The role of sources and sinks in regional and local metapopulation dynamics

Background/Question/Methods The role of landscape heterogeneity in metapopulation dynamics is a contemporary area of interest. Metapopulations are often composed of a mixture of large, good quality patches (sources) and smaller, poorer quality patches (sinks). The dynamics in these individual patches and the role they play in a patch network are likely to differ and can have profound effects on the growth rate, size and persistence of metapopulations. Furthermore, in a fluctuating environment patch characteristics will vary through time. Thus the source-sink pattern of patches across the landscape is not constant. In this study, using the bruchid beetle Callosobruchus maculatus, we examined the local and regional dynamics in metapopulations under varying spatial and/or temporal heterogeneities in resource supply.  In these regionally-persistent systems, source and sink patches were defined based on the net loss or gain of colonizers during dispersal events. The source-sink composition of landscapes at the regional scale and the dynamic source-sink behavior of individual patches at the local scale were assessed.

Results/Conclusions

Regional metapopulation size did not differ between the heterogeneity treatments. However, temporal heterogeneity compared to spatial heterogeneity in resource supply was found to have a greater impact on the variation in regional population size.  At the local scale there was a strong positive correlation between resource availability and population size. Variation in local population size was greatest in metapopulations under temporal heterogeneity.
Net dispersal was greatest in spatially heterogeneous metapopulations. In this system, dispersal was density dependent such that colonization direction was from high density patches to low density ones.
Regionally, metapopulations varied in their composition of sources and sinks with spatially heterogeneous systems supporting consistently larger numbers of sinks. At a local scale, patches in spatially heterogeneous systems were more likely to exhibit a fixed pattern, either acting as sources or sinks. Conversely temporal heterogeneity resulted in much more dynamic behavior with patches frequently changing between sources and sinks.
We propose that it is important to classify the dynamic nature of source and sink patches more carefully, differentiating the mechanisms that drive differences in patch state. Understanding the dynamics of source-sink metapopulations is relevant in the context of any fragmented landscape undergoing habitat loss or degradation. It is also essential in order to gain a comprehensive understanding of landscape function for conservation, landscape management and reserve design.