Abstract
Invertebrate diversity can be a key driver of ecosystem functioning, yet understanding what factors influence local biodiversity remains uncertain. In many marine and terrestrial systems, facilitation cascades where primary foundation and/or autogenic ecosystem engineering species promote the settlement and survival of a secondary foundation/engineering species have been shown to enhance local biodiversity and ecosystem functioning. We experimentally tested if a facilitation cascade occurs among eelgrass (Zostera marina), pen clams (Atrina rigida), and community diversity in temperate seagrass beds in North Carolina, U.S.A., and if this sequence of direct positive interactions created feedbacks that affected various metrics of seagrass ecosystem function and structure. Using a combination of surveys and transplant experiments, we found that pen clam density and survivorship was significantly greater in seagrass beds, indicating that eelgrass facilitates pen clams. Pen clams in turn enhanced local diversity and increased both the abundance and species richness of organisms (specifically, macroalgae and fouling invertebrate fauna)—the effect of which scaled with increasing clam density. However, we failed to detect an impact of pen clams on other seagrass functions and hypothesize that functioning may more likely be enhanced in scenarios where secondary foundation species specifically increase the diversity of key functional groups such as epiphyte grazers and/or when bivalves are infaunal rather than epifaunal. Our findings add to the growing amount of literature that demonstrates that secondary foundation species are important drivers of local biodiversity in marine ecosystems. Further experimentation is needed that directly examines (i) the role of functional versus overall diversity on seagrass functions and (ii) the relative importance of life-history strategy in determining when and where engineering bivalves increase biodiversity and/or functioning of seagrass beds.
Highlights
It has long been recognized that cascades of species interactions can indirectly alter ecosystem processes and community organization [1,2]
Transplantation into seagrass beds and sandflats found that 16 months post-transplant, there was a significant effect of seagrasses on pen clam survivorship with 80% mortality observed in sand flats and no pen clam mortality observed in seagrass beds (Figure 2c)
Using a combination of observational and experimental studies, we found support for a facilitation cascade among seagrasses and pen clams in coastal North Carolina by showing that (i) seagrasses support higher survivorship and densities of pen clams and (ii) pen clams act as secondary foundation species that enhance local diversity, which scales positively with clam density
Summary
It has long been recognized that cascades of species interactions can indirectly alter ecosystem processes and community organization [1,2]. Tri-trophic cascades are one well-known example that occur when double-negative feeding interactions result in predators indirectly increasing plant populations by suppressing populations of otherwise voracious grazers [3,4,5]. Facilitation cascades generated by sequences of direct positive effects between habitat-forming ecosystem engineers can have powerful, indirect effects on marine ecosystems [6]. Secondary foundation species further modify ecosystem structure and functioning and enhance community diversity via their own engineering and niche space provisioning [9].
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