Department of Applied Biology, Universidad Miguel Hernández, Carretera de Beniel, km. 3.2, 33012 Orihuela, Alicante, Spain.
Ecology. 2011 Oct;92(10):1948-58. doi: 10.1890/11-0181.1.
Theoretical and empirical evidence suggests that body size is a major life-history trait impacting on the structure and functioning of complex food webs. However, long-term analyses of size-dependent interactions within simpler network modules, for instance, competitive guilds, are scant. Here, we model the assembly dynamics of the largest breeding seabird community in the Mediterranean basin during the last 30 years. This unique data set allowed us to test, through a "natural experiment," whether body size drove the assembly and dynamics of an ecological guild growing from very low numbers after habitat protection. Although environmental stochasticity accounted for most of community variability, the population variance explained by interspecific interactions, albeit small, decreased sharply with increasing body size. Since we found a demographic gradient along a body size continuum, in which population density and stability increase with increasing body size, the numerical effects of interspecific interactions were proportionally higher on smaller species than on larger ones. Moreover, we found that the per capita interaction coefficients were larger the higher the size ratio among competing species, but only for the set of interactions in which the species exerting the effect was greater. This provides empirical evidence for long-term asymmetric interspecific competition, which ultimately prompted the local extinction of two small species during the study period. During the assembly process stochastic predation by generalist carnivores further triggered community reorganizations and global decays in population synchrony, which disrupted the pattern of interspecific interactions. These results suggest that the major patterns detected in complex food webs can hold as well for simpler sub-modules of these networks involving non-trophic interactions, and highlight the shifting ecological processes impacting on assembling vs. asymptotic communities.
理论和经验证据表明，体型是影响复杂食物网结构和功能的主要生活史特征。然而，长期以来，对于更简单的网络模块（例如竞争群体）内体型依赖性相互作用的分析却很少。在这里，我们模拟了地中海盆地过去 30 年来最大的繁殖海鸟群落的组装动态。这个独特的数据集使我们能够通过“自然实验”来检验体型是否驱动了一个生态群体的组装和动态变化，该群体在栖息地保护后数量非常少。尽管环境随机性解释了大部分群落变异性，但种间相互作用所解释的种群方差虽然很小，但随着体型的增加而急剧下降。由于我们沿着体型连续体发现了一个种群密度和稳定性随着体型增加而增加的种群梯度，因此种间相互作用的数量效应在较小的物种上相对于较大的物种比例更高。此外，我们发现，在竞争物种之间的体型比越高的情况下，种间相互作用的人均系数越大，但仅在发挥作用的物种体型较大的情况下才如此。这为长期的非对称种间竞争提供了经验证据，这种竞争最终促使在研究期间两种较小的物种在当地灭绝。在组装过程中，肉食性捕食者的随机捕食进一步引发了群落重组和种群同步性的全局衰退，这破坏了种间相互作用的模式。这些结果表明，在复杂食物网中检测到的主要模式也适用于涉及非营养相互作用的这些网络的更简单的子模块，并强调了影响组装和渐近群落的生态过程的转变。
