Natural enemies have inconsistent impacts on the coexistence of competing species

The role of natural enemies in promoting coexistence of competing species has generated substantial debate. Modern coexistence theory provides a detailed framework to investigate this topic, but there have been remarkably few empirical applications to the impact of natural enemies. We tested experimentally the capacity for a generalist enemy to promote coexistence of competing insect species, and the extent to which any impact can be predicted by trade-offs between reproductive rate and susceptibility to natural enemies. We used experimental mesocosms to conduct a fully factorial pairwise competition experiment for six rainforest Drosophila species, with and without a generalist pupal parasitoid. We then parameterised models of competition and examined the coexistence of each pair of Drosophila species within the framework of modern coexistence theory. We found idiosyncratic impacts of parasitism on pairwise coexistence, mediated through changes in fitness differences, not niche differences. There was no evidence of an overall reproductive rate-susceptibility trade-off. Pairwise reproductive rate-susceptibility relationships were not useful shortcuts for predicting the impact of parasitism on coexistence. Our results exemplify the value of modern coexistence theory in multi-trophic contexts and the importance of contextualising the impact of generalist natural enemies to determine their impact. In the set of species investigated, competition was affected by the higher trophic level, but the overall impact on coexistence cannot be easily predicted just from knowledge of relative susceptibility. Methodologically, our Bayesian approach highlights issues with the separability of model parameters within modern coexistence theory and shows how using the full posterior parameter distribution improves inferences. This method should be widely applicable for understanding species coexistence in a range of systems.同生态位物种间的共存受到它们共同天敌的调控。这一调控的影响仍备受争议。现代共存理论为这一问题提供了量化的研究框架, 但相关的实证研究还远远落后于理论进展。 我们通过实验探究了非特异性寄生蜂如何调控其宿主之间的竞争性共存, 并尝试用宿主的繁殖力与免疫力间的关系对共存结果加以解释。 在实验室的模拟实验中, 我们将六个热带雨林果蝇物种两两组合后放置于有或者没有寄生蜂的环境下, 记录子代果蝇的数量。我们用这些实验数据拟合出宿主间的物种竞争模型, 再通过现代共存理论推导宿主间的共存关系。 我们发现非特异性寄生蜂改变了不同宿主物种间的适合度差异, 但并没有改变它们的生态位差异。非特异性寄生蜂最终的调控效果依宿主组合的不同而不同, 并没有统一的规律。宿主的繁殖力和免疫力间并不普遍存在此消彼长的关系。即使在某几对宿主间存在负相关性, 寄生蜂的调控也并不总是促进共存。 我们的研究示范了如何将现代共存理论应用于多营养级生态网络,强调了寄生蜂对宿主间共存的影响依具体情况而定: 宿主物种间的竞争受到更高营养级的调控, 而此调控对宿主间共存的影响并不能简单地通过它们的对寄生蜂的免疫能力进行预测。另外, 我们的统计分析指出了在应用现代共存理论到实际系统时将面临的参数协变性问题, 并展示了如何利用贝叶斯统计方法完善对物种共存关系的推测。我们推荐这一方法被更广泛地应用于物种间共存的研究。.