Managing Interacting Invasive Predators for Biodiversity Conservation
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2022
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Melbourne Authors
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School of Ecosystem and Forest Sciences
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PhD thesis
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Open Access
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© 2022 Matthew Wayne Rees
Abstract
Predators can have devastating impacts on biodiversity when introduced beyond their native range, and so are often lethally controlled. However, control of a dominant invasive predator can backfire if it frees subordinate invasive predators from top-down suppressive and competitive effects, a process referred to as ‘mesopredator release’. Identifying whether mesopredator release is occurring is important for biodiversity conservation, particularly as integrated management of co-occurring invasive predators is often costly or infeasible.
Lethal control of the introduced red fox *Vulpes vulpes* (hereafter ‘fox’) is a common conservation strategy in Australia. Concerningly, there are reports of dramatic declines in native prey species following targeted fox control, frequently attributed to mesopredator release of the feral cat *Felis catus* (hereafter ‘cat’). However, evidence for the mesopredator release hypothesis is limited--both in regards to cats and more generally.
The primary aim of my thesis was to test whether lethal fox control (1080 poison-baiting) causes mesopredator release of cats, in terms of their population density, occurrence, and behaviour. In the process, I derived the first estimates of cat density for Australia wet forest environments as well as investigated the relative importance of fox control, fire and other environmental drivers on the occurrence of these invasive predators and two threatened native prey species.
My study was conducted across two regions in temperate south-eastern Australia, where foxes and cats are the only medium-large sized terrestrial predators (dingoes *Canis familiaris* being absent). In one region, I spatially replicated a landscape-scale control-impact experimental design three times within a long-term fox control program. In the other region, I used a before-after control-impact-paired-series design around a new fox control program. I led the deployment of 949 camera-traps (63,560 trap nights) to survey mammal communities and identify individual cats based on unique pelage markings, as well as collated data from an additional 2,831 camera-traps deployed by government research partners.
I found that fox suppression varied with spatial and temporal variation in lethal control effort. Fox occurrence declined across gradients of poison-bait density: from ubiquitous to a near-zero occurrence probability in the region with long-term and consistent fox control, but relatively weakly in the region where fox control was less frequent and had only recently commenced. In contrast to expectations, my estimates of cat density in wet forests are among the highest recorded in 'natural' Australian environments. Cat density was higher with fox control across both regions. The strength of this effect appeared dependent on the degree of fox suppression: from negligible to a 3.7-fold higher density of cats. In addition, when localised fox activity was reduced, in some areas cat (i) detectability increased, (ii) movement rates decreased, and (iii) diel activity patterns reversed, potentially facilitating spatial coexistence. There was some indication that cats effectively employed avoidance behaviours when foxes were rare, but only declined in population density when fox activity was moderate - high.
Long-term and spatially intensive fox control appeared strongly beneficial to the long-nosed potoroo *Potorous tridactylus*, but not the southern brown bandicoot *Isoodon obesulus*. This may reflect species-specific variability in susceptibility to foxes relative to cats. Further work is needed to understand the implications of mesopredator release on a wider range of shared prey species, particularly those more vulnerable to cat predation.
My thesis provides robust experimental evidence that dominant predator suppression can cause mesopredator release in terms of both population density and behaviour. The singular control of an invasive predator may therefore take the pressure off some native prey, but alone is unlikely to improve the persistence of all species in systems where multiple invasive predators co-occur.
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Keywords
1080 poison, conservation biology, diel activity patterns, feral cat, fox control, generalised additive model, invasive predator, landscape of fear, mesopredator release, population dynamics, predator ecology, red fox, spatial capture-recapture, species interactions, wildlife management