Trophic dynamics of invasive species communities

Background

Invasive species are a leading cause of biodiversity loss globally meaning considerable focus has been placed on their eradication and control. Many of these efforts have focused on single-species, often apex predators which are having the greatest impact within the ecosystem. However, increasingly, ‘unintended consequences’ are occurring where the removal of a dominant invasive predator results in a mesopredator release resulting in more detrimental impacts than the initial predator.

My research follows the successful eradication of red foxes from Phillip Island, Victoria, Australia. The eradication of foxes marked a significant achievement on a large (10,000 ha) and highly urbanised island. This success has led to concerns regarding the possible mesopredator release of feral cats. Feral cats are a significant driver of biodiversity loss, particularly within island ecosystems, with the potential to considerably impact abundant seabird populations across the island. Concerns about the possible impacts of feral cats have led to preemptive control, yet uncertainty about the influence of these control operations on feral rabbit, rat and mice populations remain unknown.

To further understand these processes, my research has developed a Qualitative model of Phillip Island’s species community, using historical data as validation, to predict the likely outcomes of future pest control programs. We are testing two broad theories of control;

1) eradication of feral cats
2) the eradication of prey species (rabbits, rats and mice).

Phillip Island Species Community: Before fox eradication

Foraging Ecology of Feral cats

The foraging ecology of feral cats within Australia is well known; however, few have investigated how feral cat foraging shifts in response to the removal of a competitively superior apex predator (i.e. red foxes). We are using a historical dataset from the 1980’s and ’90s that investigated both fox and cat diet (see Kirkwood et al. 2005), when both species were abundant across Phillip Island. Samples of current cat diet (now in the absence of foxes) have been collected from April 2016 – April 2019 inclusive. We predict that in the presence of a mesopredator release, feral cat foraging will switch in the absence of foxes, capitalising on abundant resources that formed considerable proportions of fox diet.

Meta-analysis of feral cat diet across Australia has suggested that the dominant prey items are often those species most abundant within the local environment. Across Australia, these species are often invasive prey species including European rabbits, black rats or house mice. In order to understand the relative influence of these invasive prey on the diet of feral cats; in conjunction with ephemerally abundant seabird populations we are conducting stable isotope and fatty acid analysis on feral cat tissues. These samples will enable the identification of any seasonality in feral cat diet, and highlight the reliance of cats on these ephemerally abundant seabirds compared to the frequently abundant invasive prey.

Invasive rodent populations

Invasive rodents are one of the most widely distributed animal groups globally. Black rats (Rattus rattus) and house mice (Mus musculus) are two of the most damaging and frequently co-occur within the same environment. Despite the frequency of their co-occurrence, there is still a limited understanding of the interspecific interactions between black rats and house mice. Some authors have suggested that black rats are likely to limit the population size and distribution of house mice; while others have provided evidence suggesting that black rats may influence the detectability, rather than the abundance of mouse populations.

Here, we use a systematic grid of remote sensing cameras established throughout two conservation reserves on Phillip Island (N = 127 sites). Cameras were set for one month within each season for two years from July 2016 until May 2018. Two-species multi-season occupancy models will be used to assess the influence of black rat site occupancy on the detectability and site occupancy of house mice. This model enables the explicit estimation of ‘species interaction factors’ to assist in quantifying rates of interspecific interactions. Analysis of activity periods for both mice and rats will also be conducted to determine whether mice vary their activity periods to avoid periods when rats are most active.