The effect of demographic characteristics on the success of ungulate re-introductions

Given that small populations are susceptible to extinction and inbreeding, a primary goal of re-introductions should be to maximize the initial rate of increase of a re-introduced population. We investigated how the rate of increase of newly re-introduced populations of artiodactyl species was affected by population characteristics, such as their size, sex-, and age-structure. While selecting the data of past re-introductions for our analysis, we controlled for factors that are known to affect success. The growth rate r of the populations increased with the number of animals released, up to about a population size of 20, at which point an asymptote was reached. All larger populations grew at a median r = 0.17, while several smaller populations declined. Small populations grew faster if they contained more mature individuals. Small, heavily female-biased populations were more variable in r than those of a more equal sex ratio. These female-biased populations also grew on average less well. Generally, populations of <20 were more variable in r than larger ones and both age and sex structure explained a significant portion of this variation. (C) 2000 Elsevier Science Ltd. All rights reserved.