Environ. Sustain. 48, 59–67. doi: 10.1016/j.cosust.2020.10.013
Zhang Y., Jin B., Zhang X., Wei H., Chang Q., Huang F., et al. (2023). Grazing alters the relationships between species diversity and biomass during community succession in a semiarid grassland. Sci. Total Environ. 887, 164155. doi: 10.1016/J.SCITOTENV.2023.164155
Keywords: ecosystem management, rewilding, natural grazing, semi-wild horses, vegetation structure, plant community
Citation: Ribeiro I, Aliácar S, Domingos T, McCracken D and Proença V (2025) Semi-wild horse grazing as a rewilding strategy: assessing effects on vegetation structure and composition in the Côa Valley, Portugal. Front. Ecol. Evol. 13:1596560. doi: 10.3389/fevo.2025.1596560
Received: 24 March 2025; Accepted: 20 May 2025;
Published: 18 June 2025.
Edited by:
Nadia S. Santini, Universidad Nacional Autónoma de México (UNAM), MexicoReviewed by:
Joel Segarra, University of Barcelona, SpainFernando Furquim, University of the Region of Campanha, Brazil
Gina Arena, Conservation International, United States
Copyright © 2025 Ribeiro, Aliácar, Domingos, McCracken and Proença. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Inês Ribeiro, aW5lcy5tLnJpYmVpcm9AdGVjbmljby51bGlzYm9hLnB0