Abstract
Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.
Highlights
A; hereafter mussels) commonly overlap to form facilitation cascades and hotspots of ecosystem function along the Atlantic coast of North America [2, 4,5,6,7,8,9]
Despite nuanced understanding of the population processes and ecosystem function effects of this secondary foundation species, why mussels vary in cover by orders of magnitude within and across marsh platforms in the region remains unexplored [4, 7, 9, 10]
Given the importance of mussel-derived ecosystem function hotspots to overall marsh multifunctionality [2, 9], we conducted a survey of creeksheds associated with short and long creeks (i.e., 50–75 m and 125–250 m from the creek mouth to creekhead, respectively) from northern Florida to central South Carolina (Figure 1A) with the goal of characterizing patterns in mussel abundance and distribution within and across salt marshes
Summary
A; hereafter mussels) commonly overlap to form facilitation cascades and hotspots of ecosystem function along the Atlantic coast of North America [2, 4,5,6,7,8,9]. Survey results revealed that creeksheds associated with long tidal creeks consistently support larger numbers of mussels, percent areal coverage of mussel aggregations, and both average and maximum aggregation size than creeksheds associated with shorter tidal creeks. All four of these mussel population metrics were highest at creekheads (0 m), where tidal water floods onto and drains off the marsh platform (Figure 1B), intermediate at 10 m, and lowest at 20 m onto marsh platforms (Figure S1; see figure insets for model results here and below). At all three distances from the tidal creekhead, mussel population size increased with tidal creek length
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