The presence of biota in the environment such as microbes, plankton, plants, and animals are indicative of the qualitative conditions of the environment, commonly referring to as biological indicators. Invertebrates account for the 95% of species in the animal kingdom (Smith, Ph.D., 1991) and have become widely used to evaluate and monitor major pollutant stress both inland and aquatic environments, and its rapid loss of species in both habitats led to abundant efforts on studies that focus on biodiversity (Pechenik, 2015). The insects (Class Insecta) from Phylum Arthropoda are highly adaptable having evolved to live successfully in most environments (Hoffman & Frodsham, 1993) and are responsible for many processes in the ecosystem however its loss can have a major drawback on entire communities (Almeida, et al., 2011). These bioindicators effectively respond to changes such as disturbances and stresses. According to Holt & Miller (2010), changes that attribute to pollution and land-use changes are called anthropogenic stressors. Most of the gradual changes in the environment are brought about by human activities which furthermore contribute to the endangerment of these biological indicators. This paper aims to analyze the impacts in the ecology as insect indicators face the threat of extinction brought about by human-induced drivers.
Insects as Primary Biological Indicators
According to Andrade (1998) and Peck, et al. (1998), insects are abundant and diverse, easy to be reared, and have good organism responses to small environmental changes. These characteristics make them potent representatives as bioindicators. Corbet (1980) confirms that Odonata (dragonflies) species are indicators in aquatic ecosystems like lakes and flooded drainage areas as they are sensitive to water quality change. Ephemeroptera (mayflies) and Plecoptera (stoneflies) have high adaptive capacity reflecting ecological changes (Hardersen, 2000). Order Coleoptera (beetles) have many indicator species representing 20% of the total diversity of arthropods. Scarabaeidae Family (scarab beetles) are used in forest and agricultural cultures in maintaining soil quality and formation, population regulation, and energy flow. Carabidae Family (ground beetles) are used in monitoring oil, sulfur, herbicide, CO2, and insecticide pollution (Carlton & Robison, 1998). Halobates and some species under Diptera (flies and mosquitos) and Lepidoptera (moth and butterflies) are heavy metal indicators (Almeida, et al., 2011). Meanwhile, ants act as indicators for soil quality in degraded and reforested areas (Peck, et al., 1998). Urbini, et al. (2006) showed that bees are used to monitor trace metals, radioactivity, pesticides, herbicides, and industrial pollutants and further supported by Ghini, et al. (2004) that honeybees (Apis mellifera) whereas act as pollination indicators as they show environment chemical impairment due to high mortality rate and intercept particles suspended in air or flowers. Groups under Hymenoptera (wasps), Hemiptera (aphids) and Isoptera (termites) are also pollination and pollution indicators.
Rate of Insect Decline
There are over 8 million estimated number of animal and plant species with 5.5 million insect species (United Nations, 2019) with only one-fifth of the overall insect species are named (Stork, 2018). Despite this large number, about 41% of global insect species are declining over the past decade compared with 22% of declining vertebrate species (Sánchez-Bayo & Wyckhuys, 2019). These disappearances are part of the gathering sixth mass extinction of animals – otherwise called Holocene extinction – since the last 66 million years which had lost 76% species including dinosaurs and other life forms (Greshko, 2019). IPBES (2019) were then able to estimate 10% of 5.5 million insect species and 10% of 2.5 million non-insect species with a total of 1 million animal and plant species facing the threat of extinction for the decades to come. The rate of insect extinction is 2.5% a year, which is eight times faster than that of mammals, birds, and reptiles. Most of the greatly affected terrestrial taxa are Lepidoptera (butterflies and moths), Hymenoptera (wasps, bees, and ants), and Coleoptera (beetles). In the aquatic ecosystem, four major taxa Odonata (dragonflies and damselflies), Plecoptera (stoneflies), Trichoptera (caddisflies), and Ephemeroptera (mayflies) have already had a considerable loss. Among which are valuable environment indicators (Sánchez-Bayo & Wyckhuys, 2019).
