Abstract
Understanding the impacts of consumers on the abundance, growth rate, recovery and persistence of their resources across their distributional range can shed light on the role of trophic interactions in determining species range shifts. Here, we examined if consumptive effects of the intertidal grazer Scurria viridula positively influences the abundance and recovery from disturbances of the alga Mazzaella laminarioides at the edge of its geographic distributions in northern-central Chilean rocky shores. Through field experiments conducted at a site in the region where M. laminarioides overlaps with the polar range edge of S. viridula, we estimated the effects of grazing on different life stages of M. laminarioides. We also used long-term abundance surveys conducted across ~700 km of the shore to evaluate co-occurrence patterns of the study species across their range overlap. We found that S. viridula had positive net effects on M. laminarioides by increasing its cover and re-growth from perennial basal crusts. Probability of occurrence of M. laminarioides increased significantly with increasing density of S. viridula across the range overlap. The negative effect of S. viridula on the percentage cover of opportunistic green algae—shown to compete for space with corticated algae—suggests that competitive release may be part of the mechanism driving the positive effect of the limpet on the abundance and recovery from disturbance of M. laminarioides. We suggest that grazer populations contribute to enhance the abundance of M. laminarioides, facilitating its recolonization and persistence at its distributional range edge. Our study highlights that indirect facilitation can determine the recovery and persistence of a resource at the limit of its distribution, and may well contribute to the ecological mechanisms governing species distributions and range shifts.
Highlights
Much emphasis has been placed on the importance of biotic interactions and their potential to influence the distribution of species at different spatial and temporal scales [1,2,3,4,5]
Quadrat surveys showed that both S. viridula and M. laminarioides had higher incidences across the range overlap when compared with sites north
In line with our hypothesis of positive effects, S. viridula negatively affected the percentage cover of opportunistic green algae, which are suggested to affect the growth of corticated algae
Summary
Much emphasis has been placed on the importance of biotic interactions and their potential to influence the distribution of species at different spatial and temporal scales [1,2,3,4,5]. Since range edge populations are characterized by low effective population size and generally low dispersal potential [8], they appear especially prone to local extinction triggered by both abiotic and biotic factors [3,9] In this way, edge populations of resources could potentially expand or contract depending on variation of environmental barriers to dispersal and the intensity and type of consumption experienced at different life stages; i.e. the magnitude and direction of trophic interactions. Edge populations of resources could potentially expand or contract depending on variation of environmental barriers to dispersal and the intensity and type of consumption experienced at different life stages; i.e. the magnitude and direction of trophic interactions This idea is in part related to predictions of the ‘stress-gradient hypothesis’ (SGH) [10], which suggest that the frequency of competitive and facilitative interaction between species pairs vary inversely across gradients of physical stress or ecosystem productivity [11]. The rate of predation and facilitative interactions at the geographic range of a species is a matter of great interest to understand changes in species distribution in face of global change [12]
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