Insects that live in and around our rivers are at great risk of extinction, and we don’t know why.
Despite billions of dollars spent on improving water quality, repairing river channels and releasing environmental flows, extinction rates among aquatic insects and other invertebrates remain puzzlingly and worryingly high.
While data for Australia is scant, international research suggests that aquatic ecosystems are losing species at a rate five times higher than terrestrial ecosystems.
The solution may lie in realising that we could be looking in the wrong place. The problem may not be so much the water, as the terrestrial environment around it.
Most insect species that live in bodies of water like rivers, lakes, wetlands and ponds, and indeed make up the majority of species living in these environments, have an adult stage during which they emerge from the water and live life as a fully terrestrial animal. Many of these insects are winged and can fly across landscapes.
Riverine adult insects may stay near river margins or may fly between streams, but females must return to rivers to lay their eggs.
Despite many of these at-risk species spending much of their adult life outside the water, most government monitoring programs that measure the health of river ecosystems and monitor riverine populations sample these species at the juvenile, or larval, stage when they inhabit rivers and streams.
But that’s a little like trying to study human populations by just counting the numbers of children, ignoring whether kids survive to reach adulthood, find partners and produce the next generation.
If adult insects aren’t thriving in the terrestrial environment, then it is small wonder that aquatic populations are dwindling and putting species survival at risk.
So, what might be happening in the terrestrial environment that’s putting these species at risk?
Insects usually require quite a specific habitat for egg-laying.
Long-term research with multiple postgraduate students (particularly Dr Paul Reich, Dr Ash Macqueen and Dr Wim Bovill) and my colleague Dr Jill Lancaster – funded by the Australian Research Council (ARC) and National Environmental Research Council (UK) – has shown that a whole suite of insects require emergent objects poking above the water’s surface on which to lay their eggs.
For example, female hydrobiosid caddisflies land on the dry portions of rocks, crawl down under the water and seek to lay their eggs on the rocks below the water line. If there is a shortage of emergent rocks, fewer eggs will be laid and there will be fewer larvae in the river.
Indeed, for some species we can actually predict the numbers of juveniles from the amount of suitable rocks.
So, how can emergent rocks be in short supply?
Well, when rivers fill, emergent rocks can disappear completely beneath the water. They also effectively vanish when water levels fall, leaving many rocks stranded out of the water altogether.
A recently completed ARC project with Dr Lancaster, Oxford University’s Associate Professor Louise Slater, Loughborough University’s Professor Steve Rice and Deakin University’s Professor Rebecca Lester has shown that, during high flows, insects are forced to crowd their egg masses onto the few available rocks. That crowding may reduce the survival of larvae.
Varying water levels in rivers are of course natural, but it is possible that human interference with river flows may in some places distort this natural variation and reduce egg-laying opportunities.
One of my PhD students Handoko Wahjudi, who is co-supervised with Dr Wim Bovill, is working with the New South Wales Department of Planning, Industry and Environment to examine how water flows released for irrigators during the summer is affecting insect egg-laying.
He has documented that irrigation flows drown all the emergent rocks in an area and cause complete reproductive failure among multiple species of insects.
This research may partly explain the well-known loss of species diversity that is recorded downstream of dams.
What are the implications if there is a shortage of places for an insect species to lay eggs? A recent review suggests such shortages have persistent effects on populations, but could it also cause extinction?
To answer that question we need to model the outcomes for species sharing the same habitat. Species don’t live in splendid isolation – they compete for food and living space.
Some species are inevitably better competitors than others and can drive weaker competitors to extinction.
Nevertheless, strong and weak competitors do routinely coexist in nature. If we understand how these species are able to coexist naturally, then we can turn that information around to shed light on what may be driving extinction risk.
To address this, our University of Melbourne research group has joined with researchers from Deakin University’s Centre for Rural and Regional Futures, and the University of Arizona’s and Taiwan’s National Chung Hsing University’s Professor Peter Chesson, to develop new models that link the consequences of egg-laying decisions of adults to the survival of larvae in streams.
Supported by a Discovery Grant from the ARC, these models – in company with data to test them – will highlight the ways this terrestrial-aquatic transition underpins species coexistence – something that has never been investigated before.
A final piece of the puzzle is that egg-laying is sometimes limited not by shortages of egg-laying habitat but by few – or even no – adults visiting or returning to stream and river margins. We don’t know what causes this because we have no understanding of how often adults successfully move around in the landscape.
To investigate this, in an ARC project started in 2021, Dr Lancaster and I are collaborating with Missouri State University’s Dr Debra Finn to look at whether adult insects can fly over catchment boundaries and lay eggs in a stream distant from the original stream from which they emerged.
If they can, then dwindling stream populations can be naturally rescued by adults moving to neighbouring streams.
However, if insects cannot move between streams, then extinction risk is much higher. Additionally, land clearance could make this problem far worse by removing shelter for insects and also making them more vulnerable to predators like birds and bats.
Maintaining healthy populations of our aquatic insects clearly isn’t just about maintaining high-quality water in our rivers and streams.
Our research is showing that factors beyond the quality of the water itself, like rocks emergent from the water and the surrounding terrestrial environment, are also important – potentially creating opportunities to head off extinctions in the future.
Banner: An overturned emergent rock revealing an adult hydrobiosid caddisfly caught in the act of laying her eggs on the bottom of the rock. Egg masses previously laid by other females can be seen nearby. Picture: Dr Wim Bovill