Human Activities as Drivers of Insect Extinction
The emergence of human activities over the past decades resulted in multiple impacts furthermore contributing to driving these organisms to extinction. Such activities mainly refer to dwindling and loss of habitat of insect assemblages followed by triggering drivers such as degradation and fragmentation which associates with deforestation, agricultural expansion, and urbanization (Cardoso, et al., 2020). Pollution becomes a key driver for insect extinction through the intensive use of pesticides (insecticides and herbicides) as its toxicity and lethal effects greatly impact insect population and habitat alteration (Brühl and Zaller, 2019). Insect population faces an even greater threat because of undetected effects on insect physiology and behavior posed by bioaccumulation and chronic pressure due to exposure to heavy metals (Desneux, et al., 2007). Light pollution interferes and desynchronizes natural activities of insects (e.g. fireflies) such as feeding, egg-laying, and causes mismatches in mutualistic interactions, being vulnerable to change in natural light/dark cycles (Owens & Lewis, 2018). Noise pollution changes and interferes with the acoustic and auditory surveillance of insects in its environment (Morley, et al., 2014).
Anthropogenic introduction of new invasive species outside of their natural zone also poses a threat to native insects. Vulnerable ones being those with narrow geographical distribution and specialist feeding habits having to face direct predation, competition, and vectoring of diseases (Wagner & Van Driesche, 2010). In correlation, with the addition of invasive plants, it reduces the quantity and quality of food leading to the decline in essential resources for many insects due to its monotypic nature (Cardoso, et al., 2020). Overexploitation is also a driver in insect loss as these organisms are harvested for decoration and medicinal uses. Human-induced climate change leads to multifaceted responses greatly affecting shifts in species distribution (Chen, et al., 2011), extinction, and unexpectable cumulative effects at different ecosystem levels (Peñuelas, et al., 2013). On top of that, other insects became co-dependent with endangered insects being vulnerable to co-extinction (Dunn, et al., 2009). Insect reproduction is even more disrupted due to a sudden increase in temperature and defragmentation. An example is insects like the Malaysian firefly (Pteroptyx tener) residing along the Selangor River and Rembau-Linggi Estuary that needs riverbank mangrove for adult courtship and larval habitat but has been converted to aquaculture farms thus limiting the range of firefly dispersal (Jusoh and Hashim, 2012; Khoo, et al., 2012).
Extinction of Insect Indicator Species and its Impact on Ecology
Insects are at the forefront of ecosystem services. Human survival is greatly dependent on ecosystem services like pollination, biological control, food provisioning, and recycling organic matter which are provided by insects (Noriega, et al., 2017). The sudden decline of insect abundance across the globe affects the biomass or energy flow through trophic levels. Consequentially, many ecosystem services are affected leading to the deterioration of ecosystem functionality and resilience, food web structure, species interactions such as plant-pollinators, and population persistence (Losey & Vaughan, 2006). As far as food webs are concerned, insects play a foundation role. In the ecosystem, insects are naturally eaten by frogs, reptiles, cattle, birds, pigs, poultry, and fish which makes up their diet as it consists of approximately 42 to 63% protein and up to 36% fat (Makkar, et al., 2014). Without insects, there will be no food for insectivores which includes amphibians and reptiles until such time that the population of these organisms will decrease. The animals that are dependent on amphibians and reptiles would be affected next as it will adjust to the scarcity of its food source. This process turns into a cascading effect until it further reaches to humans.
As insects provide nourishment in the food chain, they also uphold the sustenance of plant life. Approximately 80% of all known green plants on Earth are represented by flowering plants or angiosperms (Cronquist & Berry, 2019), which require pollination from insects. Hymenopterans (honeybees, solitary bees, bumblebees, pollen wasps and ants), Lepidopterans (butterflies and moths), Dipterans (flies, hoverflies, and mosquitoes), and Coleopterans (fireflies and beetles) are known insect pollinators (Bartomeus, et al., 2014). Agents like the wind may assist in the pollination process but only account for 12.5% of all flowering plants (Ollerton, et al., 2011). Insect pollinators affect 75 percent of the crop species enhancing an average crop yield between 18 and 71% (Bartomeus, et al., 2014). Nearly 78% in temperate-zone communities to 94% in tropical communities of flowering species benefit from insect-mediated pollination for production and evolution, thus, biotic pollination is functionally important for the diversification of major groups of plants (Ollerton, et al., 2011). Extinction of these insects means no major pollinators to lead the dispersal of plants. This means no fruit, vegetables, and crops like rice and wheat, consequently, no sustainable food for cows, pigs, poultry, and fish. In toto, trophic levels are greatly affected as most organisms would suffer from starvation and would cease to exist, eventually initiating a catastrophic crisis. Nearly all biotic components are insect-dependent. The disappearance of insects ultimately disrupts the biomass and energy flow of the ecosystem (Losey & Vaughan, 2006).
