Facilitation displaces hot-spots of diversity and allows communities to persist in heavily stressed and disturbed environments

Question What are the interacting effects of stress and disturbance on both competition and facilitation, and ultimately their impact on diversity and species ranges? Location Simulated data. Method We extended the spatially explicit model of Xiao et al. (Oikos, 118, 2009, 1343) to consider how stress and disturbance - operating alone or together - affect species distributions through varying biotic interactions. Results In the absence of facilitation, species ranges only occurred within the limits of a triangular zone, which can be considered equivalent to Grime's C-S-R triangle. Competitive species were distributed in low stress and disturbance conditions. Stress- and disturbance-tolerant species occurred in more stressed and disturbed environments. Species richness followed a hump-shaped relationship with a hotspot of diversity occurring close to the centre of the C-S-R triangle. In contrast, facilitation was able to dampen the negative impacts of stress and disturbance acting either solely or simultaneously, but this influence was not simply the sum of the effects of the processes operating independently on the two environmental gradients. Facilitation extended species distributions along both the stress and disturbance gradients and displaced the hotspot of diversity into the centre of the response surface. Conclusions Facilitation was able to promote the occurrence and persistence of communities in highly stressed and disturbed conditions, i.e. allowing a stable community to exist beyond the limits of the C-S-R triangle. We also showed that facilitation can be an important mechanism driving the displacement of hotspots of diversity from benign toward intermediate stressed and disturbed environments