Amarasekare P, Nisbet R M
National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, California 93101-5504, USA.
Am Nat. 2001 Dec;158(6):572-84. doi: 10.1086/323586.
Patch occupancy theory predicts that a trade-off between competition and dispersal should lead to regional coexistence of competing species. Empirical investigations, however, find local coexistence of superior and inferior competitors, an outcome that cannot be explained within the patch occupancy framework because of the decoupling of local and spatial dynamics. We develop two-patch metapopulation models that explicitly consider the interaction between competition and dispersal. We show that a dispersal-competition trade-off can lead to local coexistence provided the inferior competitor is superior at colonizing empty patches as well as immigrating among occupied patches. Immigration from patches that the superior competitor cannot colonize rescues the inferior competitor from extinction in patches that both species colonize. Too much immigration, however, can be detrimental to coexistence. When competitive asymmetry between species is high, local coexistence is possible only if the dispersal rate of the inferior competitor occurs below a critical threshold. If competing species have comparable colonization abilities and the environment is otherwise spatially homogeneous, a superior ability to immigrate among occupied patches cannot prevent exclusion of the inferior competitor. If, however, biotic or abiotic factors create spatial heterogeneity in competitive rankings across the landscape, local coexistence can occur even in the absence of a dispersal-competition trade-off. In fact, coexistence requires that the dispersal rate of the overall inferior competitor not exceed a critical threshold. Explicit consideration of how dispersal modifies local competitive interactions shifts the focus from the patch occupancy approach with its emphasis on extinction-colonization dynamics to the realm of source-sink dynamics. The key to coexistence in this framework is spatial variance in fitness. Unlike in the patch occupancy framework, high rates of dispersal can undermine coexistence, and hence diversity, by reducing spatial variance in fitness.
斑块占据理论预测，竞争与扩散之间的权衡应导致竞争物种在区域内共存。然而，实证研究发现优势和劣势竞争者在局部共存，这一结果在斑块占据框架内无法得到解释，因为局部动态和空间动态解耦。我们开发了双斑块集合种群模型，明确考虑竞争与扩散之间的相互作用。我们表明，只要劣势竞争者在定殖空斑块以及在被占据斑块之间迁移方面具有优势，扩散 - 竞争权衡就可以导致局部共存。来自优势竞争者无法定殖的斑块的迁入，能使劣势竞争者在两个物种都定殖的斑块中免于灭绝。然而，过多的迁入可能对共存不利。当物种间的竞争不对称性很高时，只有当劣势竞争者的扩散率低于临界阈值时，局部共存才有可能。如果竞争物种具有相当的定殖能力且环境在其他方面空间均匀，那么在被占据斑块之间更强的迁移能力并不能阻止劣势竞争者被排除。然而，如果生物或非生物因素在整个景观的竞争排名中造成空间异质性，即使没有扩散 - 竞争权衡，局部共存也可能发生。事实上，共存要求总体劣势竞争者的扩散率不超过临界阈值。明确考虑扩散如何改变局部竞争相互作用，将重点从强调灭绝 - 定殖动态的斑块占据方法转移到源 - 汇动态领域。在此框架下共存的关键是适合度的空间方差。与斑块占据框架不同，高扩散率会通过降低适合度的空间方差来破坏共存，进而破坏多样性。
