Exploring the “density-benefit” relationships of alien species

Abstract

The disastrous impacts of invasive alien species are well-documented. However, there is growing evidence that some alien species can also have positive effects in their new environments, adding complexity to their management. Classic density-damage relationships are used to determine cost effective densities at which to control alien populations and limit their adverse effects. In this thesis I proposed that the ecological benefits of alien species are also related to population density, and I hypothesised the different types of relationships that may exist. I then investigated the "density-benefit" relationships of three common alien species through a series of manipulative field experiments. Firstly, I examined the effect of population density on the pollination behaviour of alien black rats, Rattus rattus, that provide a pollination function for an Australian native plant. I found black rats at high densities behave in a way that may reduce their pollinator efficacy. I next investigated the effect of density upon an alien plant, Lantana camara, providing refuge for small native reptiles. I found that the abundance of reptiles was related to L. camara density in a U-shape, showing a benefit at high densities only. Then I conducted a simulation experiment to compare the digging activities of native marsupials and alien European rabbits (Oryctolagus cuniculus) at different densities. Digging density was significantly related to the germination and survival of seedlings. However, I detected a difference in the quality of the ecological function provided by these analogous alien and native species. Together, these results suggest that “density-benefit” relationships need to be integrated into the management of alien species densities. This work will advance the current understanding of alien species ecology, guide the management of alien species with complex impacts and ensure that significant ecological benefits provided by alien species are not lost unnecessarily.