Cronin James T
Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803-1715, USA.
Ecology. 2007 Dec;88(12):2977-90. doi: 10.1890/07-0253.1.
The interaction between species, mediated by a shared natural enemy (i.e., apparent competition), has been the subject of much theoretical and empirical investigation. However, we lack field experiments that assess the importance of apparent competition to metacommunity structure. Here, I conducted a series of field experiments to test whether apparent competition, mediated by shared egg parasitoids (Anagrus nigriventris and A. columbi), occurs between two abundant planthopper species (Delphacodes scolochloa and Prokelisia crocea) of the North American Great Plains. The two planthoppers feed on different plant species within prairie potholes (wet depressions) and, thus, do not interact directly. At the scale of individual potholes, a five-fold pulse increase in D. scolochloa density (relative to control potholes) resulted in a steady decline in P. crocea density over two generations. As expected in cases of apparent competition, P. crocea eggs in these potholes suffered twice the level of parasitism as P. crocea eggs in control potholes. In contrast, a sixfold increase in P. crocea density had no effect on D. scolochloa density or parasitism in those potholes. The superiority of D. scolochloa over P. crocea likely can be attributed to a larger source population size, greater amount of host habitat, and/or the presence of a phenological refuge from parasitism for D. scolochloa. In another experiment, in which small populations of P. crocea were established either in close proximity to D. scolochloa or in isolation, I found that the likelihood of P. crocea persistence was 36% lower in the former than the latter populations. This difference was attributable to very high rates of parasitism of P. crocea when adjacent to D. scolochloa. These two experiments provide clear evidence that the two planthopper species engage in apparent competition and that the shared parasitoids may play a significant role in limiting membership in a local community. Based on these findings, I argue that metacommunity studies must be broadened to include higher trophic levels.
由共同的天敌介导的物种间相互作用（即似然竞争）一直是大量理论和实证研究的主题。然而，我们缺乏评估似然竞争对集合群落结构重要性的野外实验。在此，我进行了一系列野外实验，以测试由共享的卵寄生蜂（黑腹角头姬小蜂和哥伦比亚角头姬小蜂）介导的似然竞争是否发生在北美大平原的两种常见飞虱物种（斯氏德尔菲飞虱和克罗氏原飞虱）之间。这两种飞虱以草原坑洼（湿地洼地）内的不同植物物种为食，因此不会直接相互作用。在单个坑洼尺度上，斯氏德尔菲飞虱密度增加五倍（相对于对照坑洼）导致克罗氏原飞虱密度在两代内稳步下降。正如似然竞争情形中预期的那样，这些坑洼中的克罗氏原飞虱卵遭受的寄生水平是对照坑洼中克罗氏原飞虱卵的两倍。相比之下，克罗氏原飞虱密度增加六倍对那些坑洼中的斯氏德尔菲飞虱密度或寄生情况没有影响。斯氏德尔菲飞虱相对于克罗氏原飞虱的优势可能归因于更大的源种群规模、更多的寄主栖息地，和/或斯氏德尔菲飞虱存在一个免受寄生的物候避难所。在另一项实验中，我将少量克罗氏原飞虱种群要么紧邻斯氏德尔菲飞虱建立，要么单独建立，结果发现前者中克罗氏原飞虱持续存在的可能性比后者种群低36%。这种差异归因于紧邻斯氏德尔菲飞虱时克罗氏原飞虱的寄生率非常高。这两项实验提供了明确证据，表明这两种飞虱物种参与了似然竞争，并且共享的寄生蜂可能在限制当地群落成员组成方面发挥重要作用。基于这些发现，我认为集合群落研究必须拓宽到包括更高营养级。
