Facilitation displaces hotspots of diversity and allows communities to persist in heavily stressed and disturbed environments

Abstract

QuestionWhat are the interacting effects of stress and disturbance on bothcompetition and facilitation, and ultimately their impact on diversity and species ranges?LocationSimulated data.MethodWe extended the spatially explicit model of Xiao et al. (Oikos, 118,2009, 1343) to consider howstress and disturbance – operating alone or together– affect species distributions through varying biotic interactions.ResultsIn the absence of facilitation, species ranges only occurred within thelimits of a triangular zone, which can be considered equivalent to Grime’s C-S-Rtriangle. Competitive species were distributed in low stress and disturbance conditions.Stress- and disturbance-tolerant species occurred in more stressed anddisturbed environments. Species richness followed a hump-shaped relationshipwith a hotspot of diversity occurring close to the centre of the C-S-R triangle. Incontrast, facilitation was able to dampen the negative impacts of stress and disturbance acting either solely or simultaneously, but this influence was not simplythe sum of the effects of the processes operating independently on the twoenvironmental gradients. Facilitation extended species distributions along boththe stress and disturbance gradients and displaced the hotspot of diversity intothe centre of the response surface.ConclusionsFacilitation was able to promote the occurrence and persistenceof communities in highly stressed and disturbed conditions, i.e. allowing a stablecommunity to exist beyond the limits of the C-S-R triangle.We also showed thatfacilitation can be an important mechanismdriving the displacement of hotspotsof diversity from benign toward intermediate stressed and disturbed environments.