Foundation species' overlap enhances biodiversity and multifunctionality from the patch to landscape scale in southeastern United States salt marshes

Abstract

Although there is mounting evidence that biodiversity is an important and widespread driver of ecosystem multifunctionality, much of this research has focused on small-scale biodiversity manipulations. Hence, which mechanisms maintain patches of enhanced biodiversity in natural systems and if these patches elevate ecosystem multifunctionality at both local and landscape scales remain outstanding questions. In a 17 month experiment conducted within southeastern United States salt marshes, we found that patches of enhanced biodiversity and multifunctionality arise only where habitat-forming foundation species overlap-i.e. where aggregations of ribbed mussels (Geukensia demissa) form around cordgrass (Spartina alterniflora) stems. By empirically scaling up our experimental results to the marsh platform at 12 sites, we further show that mussels-despite covering only approximately 1% of the marsh surface-strongly enhance five distinct ecosystem functions, including decomposition, primary production and water infiltration rate, at the landscape scale. Thus, mussels create conditions that support the co-occurrence of high densities of functionally distinct organisms within cordgrass and, in doing so, elevate salt marsh multifunctionality from the patch to landscape scale. Collectively, these findings suggest that patterns in foundation species' overlap drive variation in biodiversity and ecosystem functioning within and across natural ecosystems. We therefore argue that foundation species should be integrated in our conceptual understanding of forces that moderate biodiversity -ecosystem functioning relationships, approaches for conserving species diversity and strategies to improve the multifunctionality of degraded ecosystems.

An invasive foundation species enhances multifunctionality in a coastal ecosystem

“…Ecologists are becoming increasingly aware that native foundation species provide ecosystem functions in nonlinear ways (39)(40)(41). Our experimental findings demonstrate similar nonlinearity in ecosystem functions provided by a nonnative foundation species that has invaded thousands of kilometers of coastline throughout the Northern Hemisphere.…”

“…For example, functions such as wave attenuation and sediment stabilization may manifest on larger scales and at higher densities (39), whereas functions associated with biodiversity, such as productivity and nursery habitats, appear to operate on smaller scales and at lower densities (i.e., with early thresholds as shown here and in ref. 41). Although curve shapes of multifunctionality are likely to depend on the functions included in the analysis, our supplementary analysis (adding four processes measured at the end of our experiment) showed similarly curved functional responses, suggesting that our results are robust (Fig.…”

“…Because recent experiments and ecological theory suggest that relationships between the structure and function of coastal habitats are characterized by thresholds and nonlinear limiting functions (39-41), we analyzed the relationship between Gracilaria cover and each response variable individually by comparing five models (41,72). Using nonlinear least squares (73), we fit null, linear, log, hyperbolic, and power relationships for each response using the average Gracilaria cover (in %) of each plot as the explanatory variable.…”

How habitat-modifying organisms structure the food web of two coastal ecosystems

“…On cobble beaches, for instance, many studies (including this one) have revealed non-trophic facilitation of the local community [23,42] and that at a scale of metres to tens of metres, cordgrass patches function as wave breaks to facilitate wave-sensitive forb species [43]. Similarly, intertidal mussel beds in the Wadden Sea were found to facilitate the community at a scale exceeding 100 m by baffling waves and changing sediment characteristics [24,44].…”

Shorescape‐level factors drive distribution and condition of a salt marsh facilitator (Geukensia demissa)

“…The role of ribbed mussels as a salt marsh facilitator has been explored for the past couple of decades in a variety of regions along the U.S. Atlantic Coast (Bertness and Leonard 1997, Angelini et al 2015. Ribbed mussels improve the marsh not only for the Spartina, but for the other organisms found there as well.…”