The usefulness of sensitivity analysis for predicting the effects of cat predation on the population dynamics of their avian prey

Abstract

Sensitivity analyses of population projection matrix (PPM) models are often used to identify life-history perturbations that will most influence a population's future dynamics. Sensitivities are linear extrapolations of the relationship between a population's growth rate and perturbations to its demographic parameters. Their effectiveness depends on the validity of the assumption of linearity. Here we assess whether sensitivity analysis is an appropriate tool to investigate the effect of predation by cats on the population growth rates of their avian prey. We assess whether predation by cats leads to non-linear effects on population growth and compare population growth rates predicted by sensitivity analysis with those predicted by a non-linear simulation. For a two-stage, age-classified House Sparrow Passer domesticus PPM slight non-linearity arose when PPM elements were perturbed, but perturbation to the vital rates underlying the matrix elements had a linear impact on population growth rate. We found a similar effect with a slightly larger three-stage, age-classified PPM for a Winter Wren Troglodytes troglodytes population perturbed by cat predation. For some avian species, predation by cats may cause linear or only slightly nonlinear impacts on population growth rates. For these species, sensitivity analysis appears to be a useful conservation tool. However, further work on multiple perturbations to avian prey species with more complicated life histories and higher-dimension PPM models is required.

Estimated Number of Birds Killed by House Cats (Felis catus) in Canada

“…Some animals brought home and assumed to have been killed by pet cats may have been killed by other means, e.g., collisions with cars or buildings. Kays and DeWan (2004) circumvented these shortcomings of prey returns by observing hunting cats directly; they found the rate of kills by outdoor pet cats was 3.3 times as high as numbers of prey brought home, and this 3.3x factor has been used by others to adjust prey returns to prey killed (Baker et al 2005, 2008, Maclean et al 2008, Thomas et al 2012). However, sample sizes were limited (60 prey returned home, 4 kills observed directly during 31 hunts over 181 hours).…”

Birds as predators and as prey

“…Predicting the likely impacts of cats across larger scales will require detailed modelling, an approach that is already underway. Maclean et al . (2008) employed population projection matrix models that had been parameterized from field data for two species that fall prey to Domestic Cats, namely House Sparrows Passer domesticus and Winter Wrens Troglodytes troglodytes .…”