Population Viability in the Presence and Absence of Cowbirds, Catastrophic Mortality, and Immigration

Abstract

Abstract Isolation, catastrophic mortality, small average population size, and exposure to nonnative enemies all influence the dynamics of Mandarte Island song sparrows (see chapters 3-5). Several populations of song sparrows of concern to conservation also face similar threats (Chan and Arcese 2002), as do other small and isolated populations of vertebrates. In this chapter we describe a simple model to rank the potential influence of some of these factors on the expected size and viability of the Mandarte Island song sparrow population. Because catastrophic mortality events affect all ages of song sparrows on Mandarte Island, we expected that the frequency of catastrophes would have an overriding influence on population viability. We also expected that immigration to the island might occasionally rescue the population from extinction. Finally, because cowbirds reduced the reproductive rate of song sparrows on Mandarte Island, we expected their presence to reduce the average population size and increase the risk of extinction. Below, we begin by providing some background to demographic analysis and the assessment of potential threats in the context of population viability analysis (PYA). Whenever we try to conserve populations of plants or animals, questions naturally arise about the hierarchy of threats facing those populations, the alternative approaches to ameliorate those threats, and the likelihood that the amelioration of key threats will result in recovery (Beissinger 2002; Boyce 2002). Answering questions such as these requires that we understand the mechanisms of population growth and stability and their influence on population persistence. So far, however, few populations worldwide have been studied in sufficient detail to answer such questions with precision (Ludwig 1999; Beissinger 2002).