A species or group of species that respond well to environmental disturbances or a change in the status of the environment in predictable ways that can be easily observed and quantified is known as an environmental indicator. It is used to detect changes in the environment.

The species or group of species demonstrates the impact of a stressor on a biotic system and is used to monitor long-term stressor-induced change on biota (including habitat alteration, fragmentation and climate change).

A group of taxa (such as a genus, tribe, family, or order, or a particular group of species from a range of higher taxa) or a functional group that exhibits diversity (character and species richness, or level of endemism) that is reflective of the diversity of other taxa in a habitat is referred to as a biodiversity indicator (Mc Geoch, 1998; Stewart et al., 2007).

Also, the term bioindicators can mean different types of indicators, including plant, animal, and microbial indicators according to their taxonomic status (Figure 2).

In the era of urbanization, pollution continues to threaten health and welfare. The Plant shows high sensitivity to environmental stress recognition and prediction. Biomonitoring with sensitive plant species performs an effective means of attenuating this problem. Increased levels of pollutants like sulphur and nitrogen are responsible for the disappearance of lichens from forest areas (Holt and Miller, 2010). Marine ecosystem contamination is indicated by alterations in the diversity of phytoplankton species, such as Euglena clastica, Phacus tortus, and Trachelon anas (Jain et al., 2010). Cyanophyta is a biological indicator of powerful plankton that shows the rapid eutrophication of aquatic habitats through the production of bloom patterns (Thakur et al., 2013). The impact of air pollutants can be interpreted by calculating Air Pollution Tolerance Index (APTI) using biochemical parameters of plants like total chlorophyll content, ascorbic acid content, relative water content, and pH value (Singare and Talpade, 2013). The Guava cultivar “Paluma” can potentially be used as a bioindicator of phytotoxic ozone levels in the tropical region (Furlan et al., 2007). Auto fluorescence analyses of the secretory cells of vegetative tissues or microspores are suitable objects for ozone (O₃) monitoring and could early indicate damage by stress factors (Roshchina and Roshchina, 2003).

Changes in a number of an organism’s characteristics have been successfully employed as indicators. Animal indicators assist in determining the level of toxins present in an animal’s tissues (Khatri and Tyagi, 2015). Zooplanktons that are zone-based indicators of pollution include Alona guttata, Mesocyclops edax, Cyclops, and Aheyella (Jain et al., 2010; Hosmani, 2014). They are used to indicate water quality, eutrophication, and contaminated water. Amphibians, notably anurans, which include frogs and toads, are frequently used as biological indicators of contaminant accumulation in a specific ecosystem (Zaghloul et al., 2020). Earthworms, a key soil system species, can play a vital role in the formation and decomposition of soil aggregates, according to the kind of organic residues (Al-Maliki et al., 2021). Earthworms are more sensitive to temperature changes as a bioindicator of soil quality than the effects of moisture content. Invertebrates are also reliable bioindicators; insects play vital roles as indicators in ecosystems. Environmental stress can be assessed at several scales, from the individual animal to the entire invertebrate community.

Microorganisms are potentially used as indicator species to detect environmental pressures when exposed to environmental perturbations. They exhibit high sensitivity towards environmental change. Changes in the physiological system of microbes like the development of stress protein (Khatri and Tyagi, 2015) or alterations in the digestive system help to detect the presence of toxins in water (Uttah et al., 2008). A group of eukaryotic microbes, Foraminifera is used for coral reef monitoring (Hallock et al., 2003). Assessment of coral reef health through biofilms coupled with Autonomous Reef Monitoring Structures (ARMS) tools are also employed (Roitman et al., 2018). Bacterial orders such as the opitulales, chitinophagales, cytophagales and saccharimonadales may act as bioindicators of high P or nutrient-rich locations (Mason et al., 2020). Molds, including Trichoderma sp., Exophiala sp., Stachybotrys sp., Aspergillus fumigatus, A. versicolor, A. niger, Phialophora sp., Fusarium sp., Ulocladium sp., Penicillium sp., Candida albicans, and certain yeasts are frequently used as biological indicators for contaminants. According to (Dokulil, 2003) blue-green algae may be used as a biological indicator for detecting pH value variations in various ecosystems.

