Global insect declines: 33% of aquatic species threatened with extinction
68% of caddisfly species populations are declining – more than any other order of insects. Image: Katja Schulz | Flickr Creative Commons
Global insect biodiversity is in dramatic decline according to a new review of existing scientific evidence. Aquatic insects are particularly threatened, with mayfly, dragonfly, stonefly and caddisfly species all showing significant declines over recent years.
Population declines of terrestrial and aquatic insects – in 41% of all species – are roughly twice those estimated for other vertebrates (mammals, birds and amphibians – 22% of species). The total global mass of insects is falling by an average of 2.5% each year, the study suggests, with potentially severe impacts on ecosystems and their services – such as pollination – globally.
“As insects comprise about two thirds of all terrestrial species on Earth, the trends confirm that the sixth major extinction event is profoundly impacting life forms on our planet,” say study authors Francisco Sánchez-Bayo and Kris Wyckhuys.
Writing in the journal Biological Conservation, the authors reviewed 73 historical scientific reports of long-term insect population dynamics from across the world. They selected studies which considered all species in a taxon (e.g. family or order) within a large area (i.e. a region or country), or smaller areas intensively studied for more than 10 years.
Percentage of insect species in decline or extinct in four aquatic orders. Graphic redrawn from Sánchez-Bayo and Wyckhuys (2019: Fig 3b)
Freshwater species
Four orders of freshwater insects are addressed in the paper: mayflies (Ephemeroptera), dragonflies and damselflies (Odonata), stoneflies (Plecoptera) and caddisflies (Trichoptera). As most freshwater insect species have relatively inflexible life cycles, which makes them particularly sensitive to environmental pressures, the authors say. As a result, insects are vulnerable to pressures such as flow alterations, habitat fragmentation, pollution and invasive species.
The study suggests that – like for vertebrates – declines of aquatic insects are higher than those of their terrestrial counterparts (see graphics above and below). The authors estimate that 33% of aquatic insects are threatened with extinction, compared to 28% of terrestrial taxa.
‘Habitat generalist’ aquatic insects – which can occupy a number of different habitats – have been particularly affected, with major losses in all four orders of insects in large river systems across Europe and North America. However, insect communities have generally remained stable – or shown lesser declines – in near-natural mountain streams and lakes.
Annual species declines and extinction rate for four aquatic insect orders. Extinction rate denotes the percentage of species not observed in >50 years. Graphic redrawn from Sánchez-Bayo and Wyckhuys (2019: Table 1)
Drivers of insect decline
Dr Sánchez-Bayo and Dr Wyckhuys identify four key drivers of global insect population decline: habitat loss, pollution, biological factors and climate change. For aquatic species, they highlight pollution as the main driver of population declines. Common sources of aquatic pollution include fertilisers and synthetic agricultural fertilisers, sewage and landfill leaching from urban areas, and industrial chemicals from factories and mining sites. The authors highlight the damaging impact of pyrethroid, neonicotinoid and fipronil insecticides on aquatic insects, due to their acute and chronic toxicity in water bodies.
As insects are crucial parts of food webs from the tropics to the tundra, the authors suggest that urgent conservation and restoration schemes are necessary to safeguard their populations and the ecosystem services they support. In particular, they highlight the need to reduce the runoff and leaching of toxic chemicals – particularly from industrial agriculture – into water bodies, in order to support the persistence and re-colonisation of aquatic insect populations.
Speaking to ABC television in Australia, Dr Sanchez-Bayo said, “We are not alarmists, we are realists. We are experiencing the sixth mass extinction on Earth. If we destroy the basis of the ecosystem, which are the insects, then we destroy all the other animals that rely on them for a food source. It will collapse altogether and that’s why we think it’s not dramatic, it’s a reality.”
The authors state that immediate global conservation action is required, chiefly through rethinking, “current agricultural practices, in particular a serious reduction in pesticide usage and its substitution with more sustainable, ecologically based practices.”
Spraying pesticides on a lettuce field in Arizona, USA. Pesticide pollution is one of the key pressures on aquatic insect populations, according to the new study. Image: Jeff Vanuga | Creative Commons Public Domain Files
What about the tropics? Geographic variations and data deficits
The study reveals the geographic variation in detailed, long-term insect research across the world. The majority of the 73 studies selected for this meta-analysis are located in Europe and the USA, with only a handful from Central America, Brazil, South Africa, China, Australia and New Zealand. In fact, the data for China and Australia refers to managed honey bees only.
Whilst this reflects the distribution of funding and support for long-term ecological monitoring, it also restricts the certainty of making broad statements about the global health and status of insect populations. The insect declines reported here are primarily from temperate, northern hemisphere ecosystems. This isn’t to underplay the significance of the dramatic trends reported in the paper, but instead to caution about drawing global trends from the reviewed studies.
If anything, this study suggests that in addition to conserving the populations we know about, there is significant work to be done in studying (and mostly likely, protecting) those for which there are currently data deficits. Such work is unlikely to provide any good news, according to Georgina Mace, who told the New Scientist recently that Sánchez-Bayo and Wyckhuys’s review could in fact underestimate the declines in insect populations across the tropics.
So, in short, the picture of global insect declines painted by Dr Sánchez-Bayo and Dr Wyckhuys is alarming, but may not show the full extent of global declines. What is clear is that insect conservation must become a key focus for environmental policy with immediate effect if species declines – both documented and undocumented – are to be halted.
The Freshwater Blog 
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