Insect Declines and Case for Long-term Insect Monitoring in India
|Separated by 15,000 km, tied to the same fate. Left: Antioch dunes shieldback katydid Neduba extincta, declared extinct by the time of describing the species from the Antioch sand dunes of USA (Rentz, 1977; see archive.org; under CC BY-NC 3.0); Right: Enigmatic tiger beetle Apteroessa grossa (Iconographia Zoologica - Special Collections University of Amsterdam; see Wikimedia Commons), not seen alive along the coastal and wetland areas of southern India; both gone because of severe land-use changes from ill-informed infrastructure developments.|
‘Nature is under siege’ begins a collective of publications (Wagner et al., 2021) addressing the inquiries on the ‘validity of claims of rapid insect decline’. Within the last two decades, several studies established declines in insect abundances based on long-term monitoring data: In Great Britain, aerial biomass of insects studied between 1973 and 2002 showed significant decline in one of the four sites (Shortall et al., 2009), suggested to be linked with changes in agricultural practice at that site; between 1970 and 2010, Fox et al. (2014) found a significant decline in frequency of occurrence of 260 moth species and an increase for 160 moth species, linked to land use change and climate change as major drivers of changes in moth diversity. A study spanning 27 years in protected areas of Germany recorded a seasonal decline of 76% in insect biomass, with as much as 82% decline in flying insect biomass (Hallmann et al., 2017), particularly butterflies, bees, and moths during summer, suggested to be linked to agricultural intensification as one of the causal factors responsible for the decline. In 2018, Lister & Garcia proposed a climate-driven decline in arthropod abundance – especially insects – in Puerto Rico’s Luquillo rainforest based on data collected between 1976 and 2012; the authors found that the arthropod biomass had declined 10 times, synchronous with the declines in lizard, frog, and bird abundances dependent on arthropods. The Living Planet Report (WWF, 2016) indicates a 33% decline in grassland butterfly species monitored over 22 years in 12 countries. Dirzo et al. (2014) argue that nearly half of the monitored insect populations have shown a decline in mean abundance, especially with respect to Lepidoptera (butterflies and moths) showing 35% decline in abundance globally over a period of 40 years, and suggest that non-lepidopteran invertebrates have declined considerably more. Long-term monitoring data on 452 invertebrate species have shown an overall decline in abundances since 1970 (ibid).
Dubbed ‘hyperalarming’ (Guarino, 2018) and ‘insect apocalypse’ (Jarvis, 2018) by the mainstream media, the decline in abundances – of diversity and numbers – vary across taxa, time, and space (Wagner et al., 2021). The annual abundance declines are estimated at 1-2%, with some regions seeing ‘faunal subtractions’ of insects over 10% per decade (ibid). Among invertebrate threatened species, 31% are due to habitat loss resulting from agriculture and logging, infrastructure development (mining and linear infrastructure), alien invasive species, change in fire regime, pollution (air, water, and soil), and climate change (Collen et al., 2012). It must be noted, however, that not all insect taxa show a declining trend. A review spanning 41 countries and 1676 sites for terrestrial and aquatic insects showed 8.81% decline in terrestrial insects per decade against a 11.33% increase in aquatic insect abundance and biomass per decade (van Klink et al., 2020). Some insects such as moths in Great Britain and the western honey bee (Apis mellifera) in North America have thrived or are likely to thrive in response to warmer global temperatures (Wagner et al., 2021). We must not forget the increase in frequency of tropical cyclones in east Africa which is in turn encouraging grasshopper (Schistocerca gregaria) to transform into locusts (Salih et al., 2020) and invade as far eastwards as India.
The declining trend directly a result of climate change (i.e., increase in temperature) is contested due to lack of comprehensive studies. A study by Willig et al. (2019) using Lister & Gracia (2018) analytical approach showed contrasting results. The authors note a significant increase in canopy arthropod density with increasing temperature for 10 of the most abundant taxa; in case of a stick insect, the reanalysis found no significant decline statistically related to temperature. Collen et al. (2012) identified climate change as a major threat to 12% of threatened terrestrial invertebrates, with threats differing geographically; for instance, Australasia, Neotropical and Nearctic realms are likely affected by alien invasive species whereas the Palearctic, Indomalayan and Afrotropical realms by habitat loss due to development, agriculture and allied activities; the distinctions in these biogeographic realms being suggestive. In 1977, D. C. F. Rentz described a new katydid from a holotype specimen collected in 1937 from Antioch sand dunes in the eastern bay area of California. While describing the species, the author made repeated visits to find katydids unique to the Antioch sand dunes in 1960s but none of the Neduba katydids were recorded. This area was then under development for industries that required sand dredging and levelling for land reclamation. Grazing, encroachment of introduced plants, and reduction in sand deposition, and a bushfire in the summer of 1976 was said to have destroyed this habitat. Long before this species was named, it had become extinct, the author naming the holotype specimen Neduba extincta. In 1781, Fabricius labelled a tiger beetle from the Coromandel coast in India Apteroessa grossa. This large, flightless tiger beetle was found from several localities in the state of Tamil Nadu, however, subsequent searches for this species proved futile. One of the major drivers was development in the coastal and wetland habitats of this beetle that has not been seen till the present day (see Shyamal, 2019).