Coleopterans are considered as potential bioindicators and mainly belong to the group Carabidae, Staphylinidae, and Curculionidae. The most popular insect utilized in environmental biomonitoring is the ground beetle, especially for evaluating environmental pollutants and forest management (Ghannem et al., 2018). They are collected by using pitfall traps from the region of chronic heavy metal pollution, which has resulted in an accumulation of various heavy metals in the soil. Multiple morphological traits are assessed to monitor heavy metal pollution like elytra length, body size, etc. (Lagisz, 2008). The toxic effect of copper has resulted in changes in the locomotory behaviour of adult carabids and a high degree of larval mortality. It is believed to be that behavioural changes are associated with copper exposure during larval development (Bayley et al., 1995). With research on Pterostichus oblongopunctatus (Simon et al., 2016), observed high BAF (Bioaccumulation factor) values for Copper (Cu) and Zinc (Zn) indicating that this species is preferable in metal contamination assessment. Additionally, it has been demonstrated by (Conti et al., 2017) that the carabid sp. Parallelomorphus laevigatus is a reliable indicator of toxic substances. In forests or on farms, scarabaeid beetles have a tremendous amount of potential as environmental indicators (Niemela et al., 1993). Dung beetles are likewise affected by forest fragmentation, and their abundance and species richness are positively correlated with the size of the fragments (Rainio and Niemela, 2003). To evaluate the genotoxic effects of herbicides on non-target soil-dwelling species, Harpalus rufipes provides early warning signs (Cavaliere et al., 2019).

Butterflies are significant bioindicators on account of their sensitivity towards the slightest alteration in environmental factors. They have been extensively used as heavy metal and environmental pollution bioindicators that are close to industrial states and even inside metropolitan regions (Da Renato et al., 2010). Butterflies are very sensitive to climatic variables. The temperature has a significant impact on butterfly range shifts, oviposition sites, egg-laying rates, larval development, and survival rates (Sharma and Sharma, 2017). Pupal size of Noctuidae and Geometridae species changes with levels of industrial air pollutants (Heliövaara et al., 1989). According to (Kyerematen et al., 2018), different environmental stressors in Sierra Leone’s wetlands affect butterfly communities, and the higher the stress, the lesser the diversity of butterflies in a specified place. According to the estimates, the above observation is the consequence of significant environmental stresses in the Sierra Leone River Estuary (SLRE), including mining, agricultural, and factory pollution. The results further show that Mamunta Mayosso Wildlife Sanctuary (MMWS) is made up of a mosaic of several vegetation patches which supports higher diversity of butterfly species. This study also reveals that anthropogenic activities have a deleterious impact on butterfly diversity. Moths have been utilized as bioindicators to monitor the recovery of vegetation following environmental stress (Kumar et al., 2015). Arctiinae, Catocalinae, Heliothinae, Noctuinae, Hermeniidae, and Phycitinae are some moth families/subfamilies that respond positively to disturbances, while Ennominae, Geometrinae, Lymantriidae, Epipaschiinae, and Anthelidae respond negatively (David, 1989). Kozlov et al. (2022) also noted that the overall abundance of Lepidoptera (moths and butterflies) did not alter along the pollution gradient. The life history characteristics of each species were linked to how it responded to pollution. Insect ability to withstand heavy metals varies widely. Despite the global decline in pollinators, little is understood about how heavy metals affect insects. According to Skaldina and Sorvari (2019), the heavy metal pollution alters the morphology and physiology of insect pests including aphids and butterfly larvae.

Ants are ubiquitous in almost all the trophic levels of the web, food chains, and other ecological functions. They are used as a good bioindicator group for various types of pollution (Kaspari and Majer, 2000; Andersen et al., 2002) including soil-heavy metal contamination (Gramigni et al., 2013; Khan et al., 2017), aerial phthalate pollution (Lenoir et al., 2014) and land rehabilitation (Khan et al., 2017). Red wood ant Formica lugubris accumulates heavy metals such as Aluminum (Al), Cadmium (Cd), Cobalt (Co), Copper (Cu), Iron (Fe), Nickel (Ni), Lead (Pb), and Zinc (Zn) both in worker ant bodies and nest material (Skaldina et al., 2018). Ants are increasingly being used as bioindicators to monitor ecosystem health conditions (Akhila and Keshamma, 2022). The ant fauna also somewhat reflects the other invertebrate taxa found in a region (Majer, 1983). Ants are frequently employed as efficient disturbance bioindicators for restoring biodiversity and managing ecosystems (Underwood and Fisher, 2006) because of their ecological relevance (Tibcherani et al., 2018) and sensitivity to ecosystem disruption brought on by species invasion, grazing, thinning of the forest, conversion of the forest, fragmentation of the forest, and other disturbances (Da Renato et al., 2010).

