A keystone mutualism underpins resilience of a coastal ecosystem to drought

Abstract

Droughts are increasing in severity and frequency, yet the mechanisms that strengthen ecosystem resilience to this stress remain poorly understood. Here, we test whether positive interactions in the form of a mutualism between mussels and dominant cordgrass in salt marshes enhance ecosystem resistance to and recovery from drought. Surveys spanning 250 km of southeastern US coastline reveal spatially dispersed mussel mounds increased cordgrass survival during severe drought by 5- to 25-times. Surveys and mussel addition experiments indicate this positive effect of mussels on cordgrass was due to mounds enhancing water storage and reducing soil salinity stress. Observations and models then demonstrate that surviving cordgrass patches associated with mussels function as nuclei for vegetative re-growth and, despite covering only 0.1–12% of die-offs, markedly shorten marsh recovery periods. These results indicate that mutualisms, in supporting stress-resistant patches, can play a disproportionately large, keystone role in enhancing ecosystem resilience to climatic extremes.

Mutualistic interactions amplify saltmarsh restoration success

“…For example, in low‐energy saltmarsh locations were sulphide stress poses a major threat to new transplants, a design including mussels and cordgrass may be most efficient. The mutualism alleviates this stress, and allows for transplanting cordgrass in widely spaced arrays, thereby maximizing the contribution of these patches to mudflat re‐colonization via clonal expansion (Angelini et al., ). However, in more hydrodynamically active locations, a clumped design, which is more effective in attenuating waves and currents than widely spaced arrays (Silliman et al., ), may yield high success rates.…”

“…Nevertheless, a recent survey among restoration agencies in the United States revealed that only 1 of 25 was implementing designs that could harness intraspecific facilitation . In a similar survey of 20 US restoration agencies conducted for this study, we found that 85% of the surveyed practitioners do not currently include positive interspecific interactions in saltmarsh restorations (Appendix S1), despite clear indications that these interactions may yield high benefits for restoration success (Angelini et al, 2015(Angelini et al, , 2016.…”

“…In this study, we test the general hypothesis that interspecificpositive interactions can enhance coastal ecosystem restoration success. We focus on southeastern US saltmarshes where sudden dieback events are known to cause vegetation mortality, resulting in marsh areas of several m 2 to several km 2 being converted to bare mudflat (Alber, Swenson, Adamowicz, & Mendelssohn, 2008;Angelini et al, 2016;Silliman, van de Koppel, Bertness, Stanton, & Mendelssohn, 2005). In these saltmarshes, Spartina alterniflora (hereafter cordgrass) is the primary habitat-forming foundation species, supporting a diverse faunal community (Angelini et al, 2015;Bertness, 1991;Dayton, 1972;Paterson & Whitfield, 2000).…”

“…The base of cordgrass stems provide settlement substrate to facilitate the establishment of clumped mussel aggregations (Bertness & Grosholz, 1985) and cordgrass leaves provide shade and reduce otherwise lethal temperatures (Angelini et al, 2015). Once established, mussels further enhance biodiversity and ecosystem functioning (Angelini et al, 2015) and engage in a mutualistic interaction with cordgrass, by enhancing cordgrass' growth and drought resistance (Angelini et al, 2016;Bertness, 1984;Bertness, Brisson, & Crotty, 2015). However, whether this mutualism could aid in increasing restoration success is unclear.…”

A Global Synthesis Reveals Gaps in Coastal Habitat Restoration Research

“…Empirical research in coastal ecosystems over recent decades, however, has shown that biotic interactions such as predation, competition, and, more recently, positive species interactions are also important controls, especially under high physical stress scenarios [37][38][39][40]. In particular, recent small-scale, experimental tests in marsh restoration have demonstrated that incorporating indirect positive interactions among transplants and between foundation species can increase ecosystem recovery and resilience [41,42]. These results indicate that expanding restoration designs to also consider trophic controls and indirect species interactions in addition to the physical template may enhance restoration efficacy and yield [41,43].…”