Edge effects reverse facilitation by a widespread foundation species.

Laura J Jurgens,Brian Gaylord

doi:10.1038/srep37573

Abstract

Dense aggregations of foundation species often mitigate environmental stresses for organisms living among them. Considerable work documents such benefits by comparing conditions inside versus outside these biogenic habitats. However, environmental gradients commonly arise across the extent of even single patches of habitat-forming species, including cases where stresses diverge between habitat interiors and edges. We ask here whether such edge effects could alter how habitat-forming species influence residents, potentially changing the strength or direction of interactions (i.e., from stress amelioration to exacerbation). We take as a model system the classic marine foundation species, Mytilus californianus, the California mussel. Results demonstrate that mussel beds both increase and decrease thermal stresses. Over a distance of 6 to 10 cm from the bed interior to its upper surface, peak temperatures climb from as much as 20 °C below to 5 °C above those of adjacent bedrock. This directional shift in temperature modification affects interactions with juvenile mussels, such that thermal stresses and associated mortality risk are higher at the bed surface, but substantially reduced deeper within the adult matrix. These findings provide a case example of how stress gradients generated across biogenic habitats can markedly alter ecological interactions even within a single habitat patch.

Highlights

We ask whether edge effects in habitats formed by M. californianus could fundamentally alter the nature of facilitative interactions with their residents
Interior mussel bed microhabitats remained below 25 °C even on the hottest days (Fig. 1), and peak values were reduced by an average of 10 to 15 °C compared to adjacent bedrock
Our results demonstrate that differences in how foundation species modify physical conditions at habitat edges versus interior locations can cause environmental stressors to vary as strongly within a habitat patch as they do between locations inside and outside it

Summary

Introduction

We ask whether edge effects in habitats formed by M. californianus could fundamentally alter the nature of facilitative interactions with their residents. Since physiological tolerances and microhabitat preferences commonly vary by body size and life stage[27,28,29], we ask whether size-related differences in thermal tolerance or habitat use might shift the relative importance of facilitation by the mussel bed[30]. Through this linked series of experiments, we provide an empirical examination of how edge effects may alter the strength and/or existence of facilitative interactions inside assemblages of a widespread habitat-forming species

Methods

Results

Conclusion