Social insects can consume pollutants while foraging and then pass them on to the larvae or mix them into the materials used to build their nests. Pollutants in bees can also wind up in food that has been kept, such as honey or bee bread (Feldhaar and Otti, 2020). Honey bees are reliable and efficient biological indicators because they exhibit environmental chemical impairment due to a high rate of mortality and intercept particles suspended in air or flowers. They were incredibly effective in ground surveys due to their high mobility. Honey bees are used to monitor pesticides, heavy metals, and radionuclides (Porrini et al., 2003). Studied by (Di Fiore et al., 2022) the concentrations of different elements viz., Beryllium (Be), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Nickel (Ni), Lead (Pb), Copper (Cu), and Vanadium (V) in urban and rural areas. Later, they concluded that the honeybee can accumulate environmental pollutants and provide data on the progression of environmental pollution across time and space. Honey bees use two signals to indicate the chemical disruption of the environment, i.e., mortality (mostly due to pesticide residues) and residues detected in their bodies or bee hive products (pesticides and other contaminants like heavy metals and radionuclides), that can be determined employing appropriate laboratory analysis (Barganska et al., 2016). There is a possibility that the honey bee colony will absorb pesticides in layers which represents the yearly treatment strategy throughout the colony’s territory because of its diverse range of feeding preferences and extended foraging season. Careful pesticide content monitoring can reveal variations in the persistence of agrochemicals in particular hive compartments (Quigley et al., 2019). When compared to colonies that were taken in less disturbed habitats, those that were obtained in heavily anthropogenic areas show more asymmetry. Indeed, morphological characteristics can also reveal the habitat quality (Nunes et al., 2015). Cunningham et al. (2022) stated that the pollinator European honey bee, Apis mellifera exposed to pollutants and diseases while foraging they bring these things back to their hives where they may be identified and used as biomonitoring. Polluted substance is found in bees and their hive components such as honey, wax, and stored pollen, as well as heavy metals, air pollution, pesticides, and plant diseases. Honey bees are also used as indicators on new dangers like climate change and antibiotic resistance.

Recently, parasitic wasps have been employed as bioindicators for habitats in woodlands (Hilszczanski et al., 2005). On a landscape scale, the parasitoids were more prevalent in habitats of mixed woodlands with a variety of species than in coniferous woodlands. The parasitoid assemblage was more strongly influenced by forest type and deadwood attributes, indicating that it is essential to maintain the diversity of deadwood habitats for parasitoid conservation. Due to their high trophic position, complex biology, and restricted host ranges, parasitic wasps are intricate and highly specialized (Maleque et al., 2009). Aculeate wasps can accurately quantify the variety of all arthropod species in an ecosystem, both social and solitary wasp species, and some social wasp species have also been found to be reliable indicators of heavy metal contamination (Brock et al., 2021).

Chironomids have been used as a biomonitoring model for ecotoxicological tests. Antennal deformities serve as great early warning indicators for the detection of toxic contaminants in the environment (Warwick, 1990). Both somatic aberrations (deletions, inversions, and deficiencies) and functional abnormalities (reduction in the amount of BRc, BRb, and the NOR activity) caused by trace-metal contamination can be easily recognized and utilized as reliable markers of genotoxicity (Michailova et al., 2012). Mouthparts deformities of chironomids are also observed under elevated metal concentrations (Arimoro et al., 2018). The family Chironomidae has the potential to be a bioindicator of changes in aquatic ecosystems on a global scale due to their species richness, including the indirect effects of drought (Cañedo-Argüelles et al., 2016).

