Using a stochastic model to examine the ecological, economic and ethical consequences of population control in a charismatic invasive species: mute swans in North America.
Abstract
Although invasive species are widely recognized as a threat to native ecosystems, control or eradication plans can create controversy, particularly when target species are considered attractive or desirable by members of the public. Detailed consideration of both biological and non-biological criteria when evaluating management proposals can better inform managers working with controversial species. In this study, alternative strategies for controlling introduced populations of mute swans Cygnus olor during the first few decades of population growth were compared in terms of ecological, economic and ethical considerations using a simple population model. By quantifying parameter uncertainty with a stochastic model, we identified, in quantitative terms, management strategies that were predicted to have a high certainty of succeeding. Survival rates should be reduced by more than 17% to be 90% certain of a decline; reproductive rates would need to decline by more than 72% to achieve the same goal. Populations in which density dependence occurs are expected to need even greater effort for management to work. Reducing survival rates was also more efficient economically; reducing population growth via reduced reproduction required three to four times more person-days within the first year across a range of simulated growth rates. Prolonging timelines for implementing management increased the number of swans that would have to be killed and the costs that would be required for reaching target population sizes. Synthesis and applications. Because of controversy surrounding swan management, various considerations must be addressed when weighing control options. Our modelling suggests that, if a population is to be controlled, a period of intensive culling of adults will be the most efficient option in terms of biological effectiveness, economic costs and minimizing the total number of swans that ultimately are killed.