This blog post is provided by John Donohue, Petri Piiroinen and Adam Kane and tells the #StoryBehindThePaper for the paper “Predicted short-term mesoscavenger release gives way to apex-scavenger dominance” which was recently published in the Journal of Animal Ecology. In their study, they explore whether and how vultures would recover from near extirpation.
Most, if not all, modern-day ecosystems bear the hallmarks of human disturbance which can be orders of magnitude more impactful than natural disturbances. This is especially true in the Anthropocene, the block of time we now find ourselves in which extends back to when humans took over as the predominant driver of global change. Given ecologists spend their days trying to work out why species occur where they do and, why they occur in the numbers that they do, it’s valuable both for theory and conservation practice to consider what our ecosystems would look like if human impacts were lessened or removed altogether.
In our paper we developed a mathematical model to explore some of these questions with a specific system in mind, a vertebrate scavenging guild roughly based on Kruger Park in South Africa which includes, vultures, jackals, hyenas and lions, all of which consume carrion to a greater or lesser extent. Here, vultures, who are the dominant scavenging species, are suffering from poisoning that is devastating their populations, with their social behaviour exacerbating the decline. Specifically, we asked how the scavenging guild would respond to a sudden collapse in vulture numbers to near extirpation. We could imagine several scenarios, 1. that the birds would never recover because their mammalian counterparts take over, 2. that the birds recover to a status quo, or 3. something entirely different to what we see today.
We drew on a wealth of biological data from the species in Kruger including estimates of carrion availability to build our model and showed that vultures rapidly respond from population lows to wrestle back control of carrion from a transient period of mammal dominance that lasts for a decade. This period of mammal dominance would give rise to a novel system where many carcasses would remain undiscovered by vertebrates leaving a substantial resource base for invertebrates.
We were keen to generalise beyond Kruger as much as possible, as Old World vultures are threatened (and important) across much of their range. A key part of that generalisation was using techniques from mathematics to understand the influences of various model parameters. For example, how fast would the vulture recovery be if the ecosystem contained more food? Would the period of mammal dominance last longer if the ecosystem could support more mammals? The quantitative extremes inherent in this kind of scavenging community (e.g. the gap between capabilities of vultures and other scavengers and the gap between present and potential vulture density) proved very useful—they allowed us to address these kinds of questions mathematically.
What’s more we show that vultures arrive at a new ceiling of population density well in excess of current estimates with almost 10 birds per km2. This suggests that humans are keeping a lid on vulture populations. What this lid is we don’t know, because other potential limiting factors like nesting sites or food are in plentiful supply. This is a promising finding because it implies that if humans reduce their influence on such systems vultures have the traits to allow them to flourish once more.
Read the paper
Read the full paper here: Donohue, J. G., Piiroinen, P. T., & Kane, A. (2024). Predicted short-term mesoscavenger release gives way to apex-scavenger dominance. Journal of Animal Ecology, 00, 1–13. https://doi.org/10.1111/1365-2656.14180
Animal Ecology in Focus 