Native predator loss opens all-year roadkill buffet for remaining scavengers

This blog post is provided by Matthew Fielding and tells the #StoryBehindThePaper for the paper ‘Roadkill islands: Carnivore extinction shifts seasonal use of roadside carrion by generalist avian scavenger’, which was recently published in the Journal of Animal Ecology. Matthew Fielding is a PhD student at the School of Natural Sciences at UTAS. A self-confessed “bird nerd”, you can usually find him in the Tasmanian wilderness behind a pair of binoculars. He is passionate about the conservation of bird species and is particularly interested in how humans impact bird communities.

When we think of roads and animals, we are often plagued with negative thoughts – roadkill, fragmentation, pollution. However, some animals do benefit from our growing network of roads. In the natural environment, carrion is often scarce and difficult to locate. Yet, roadsides often provide a reliable source of animal carcasses from vehicle-animal collisions, commonly known as roadkill. Scavengers will often exploit these resources, using roads as a regular food source.

A forest raven (Corvus tasmanicus), a common scavenger, feeds on roadkill in Tasmania. (Credit: JJ Harrison (https://www.jjharrison.com.au/), CC BY-SA 3.0 , via Wikimedia Commons)

Tasmania, a large island off the southeast of Australia, is often regarded as the ‘roadkill capital of Australia’; however, the offshore Bass Strait islands could arguably steal that title. The two largest islands in the region, King Island and Flinders Island, have been heavily modified for cattle and sheep farming. The extensive open pastures across the islands have resulted in incredibly high numbers of macropods (wallabies and pademelons), meaning vehicle-animal collisions are often difficult to avoid.

In Tasmania, carcasses, like roadkill, are often quickly munched down by apex scavengers like the Tasmanian devil (Sarcophilus harrisii) and meso-predators, like the spotted-tailed quoll (Dasyurus maculatus). However, on the Bass Strait islands, these native mammalian predators are absent, either due to historical extermination by European settlers or past processes that we are yet to understand (e.g., the devil has only been found in the recent fossil record on Flinders Island). There are now two main scavengers remaining on the islands, the feral cat (Felis catus), an invasive carnivore, and the forest raven (Corvus tasmanicus), a native omnivore. With no great competition for carcasses, the two species are left with a smorgasbord of roadkill to feed on. So, what happens when we lose the main scavenger in an environment? Does this impact the foraging behaviour of the remaining scavengers?

The Tasmanian devil is the apex scavenger across the Tasmanian mainland (Credit: JJ Harrison (https://www.jjharrison.com.au/), CC BY-SA 3.0 , via Wikimedia Commons)

In our recent paper in the Journal of Animal Ecology, we used this naturally occurring experiment to examine how the loss of native carnivores, like the devil and quoll, within a community impacts the use of roadkill by a remaining scavenger, the forest raven. We monitored the locations of roadkill and ravens on eight road transects across four seasons on the Tasmanian mainland and the Bass Strait islands. Unsurprisingly, we found that roadkill was a strong predictor of raven presence along roads in general. However, the effect of roadkill was greater on the Bass Strait islands, where ravens used the resource heavily throughout the entire year. In contrast, ravens were mostly clustering around roadkill on Tasmanian mainland roads in the autumn, when other resources were low. This suggests that under lower levels of competition, ravens can, and choose to, feed on roadkill throughout the year, even in seasons when other resources are readily available.

A pair of forest ravens feeding on a wallaby carcass on King Island (Credit: Matthew Fielding).

The loss of apex predators within ecosystems has been found to trigger increases in the abundance of smaller predators (meso-predator release), leading to greater hunting pressure on smaller prey. On the Bass Strait islands, the lack of competition for roadkill could be disproportionately benefiting forest ravens, explaining the apparent increase in raven abundance across the islands. This is bad news for the many threatened birds on the islands with nest depredation by ravens on the rise. The local farmers are also disadvantaged, as an overabundance of ravens leads to heightened attacks on livestock, such as sick or newborn lambs.

So, what can we do about this? Well, one important answer could be trophic rewilding – the restoration of ecological function through the introduction of targeted animal species. By restoring these ecosystem processes and species interactions, the goal is to promote complexity and self-sustaining ecosystems. In a recent paper, we argued that bringing back quolls or introducing devils on the Bass Strait islands could keep raven and feral cat numbers down by limiting their access to food and modifying their behaviour. We’ve seen many rewilding success stories around the world, including the iconic return of grey wolves to Yellowstone National Park. The impacts of their reintroduction trickled throughout the food web with the wolves controlling numbers of the overabundant elk in the park, which in turn led to the restoration of beaver populations. Admittedly, rewilding can seem idealistic, but we are faced with the escalating threat of further extinctions across the world. We need novel, practical approaches that can be implemented now and, in many cases, bringing back those top predators can be the solution.

Further Reading:

Fielding, M.W., Buettel, J.C., Brook, B.W., Stojanovic, D. and Yates, L.A. (2021), Roadkill islands: carnivore extinction shifts seasonal use of roadside carrion by generalist avian scavenger. Journal of Animal Ecology. In Press. https://doi.org/10.1111/1365-2656.13532

Fielding, M. W., Buettel, J. C. & Brook, B. W. (2020) Trophic rewilding of native extirpated predators on Bass Strait Islands could benefit woodland birds. Emu – Austral Ornithology, 120(3), 260-262. https://doi.org/10.1080/01584197.2020.1797509

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s