Burrowing Crabs Weaken Mutualism Between Foundation Species

Jasper R H Hoogveld,Marlous Derksen-Hooijberg,Annieke Borst,Tjisse Van Der Heide,Laura L Govers,Alfons J P Smolders,Leon P M Lamers,Christine Angelini

doi:10.1007/s10021-018-0301-x

Abstract

Foundation species often interact with each other and co-create habitat upon which other species depend. Whether the presence of these facilitated species feeds back to mediate the growth and resilience of the foundation species themselves, and influence the strength of their interactions, remains poorly understood. In a 16-month field experiment in a southeastern US salt marsh, we tested how the overlapping presence of two foundation species, cordgrass (Spartina alterniflora) and ribbed mussels (Geukensia demissa), influences the abundance of facilitated species, specifically burrowing crabs (mainly Uca pugnax), and how crabs, in turn, affect each foundation species and their mutualistic interaction. Mussel aggregations enhanced crab abundance 3.9-fold, which in turn reduced both mussel and cordgrass growth and stifled cordgrass recovery after a simulated disturbance. Porewater and plant tissue analyses suggest crabs reduced cordgrass growth by reducing nitrogen availability, damaging roots, and potentially interfering with mussel deposition of nutrient-rich pseudofeces. A five-site field survey along 700 km southeastern US coastline revealed that cordgrass biomass and crab abundance are consistently higher in mussel aggregations. Furthermore, cordgrass biomass correlated negatively with crab abundance, supporting our experimental findings and the hypothesis that facilitated biota can negatively impact the foundation species upon which they depend. We anticipate that such negative, but non-lethal, feedbacks between foundation species and the biota they facilitate may be a common but overlooked phenomenon controlling foundation species growth and interactions in a wide range of ecosystems.

Highlights

Foundation species strongly control ecosystem dynamics and functioning through their enhancement of habitat complexity, reduction of physical stress and modulation of resource availability (Dayton 1972)
To examine the generality of the experimental results and their broader implications for understanding the stability of this mutualism in salt marshes across the southeastern US seaboard, we conducted a field survey across five salt marshes distributed from Florida to North Carolina, in which we correlatively examined the relations between cordgrass, mussels and crabs
We demonstrate that the ecosystem resilience- and recovery-enhancing mutualism between mussels and cordgrass in southeastern US salt marshes (Bertness and others 2015; Angelini and others 2016; Derksen-Hooijberg and others 2018) is dampened by fiddler crabs that themselves profit from the positive cordgrass–mussel interaction

Summary

Introduction

Foundation species strongly control ecosystem dynamics and functioning through their enhancement of habitat complexity, reduction of physical stress and modulation of resource availability (Dayton 1972). Mutualists can be indirectly vital to ecosystem functioning by providing essential support to a foundation species (Loya and others 2001; Angelini and others 2016; de Fouw and others 2016; Peres and others 2016), their role can be of direct importance when the mutualist is a foundation species and strongly facilitates associated species In these cases, the foundation species partners co-create a complex and low-stress habitat that is suitable for many different species, resulting in increased species richness and abundance (Thomsen and others 2010; Bishop and others 2013; Angelini and Silliman 2014; Bell and others 2014; Angelini and others 2015). How the increased abundances of facilitated species feed back to influence the growth and persistence of foundation species is not well understood

Methods

Results

Discussion

Conclusion