In ‘Defaunation in the Anthropocene’, Dirzo et al. (2014) argue that drivers, including overexploitation, habitat destruction and impacts from invasive species as increasing, and climate change as an emerging threat ‘which will likely soon compete with habitat loss as the most important driver of defaunation’ – especially in context of vertebrates and some habitat and niche-specific invertebrates. A case in point of climate change driven recurring and high intensity wildfires is the 2019-2020 Australian bushfire crisis that burnt across 186,000 sq. km area reportedly endangering over 500 species of animals, 230 of which belong to invertebrates including molluscs, millipedes, spiders, and insects; the Kangaroo Island assassin spider (Zephyrarchaea austini), an endemic, unique spider, is considered likely to be extinct because of the 2019-2020 bushfire (Pickrell, 2021). The Beydaglari bush-cricket (Psorodontus ebneri), which is already facing extinction, is showing a decline from an already restricted population range in the south-western Anatolia mountains in Turkey due to increasing intensity of droughts leading to drastic changes to the species’ habitat (Collen et al., 2012). A phenological (emergence of insects) study of 14 species of insects in response to temperature and precipitation over a period of 40 years across several sites in Japan (Ellwood et al., 2012) showed earlier phenology during warmer years, especially among cicadas. The authors note that responses to rising temperatures by insects were weaker than that of plants, suggesting that although temperature may be a major driver of changing phenology for some group of insects, there could be other factors, including overall declining populations, contributing to variability in phenology – a seemingly trivial change likely to have a cascading effect on the food web of the region.
The ‘gap’ between uncertainty concerning insect diversity and abundance and the insect apocalypse in context of Indian insect (and invertebrate) diversity is vast and remains largely unknown. With an assumption that insect extinction rate is similar to that of other – higher – taxa, Dunn (2004) suggests that over the last 600 years, 44,000 insect extinctions have occurred worldwide, or, roughly 73 species per year, with only 70 insect extinctions documented in the last 600 years. Compared to the average species description rate of India according to the Zoological Survey of India compilation – at 38 (SE ±4.32) insects per year between 1971 and 2015 (see Sen et al., 2016) – it is likely that we are missing the big picture.
Wagner et al. (2021) give emphasis on more long-term studies: ‘There remains an urgent need for time-series data so that temporal and spatial population trends can be assessed. Such data can be used to identify stressors, rate of insect population changes, and lineages and ecological guilds that are changing in abundance.’ Establishing long-term monitoring system for insects (and other invertebrates) is pertinent in Indian context, both ecologically and economically. Ecologically because India supports two biodiversity hotspots and about 6% of identified insect diversity on a landmass of 2.4% and economically because India is an agrarian nation in terms of employment (42.6%) as of 2019 (The World Bank, 2021), where many of the crops such as mango, cashew, coffee, apple, gourds, oilseeds, several spices and vegetables, etc. being pollinated by insects (87 out of 115 major world food crops – about 75% – require insect pollination; see Klien et al., 2007); on the other hand, India also has regions suffering from sudden outbreaks of insect-borne diseases such as malaria, dengue, leishmaniasis, chikungunya, as well as secondary diseases such as diarrhoea, typhoid, salmonella, wherever the ecological dynamics between pests, predators, and parasitoids are disproportionately imbalanced, as indicated by some dipteran species (eg. Dhamorikar, 2017). In other words, it is as much a concern to the ecologist as it is to the farmer, the consumer, and in some cases, the doctor.
Global studies that show a general decreasing trend in insect abundances are largely restricted to the western hemisphere. Wagner et al. (2021), Drizo et al. (2014), and van Klink et al. (2020) identify a considerable bias in long-term insect demographic studies, with most studies particularly from Europe and United States that collectively support less than 20% of global insect species diversity on 13.11% of Earth’s land area compared to India’s 6% global insect species diversity on 2.4% land area.
