The spatial insurance hypothesis in spatially explicit networks

Background/Question/Methods

The spatial insurance hypothesis demonstrated how dispersal between metacommunity patches can increase ecosystem productivity and stability in changing environments. Dispersal of organisms between local patches that experience asynchronous environmental change allows species to better track conditions to which they are tolerant. This allows for greater persistence of local and regional diversity, which promotes compensatory dynamics, leading to greater, and more stable metacommunity productivity. However, the spatial insurance hypothesis is based upon a model that assumes global dispersal and thus does not consider space explicitly.

In nature, metacommunities are likely composed of networks of communities with spatially explicit patterns of dispersal, and so are affected by the pattern of connectivity and the physical distances between patches. We combined the original spatial insurance hypothesis model with network structure and distance-dependent dispersal to provide a spatially-explicit test of the spatial insurance hypothesis. We compared network structures ranging from completely regular (only nearest neighbor connections) via small world (some random connections) to completely random. We predict that the small-world network structure will increase ecosystem stability by simultaneously maintaining high levels of short-distance dispersal among neighboring patches and allowing for corrective long-distance dispersal from patches of lower productivity to non-neighbouring patches of higher productivity.

Results/Conclusions

We found that small-world network structure can increase the stability of productivity and preserve species richness, if dispersal distance limitation is not so high as to prevent the use of the ‘shortcut.’ This stabilizing effect of shortcuts is only the case when most (>70%) of the connections are between neighboring patches. We found no beneficial effect of shortcuts on the rate of metacommunity productivity, despite the increase in stability. In all network structures, a higher proportion of dispersers was required for spatial insurance to occur than predicted from prior spatially-implicit metacommunity models.

These results demonstrate the context dependence of spatial insurance; they suggest that the both the pattern of connectivity between habitat patches as well as the ability of organisms to use those connections will greatly affect the stability of metacommunities. This has important implications for our understanding of the effect of habitat fragmentation on ecosystems and on strategies for designing ecological networks that will be resilient in changing environments.