Demographic stochasticity and social mating system in the process of extinction of small populations: The case of passerines introduced to New Zealand

Underlying the many causes of extinction of small populations is the random fate of each constituent individual or, in other words, demographic stocha sticity. Demographic stochasticity is inherent to any demographic process, regardless of the environment, and its strength increases as population siz e gets smaller. In particular, random fluctuations in the proportion of mal es and females and the: way they pair for reproduction (i.e., the social ma ting system) are usually neglected. To assess the potential importance of d emographic stochasticity to the extinction process, a two-sex model with an explicit mating system was built. Extinction probabilities computed via Mo nte Carlo simulation were compared to real data, the case of passerines int roduced to New Zealand a century ago. This minimal model of extinction allo wed assessment of the importance of the mating system in the colonization p rocess. Monogamous mating led to a higher extinction risk than did polygyno us mating. Demographic uncertainty imposes high extinction probabilities on short-lived bird species as compared to long-lived bird species. Theoretic al results for two-sex models are provided.