In India, the rate of species description increased to about 60.6 (SE ±17.35) per year between 2011 and 2015 (from Sen et al., 2016). As India gains momentum in invertebrate taxonomy, ecological studies that look at population dynamics, ecosystem function and ecosystem services, are few and far between. Ghorpade (2011) notes that the ecology of an important order of insects, Diptera, is ‘shamefully side-lined’. While the checklist of ants is up-to-date, Bharti et al. (2016) mention undersampling and poor knowledge gap given high level of endemism in ants of India. Smetacek (2011) suggests that although there are no records of an Indian butterfly becoming extinct, there are no studies on the relatively lesser-known taxa, with the existence of some species, especially the endemic butterflies, being uncertain. Like butterflies, Odonates (dragonflies and damselflies) are well documented in India, with an active taxonomic interest group, however, their role in wetland ecosystem health, as evidenced in some works (see Subramanian, Ali & Ramachandra, 2008) remains poorly known. Even as wild bee populations (Mathaisson & Rehan, 2019) and some apiaries (Oldroyd, 2007) are showing range shifts and population declines, and the effects of air pollution (Thimmegowda et al., 2020), pesticides (Chan & Raine, 2021; Mullin et al. 2010), and diseases (Conte & Navajas, 2008) are documented, and given that some geographies (eg. Kenya, see Muli et al., 2014; eastern France, see Mouret et al., 2013) show that in spite of high prevalence of disease load bees show low mortalities, and the fact that there is an overall global increase in honey production (Statista, 2021), status of Indian bees remains to be explored although anecdotal records from some regions suggest declining bee populations (Chari, 2017). For the tropics, Godfray, Lewis & Memmott (1999) argue that ‘progress in estimating insect diversity and in understanding insect community dynamics will be enhanced by building local inventories of species diversity, and in descriptive and experimental studies of the trophic structure of communities.’ Identification of ecosystem function specific to a species or a community is important to address the gap between species diversity and trophic structures.
Termed ‘functional diversity’, it is ‘the value and the range of those species and organismal traits that influence ecosystem functioning’ (see Laureto et al., 2015). Although species richness dominates studies that look at biodiversity patterns, species contribution towards ecosystem processes and services are not equal (Smith et al., 2013). The resilience of an ecosystem is a function of aspects of diversity beyond species numbers and is inherent to the functional traits of species or a community of species dependent on the niche, trophic-level, functional traits, and phenology. Conceived as an alternative classification to measure the ecological importance of species or a community, functional diversity paves way for ecological studies of taxonomy-deficient species based on field observations and natural history of related species wherever information can be validated. For eg. a rare and inconspicuous parasitoid wasp of crickets, Olixon sp. (undescribed) was first documented in India (unpublished, see Dhamorikar, 2019) in 2016, although undescribed, its ecosystem process based on functional traits are known as being one of the parasitoids affecting cricket populations wherever they exist. Orford, Vauhan & Memmott (2015) suggest that while bees are important pollinators, flies (Order Diptera) play a significant role in pollination en masse; in this context, the functional diversity of Diptera vis-à-vis changes to bee populations are important parameters to consider. Smith et al (2013) showed higher functional diversity relative to species diversity of fishes in temperate compared to tropical sites, although the temperate latitudes showing relatively few marine protected areas –the authors identify it as ‘unrecognized biodiversity values’ for the conventional way of declaring a protected area. A long-term monitoring system serves as a platform to study functional diversity dynamics in the wake of changing landscape and climate.
|The Indian giraffe stag beetle Prosopocoilus girafa nilgiriensis (left) and Indian rhinoceros beetle Xylotrupes gideon (right) are the largest beetles of Madhya Pradesh; both facing threat from pet trade and specimen collectors across the world and need to be on WLPA, 1972 list. Their functional role as a grub is turning dead wood to soil. As adults they are charismatic insects symbolising beauty, strength, and the need for insect conservation.|
With over 64,000 species described from India, only 0.8% are protected under the Wildlife Protection Act, 1972 and with only one insect Critically Endangered on the IUCN Red List of 2018 – an ectoparasitic lice on its critically endangered host, the pygmy hog – there is dire need to document insect diversities and abundances vis-à-vis the changing landscape and climate. A long-term monitoring system would also help develop data for Least Concern and Data Deficient species. The Living Planet Report (WWF, 2016) identifies developing methodologies to study invertebrates in its assessments which can also be addressed through long-term monitoring systems regionally as well as nationally.
As we debate economic growth at the cost of conservation of ecosystems, insects remain the unlikely conversation starters – the reasons, among many, is that we simply don’t know where most of them are and what they do to keep ecosystems functioning.
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