Stochastic dynamics in populations with nonbreeders

Background/Question/Methods

Many populations, particularly of birds, have large numbers of individuals that fail to obtain breeding opportunities in a given year. These nonbreeders often go undetected in population studies, which tend to be focused on the breeding population only. Consequently, little is known about how nonbreeders might influence population processes, and estimates of population growth, dynamics and extinction risk could be biased in populations with high proportions of nonbreeders. We have developed a stochastic matrix model that is suitable for a wide variety of systems with different types of nonbreeders. Using this model, we have tested how survival in different classes and transition probabilities between classes affect population growth in systems with and without density dependent effects of nonbreeders on the breeding population. In addition we have quantified how changes in various population parameters affect the demographic variance, and thus potentially the extinction risk, of small populations. These results have been compared to estimates from an equivalent Leslie matrix model in which nonbreeders are included but not explicitly modeled.

Results/Conclusions

We find that a variety of systems with different types of nonbreeders show similar population growth mechanisms. The probability of becoming a breeder at first maturity is a particularly important parameter for population growth, along with juvenile survival and breeder survival. The relative influence of these three parameters depends on the age at first possible breeding and the presence or absence of density dependence. These results have implications for the focus of conservation efforts in populations with nonbreeders. The parameters we studied have different effects on the demographic variance, some positive and some negative. In some cases a Leslie matrix model predicts the opposite effect on the demographic variance of that found in our stage structured model. This indicates that the way in which nonbreeders are incorporated into population models could affect predictions of population viability and extinction risk. Our results will be discussed in relation to new knowledge about nonbreeders in a well-studied population of green-rumped parrotlets (Forpus passerinus) in Venezuela.