Cronin James T
Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803-1715, USA.
Ecology. 2007 Dec;88(12):2966-76. doi: 10.1890/07-0070.1.
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 (Prokelisia crocea) and egg parasitoids (Anagrus columbi) 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 vs. brome-grass matrix that facilitates dispersal) connectivity. Planthoppers and parasitoids on caged patches exhibited density-dependent growth rates, achieved high equilibrium densities, and rarely went extinct. Therefore, experimental cordgrass patches were classified as population sources. Because access to immigrants did not result in elevated population densities, source populations were not also pseudosinks, i.e., patches whose densities occur above carrying capacity due to high immigration. Planthoppers and parasitoids in open patches in mudflat had dynamics similar to those in caged patches, but went extinct in 4-5 generations in open patches in brome. Brome-embedded patches leaked emigrants at a rate that exceeded the gains from reproduction and immigration; populations of this sort are known as population sieves. For species whose suitable patches are becoming smaller and more isolated as a result of increased habitat fragmentation, emigration losses are likely to become paramount, a condition favoring the formation of population sieves. An increase in the proportion of patches that are sieves is predicted to destabilize regional population dynamics.
考察移民、迁出或景观结构（如基质组成）对破碎种群源 - 汇动态影响的田间实验很少。在这里，将稻飞虱（Prokelisia crocea）和卵寄生蜂（Anagrus columbi）释放到寄主植物斑块中，这些斑块在结构（笼养、孤立或处于其他斑块网络中）和功能（阻碍扩散的泥滩基质与促进扩散的雀麦草基质）连通性方面存在差异。笼养斑块上的稻飞虱和寄生蜂表现出密度依赖的增长率，达到高平衡密度，且很少灭绝。因此，实验性的互花米草斑块被归类为种群源。由于接入移民并未导致种群密度升高，源种群也不是伪汇，即由于高移民率导致密度高于承载能力的斑块。泥滩开阔斑块中的稻飞虱和寄生蜂动态与笼养斑块中的相似，但在雀麦开阔斑块中4 - 5代后灭绝。嵌入雀麦的斑块以超过繁殖和移民增益的速率泄漏迁出者；这种种群被称为种群筛。对于那些由于栖息地破碎化加剧而适宜斑块变得更小且更孤立的物种，迁出损失可能变得至关重要，这种情况有利于种群筛的形成。预计种群筛斑块比例的增加会破坏区域种群动态的稳定性。
