Department of Biology, University of Nevada, Reno, Nevada 89557, USA.
Ecology. 2010 Dec;91(12):3707-18. doi: 10.1890/08-1634.1.
Indirect effects of trophic interactions on biodiversity can be large and common, even in complex communities. Previous experiments with dominant understory Piper shrubs in a Costa Rican rain forest revealed that increases in herbivore densities on these shrubs caused widespread seedling mortality as a result of herbivores moving from Piper to seedlings of many different plant genera. We tested components of the Janzen-Connell hypothesis by conducting focused studies on the effects of specialist and generalist Piper herbivores on local seedling diversity. Whereas specialist herbivores are predicted to increase mortality to neighboring seedlings that are closely related to the source plant, true generalists moving from source plants may cause density-dependent mortality of many species, and possibly increase richness if new species replace abundant species that have been thinned by herbivores. Therefore, we hypothesized that seedling richness would be greater in understory control plots created in patches of Piper that had normal densities of generalist herbivores compared to plots from which we removed generalist herbivores manually from all Piper shrubs. After 15 months, generalist-herbivore-removal plots had > 40% fewer seedlings, > 40% fewer species, and 40% greater seedling evenness, on average, than control plots with generalist herbivores intact. Using a complementary approach in unmanipulated plots in four forests, we used path analysis to test for a positive association between seedling diversity and herbivore damage on Piper species. In unmanipulated plots, for both generalist and specialist herbivores, our data were significant fits to the causal model that Piper herbivores decrease evenness and increase plant species richness, corroborating the experimental results. Because herbivores changed how individuals were apportioned among the species and families present (lower evenness), one interpretation of these associations between herbivores on Piper shrubs and local seedling richness is that high seedling mortality in dominant families allowed the colonization or survival of less common species. If interspecific or apparent competition allowed for a relative increase in species richness, then the Janzen-Connell hypothesis may extend its predictions to generalist seedling predators. We speculate that apparent competition may explain some of the deviations from neutral model predictions, especially at small scales.
营养相互作用对生物多样性的间接影响可能很大且很常见，即使在复杂的群落中也是如此。先前在哥斯达黎加雨林中对优势下层林 Piper 灌木进行的实验表明，食草动物密度的增加会导致这些灌木上的幼苗广泛死亡，因为食草动物会从 Piper 转移到许多不同植物属的幼苗上。我们通过对专食性和广食性 Piper 食草动物对当地幼苗多样性的影响进行重点研究，测试了 Janzen-Connell 假说的组成部分。专食性食草动物预计会增加与源植物密切相关的邻近幼苗的死亡率，而从源植物转移的真正广食性食草动物可能会导致许多物种的密度依赖性死亡率，如果新物种取代了因食草动物而变薄的丰富物种，则可能会增加丰富度。因此，我们假设在具有正常密度广食性食草动物的 Piper 斑块中创建的下层林对照样地中，幼苗丰富度会高于我们从所有 Piper 灌木中手动去除广食性食草动物的样地。15 个月后，与保留广食性食草动物的对照样地相比，广食性食草动物去除样地的幼苗数量减少了>40%，物种数量减少了>40%，幼苗均匀度增加了>40%。在四个森林中的未受干扰的样地中，我们采用补充方法，通过路径分析来检验 Piper 物种上的幼苗多样性与食草动物损害之间的正相关关系。在未受干扰的样地中，对于专食性和广食性食草动物，我们的数据与 Piper 食草动物降低均匀度并增加植物物种丰富度的因果模型拟合良好，这与实验结果相符。由于食草动物改变了个体在存在的物种和科之间的分配方式（均匀度降低），因此，Piper 灌木上的食草动物与当地幼苗丰富度之间的这些关联的一种解释是，优势科的高幼苗死亡率允许较少常见的物种的定植或生存。如果种间或明显的竞争允许物种丰富度相对增加，那么 Janzen-Connell 假说可能会将其预测扩展到广食性幼苗捕食者。我们推测，明显的竞争可能解释了一些与中性模型预测的偏差，尤其是在小尺度上。