Syrphid flies are a strong bioindicator of landscape-level forest management practices due to their widespread geographic distribution (Maleque et al., 2009). Eristalis spp. and Sphaerophoria spp. accumulate heavy metals like Mn, Pb, and Cd of the industrial region in their body (Markova and Alexiev, 2002). Additionally, due to their high adult mobility, flies are the best tool to assess the biodiversity of landscapes (Sommaggio, 1999).

Ephemeroptera, Plecoptera, and Trichoptera (EPT), more commonly referred to as mayflies, stoneflies, and caddisflies are great bioindicators of good water quality. Mayflies are sensitive to oxygen depletion in flowing water. Stoneflies indicate highly oxygenated water and caddisflies are sensitive to water pollution and used as bioindicators of water purity (Parikh et al., 2020). Caddisflies and mayflies being abundant in waters subject to little toxic stress, it has been hypothesized that the relative numbers of caddisflies, and mayflies may indicate various degrees of heavy-metal stress (Winner et al., 1980).

The most reliable ecological indicator in riparian and aquatic environments is thought to be dragonflies. They respond sensitively to habitat disturbance and heavy metal deposition, particularly in lakes and flooded drainage areas (Shafie et al., 2017). Their presence in any aquatic body indicates that it is synthetic pollution-free (Azam et al., 2015).

Termites are “ecosystem engineers”, which act as bioindicators of soil fertility through the determination of soil physico chemical parameters of termite mound soil in comparison with chemically fertilized soil (Nithyatharani and Kavitha, 2018). Termites also show indicator capacity for soil ecosystem services such as chemical fertility, hydrological functions, macro-aggregation, and biodiversity (Duran-Bautista et al., 2020). Termites accumulated high amounts of heavy metals such as Ca, Mg, Al, Fe, Zn, Cu, Mn, Be, Ba, Pb, Cr, V, Ni, and Cd. Alajmi et al. (2019) investigated and recorded a significant direct relationship between the presence of termites on the concentrations of Al, Cu, Zn, Be, Cd, Mn, Ca, Mg, Pb, V, and Mo, while, a significant indirect correlation existed for Ba, Cr, Ni, Co, and Fe.

Springtails are abundant soil arthropods which are very sensitive to changes in the soil properties and the impact of anthropogenic activities. The decline in springtail species richness reflects soil water acidity brought on by organic pollutants and wastes, pesticide use in agricultural soils, and heavy metal pollution (Rusek, 1998). The QBS-adapt index, which measures the soil’s biological quality in response to seasonal changes, is a sensitive tool for distinguishing between various land use patterns for different seasons. The QBS-adapt index can be determined by the morphological traits of springtails (Machado et al., 2019).

Aphids are indications of pollution even though surge in population density while feeding on hosts that are exposed to environments with high CO₂ concentrations (Zaghloul et al., 2020). Also, Nepidae, Corixidae, Notonectidae, Gerridae, Belostomatidae, Veliidae, and Mesoveliidae are hemipteran insect families that serve as bioindicators used to assess the water quality (John and Polhemus, 2008). Gerridae family members were used to monitor different iron and manganese concentrations, however, it seems that they behaved less well enough for nickel and lead (Nummelin et al., 2007).

Changing consumer demand, the need for higher production, and climate change are all exerting strain on agricultural landscapes. Environmentally significant creatures that inhabit agricultural land and its surroundings will be impacted by decisions made about land management. Agricultural landscape quality changes can be detected and followed over time and space using honey bees as bioindicators (Quigley et al., 2019). Agro-ecosystems are primarily performed as both providers and consumers of ecological services including carbon sequestration, soil conditioning, decomposition of organic matter, nutrient recycling, predation, pollination, support biodiversity, and cultural services. Intensive chemical input has subjected to serious degradation of these services. Monoculture provides an optimum environment for the growing population of herbivorous insects. Insect pollinators include hundreds of solitary bee species, bumble bees, flies, beetles, and butterflies, mainly hymenoptera communities that are common for pollination in agricultural areas. Furthermore, predatory insects such as some beetles, true bugs, lacewings, flies, midges, and wasps contribute to crucial ecosystem functions by acting as biocontrol agents. Moreover, through interacting with the soil, insects play a crucial role in enhancing agricultural soil. Dung beetle increases nitrogen, phosphorus, potassium, calcium, and magnesium or total protein content in the soil which elevates the crop yield in relation to chemicals (De Groot et al., 2002). Bioturbation agents, like ants, play a major role in bioindication due to their greater diversity in agro-ecosystem. The activity of terrestrial decomposer termites boosts infiltration potential, which results in water retention and improved productivity.

