COS 31-6
Host-associated microbial diversity facilitates the ecological rescue of host species experiencing rapid environmental change
Host-associated microbial communities are characterized by a large diversity of microbial species, many of which are involved in mutualistic interactions with the host organism. Mutualistic microbes in marine organisms such as corals have been shown to be critical in allowing the host species to expand into new habitats, and to adapt to otherwise inhospitable environments. The rapid evolutionary emergence of novel microbial mutualists capable of facilitating a host species’ adaptation to new environments represents a potentially potent form of evolutionary rescue. Because such microbial mutualists are always at risk of being replaced by non-mutualist (i.e., “cheater”) microbes that are capable of reaping the benefits of the mutualistic interaction without enduring any of the costs, current theory assumes that the host must actively regulate the microbiome by imposing sanctions against potential cheaters. Yet, with drastic or rapid environmental changes, slow-growing hosts may not have enough time to evolve the required regulatory mechanisms for blocking cheaters, and for facilitating the mutualists that would allow them to adapt to changing environmental conditions. Here we use mathematical and simulation-based metacommunity models to study the role of microbial mutualists in the evolutionary rescue of their more slower-growing hosts when host selection for mutualism is absent.
Results/Conclusions
We demonstrate that in the absence of host regulation of the microbiome, the emergence of microbial mutualists alone will not lead to the evolutionary rescue of the host species in the face of increasingly harsh environmental changes due to the rapid displacement of mutualists by strong cheater strategies. However, the presence of numerous non-mutualistic weak cheater strategies in the host microbiome can help facilitate the persistence of microbial mutualists by competing with the strong cheater, and thereby decreasing its host-occupancy at the metacommunity scale. In systems with a high microbial (weak cheater) diversity, the stabilization of the mutualist gives rise to the familiar U-shaped curve in the host species as a response to environmental change. However, now the host species’ U-shaped curve represents a form of ecological rescue that arises from ecological or community-level interactions within the host microbiome, and not just adaptive changes in the genome of a single microbial species. For more complex holobiont species, such potent ecological rescue is more likely to play a crucial role in staving off potential extinction than evolutionary rescue sensu stricto.