Evolution and Ecology Program, International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria.
PLoS Comput Biol. 2012;8(11):e1002660. doi: 10.1371/journal.pcbi.1002660. Epub 2012 Nov 15.
There is continuing interest in understanding factors that facilitate the evolution and stability of cooperation within and between species. Such interactions will often involve plasticity in investment behavior, in response to the interacting partner's investments. Our aim here is to investigate the evolution and stability of reciprocal investment behavior in interspecific interactions, a key phenomenon strongly supported by experimental observations. In particular, we present a comprehensive analysis of a continuous reciprocal investment game between mutualists, both in well-mixed and spatially structured populations, and we demonstrate a series of novel mechanisms for maintaining interspecific mutualism. We demonstrate that mutualistic partners invariably follow investment cycles, during which mutualism first increases, before both partners eventually reduce their investments to zero, so that these cycles always conclude with full defection. We show that the key mechanism for stabilizing mutualism is phase polymorphism along the investment cycle. Although mutualistic partners perpetually change their strategies, the community-level distribution of investment levels becomes stationary. In spatially structured populations, the maintenance of polymorphism is further facilitated by dynamic mosaic structures, in which mutualistic partners form expanding and collapsing spatial bubbles or clusters. Additionally, we reveal strategy-diversity thresholds, both for well-mixed and spatially structured mutualistic communities, and discuss factors for meeting these thresholds, and thus maintaining mutualism. Our results demonstrate that interspecific mutualism, when considered as plastic investment behavior, can be unstable, and, in agreement with empirical observations, may involve a polymorphism of investment levels, varying both in space and in time. Identifying the mechanisms maintaining such polymorphism, and hence mutualism in natural communities, provides a significant step towards understanding the coevolution and population dynamics of mutualistic interactions.
人们一直有兴趣了解促进物种内和物种间合作进化和稳定的因素。这种相互作用通常涉及投资行为的可塑性，以响应相互作用伙伴的投资。我们的目的是研究种间相互作用中互惠投资行为的进化和稳定性，这是一个得到实验观察强烈支持的关键现象。特别是，我们在混合种群和空间结构种群中，对互惠者之间的连续互惠投资博弈进行了全面分析，并展示了维持种间互惠的一系列新机制。我们证明，互惠伙伴总是遵循投资周期，在这个周期中，互惠首先增加，然后双方最终将投资减少到零，因此这些周期总是以完全背叛结束。我们表明，稳定互惠的关键机制是投资周期中的相位多态性。尽管互惠伙伴不断改变他们的策略，但社区层面的投资水平分布保持稳定。在空间结构种群中，投资水平的多态性通过动态镶嵌结构进一步得到维持，其中互惠伙伴形成扩展和收缩的空间泡沫或簇。此外，我们揭示了混合和空间结构互惠社区的策略多样性阈值，并讨论了满足这些阈值的因素，从而维持了互惠。我们的结果表明，种间互惠作为一种可塑的投资行为，可能是不稳定的，并且与经验观察一致，可能涉及投资水平的多态性，在空间和时间上都有变化。确定维持这种多态性的机制，从而确定自然群落中的互惠关系，是理解互惠相互作用的共同进化和种群动态的重要一步。
