Field experiments that examine the impact of immigration, emigration or landscape structure (e.g., the composition of the matrix) on the source-sink dynamics of fragmented populations are scarce. Here, planthoppers and egg parasitoids were released among host-plant patches that varied in structural (caged, isolated, or in a network of other patches) and functional (mudflat matrix that impedes dispersal versus brome-grass matrix that facilitates dispersal) connectivity. Planthoppers and parasitoids on caged patches achieved high equilibrium densities, indicating that they were population sources. Because access to immigrants did not result in elevated population densities, source populations were not also pseudosinks. Planthoppers and parasitoids in open patches in mudflat had dynamics similar to caged patches, but went extinct in 4-5 generations in open patches in brome. Emigration losses in brome-embedded patches were so high that these patches functioned as population sieves. This study provides strong evidence that matrix composition can greatly alter source-sink structure.
Keywords: Anagrus columbi, emigration, extinction, host-parasitoid interactions, matrix, metapopulation, Prokelisia crocea, pseudosink, sieve, source-sink structure.