Bioindicators are a potentially useful tool for Sustainable Forest Management (SFM). An increase or decrease in the population of insect species may indicate significant changes in the ecosystem. The predominance of houseflies and mosquitoes indicates unhygienic environmental conditions. Butterflies are regarded as reliable ecological indicators and are tolerant to changes in topography and moisture. Butterflies are a sign of healthy environmental conditions (Weiss et al., 1988). Due to the intensive competition for resources, which prevents the predominance of a few dominating species, areas that are either cultivated or heavily forested with a diverse range of plant species demonstrated significant insect species diversity and higher ecological stability (Da Rocha et al., 2010). An extensive study of the consequences of forest fragmentation has been conducted utilizing a variety of arthropod indicator species. For example, (Fujita et al., 2008) revealed that urban forest remnants’ richness in carabid species increased with the rise in fragment area but remained largely constant with the increase in isolation distance from major forests. Dung beetles are likewise more affected by forest fragmentation, and their number and species richness were positively associated with the extent of the fragments (Feer and Hingrat, 2005). Syrphid flies are the best tool for assessing landscape-level biodiversity due to their great adult mobility and species richness (Maleque et al., 2009). Furthermore, climate change will have a significant impact on increasing frequency, and severity of non-indigenous invasive insect species in forest ecosystem. Invasive species are introduced beyond their native regions to a completely new region of less or no natural enemies and capable of causing vast invasion owing to their great dispersal and adaptation ability. Invasion of insect species is a serious threat like emerald ash borer (Agrilus planipennis) causes severe destruction of ash trees (Herms and Mc Cullough, 2014), balsam wooly adegid (Adelges tsuga) infestation resulting in death of hemlock tree species (Havill et al., 2011), spread of gypsy moth (Lymantria dispar) causes Quercus sp. mortality (Davidson et al., 1999), and Asian long horned beetle (Anoplophora glabripennis) is responsible for the death of hardwood species of Acer, Salix, Aesculus, etc. (Dodds and Orwig, 2011) across the forests of eastern N. America.

Ephemeroptera, Plecoptera, Trichoptera, and Diptera (EPT&D) are aquatic invertebrate insect species that are extremely sensitive to changes in water quality and are only able to survive in streams with very little pollution, cool temperatures, and well-oxygenated waters. Baetis and Paraleptophlebia (Ephemeroptera), and Chironomidae (Diptera) exhibit a considerable rise in abundance, and therefore density, immediately after an anthropogenic disturbance (Relyea et al., 2000). Insects are less typically utilized as biological indicators of metal contamination, despite species of the genus Halobates being suited to detect Cd and Hg (Nummelin et al., 2007). There are several benefits of using aquatic insects to biomonitor the health of aquatic ecosystems. One of these is the potential for detecting changes in water quality, both during sampling and changes that have occurred, as a result of these species’ largely sedentary lifestyles and extended life spans (Wahizatul et al., 2018).

In alpine regions, anthropogenic environmental degradation is still prevalent. According to their specific environmental tolerances, invertebrate communities are dispersed along with these sharp altitudinal gradients, and they respond accordingly when environmental conditions change. Only a few reliable invertebrate biomonitoring systems, particularly those which focus on terrestrial invertebrates, are regularly and extensively employed to track the changes in the mountain environment. Neophilaenus lineatus, a species of spittlebug, responds to shifting mean temperatures by exhibiting fast, often yearly changes in their upper altitudinal limit. Craspedolepta schwarzi, a species found at high elevations, may become extinct if suitable alpine habitats disappear. C. nebulosa and C. subpunctata, two lower-altitude species, may be anticipated to demonstrate an upward extension of their overlapping ranges, however, C. nebulosa always inhabits an elevation that is slightly higher than C. subpunctata (Hodkinson and Bird, 1998).