Near extinction of valuable indicators also allows persistent invasive pest species to emerge. Insects also occupy in a vertical manner (Stork, et al., 2016), composed of few common and rare species which contribute to hyper-diversity or called beta-diversity. Anthropogenic pressures that homogenize natural systems decrease beta-diversity by removing rare species and native species. These causes to allow secondary invasions of dominant alien invasive insects that outcompete and effectively replace rare species and reduce niche space (Swart, et al., 2019). This also causes insects to partition themselves across time. Most insects like cicadas and bush-crickets call at different frequencies to avoid overlap. Warmer temperatures interfere with arthropod phenology. Due to the alteration of host tree cycles (Karban, et al., 2000), 17-year periodic cicadas emerged after just 13 years in 2017 (Sheikh, 2017). Simultaneously, insects as decomposers are also an essential part of the biosphere. Coleopterans (carrion beetles, rove beetles, hister beetles, dermestid beetles, and dung beetles), Dipterans (flies), and Isopterans (termites) are key families in decomposition. The disappearance of these valuable decomposers means an overload volume of carcasses and dung. A historic example would be the introduction of cattle to Australia from Britain back in 1788 (Losey and Vaughan, 2006). In Britain, dung beetles normally eat and break down cattle dung while Australia had no insect fauna to process it. The dung fouled the rangeland and provided fodder for pest species (Dadour & Allen, 2001). Dung as moist habitats encourages the density of cattle parasites and pest flies (Fincher, 1981) like the pestiferous bush fly (Musca vetustissima) (Dadour & Allen 2001). Dung beetles from family Scarabaeidae are efficient in decomposing wastes, enhancing forage palatability, recycle nitrogen, and reduced pest habitat (Fincher, 1981). With dung beetles absent, cattle feces that remain undecomposed dries up and lose a large proportion of inorganic nitrogen into the atmosphere (Gillard, 1967).
Insects, being the forefront of ecosystem services and functional diversity, are crucial in maintaining communities and cohesion in ecological networks (Guimarães Jr., et al., 2017). Beetles, flies, and bees are common connectors in the modular structure that form inter-module interactions and relations (Olesen, et al., 2007). In this case, major insect indicator extinctions not only reduce species diversity but also simplify networks (Tylianakis, et al., 2008), however, they may vary according to the role of species. Loss of specialist insects although will keep the structure remain. In contrast, the loss of generalist insects erodes and changes the architecture, resulting in more extinction cascades and fragmentation of networks into isolated modules (Dunne, et al., 2002). Insect drives coevolution between interactions with plants resulting in remarkable trait complementarities like pollination and ant protection of plants (Bronstein, et al., 2006). Insect extinction will affect direct partners, ultimately incapacitating and severing entire community-wide trait integration.
Insects as biological indicators are greatly affected at an alarming rate due to drivers caused by anthropogenic stressors. Known insect indicators provide services like pollination, biological control, food provision, and organic matter recycling in which biotic components are greatly dependent on. Being largely essential to the biosphere, extinction of these organisms pose catastrophic threats to the dynamics of the ecosystems worldwide. While entomologists regard this as frightening and cause serious concerns, other researchers add that there is no need as scientists still have 80 percent of the estimated insect species that are undescribed. However, with the rate of extinction per year, it is appropriate to assume immediate action and mitigate detrimental human activities as some insects undergo extinction even before being described.