Different components of biodiversity mediate two ecosystem functions derived from naturally assembled filter‐feeder communities

Abstract

Positive biodiversity–ecosystem functioning (BEF) relationships observed in experiments can be challenging to identify in natural communities. Freshwater animal communities are disproportionately harmed by global change that results in accelerated species loss. Understanding how animal‐mediated ecosystems functions may change as a result of global change can help determine whether biodiversity or species‐specific conservation will be effective at maintaining function. Unionid mussels represent half of imperiled species in freshwater ecosystems globally and perform important ecological functions such as water filtration and nutrient recycling. We explored BEF relationships for 22 naturally assembled mussel aggregations spanning three river basins. We used the Price equation to partition the contributions of species richness, composition, and context dependent interactions to two functions of interests: spatially‐explicit standing‐stock biomass (indirect proxy for function) and species‐specific nitrogen (N) excretion rates (direct measure of N recycling). Random and non‐random species loss each reduced biomass and N recycling. Many rare species with low contributions to biomass contributed to standing‐stock biomass in all basins. Widespread species had variable function across sites, such that context dependent effects (CDEs) outweighed richness effects on standing‐stock biomass in two basins, and were similar to richness effects in the third. Richness effects outweighed CDEs for N recycling. Thus, many species contributed a low proportion to overall N‐recycling; a product we attribute to the high evenness and functional effect trait diversity associated with these communities. The loss of low‐functioning species reduced the function of persisting species. This novel result using observational data adds evidence that positive species interactions, such as interspecific facilitation, may be a mechanism by which biodiversity enhances ecosystem functions. Our work stresses the importance of evaluating species‐specific contributions to functions in diverse systems, such as nutrient cycling when maintaining specific animal‐mediated functions is a management goal because indirect proxies may not completely characterize BEF relationships.

Species identity and diversity of filter‐feeding bivalves impact green and brown food webs

“…Our study is among the first to examine consumer driven nutrient dynamics of freshwater communities, while linking both brown and green food webs in the same study system. Our results highlight the importance of measuring multiple functional metrics across a gradient of diversity in ecologically similar consumers (Hopper et al, 2023). We observed many positive effects when mussels were present in components of both green and brown food webs.…”

“…Beyond the results of our experiment, this functional trait complementarity effect has implications for natural systems where many species of freshwater mussels co-occur, but do not occur at equal abundances or biomass (Hopper et al, 2021). Thus, through niche complementarity or synergistic interactions, the overall contribution to ecosystem functions may be enhanced as a result of species-specific functional traits as a result of differential biomass and resulting trait expression (e.g., Hopper et al, 2023). Such species-specific interactions means that the loss of certain species within the mussel community may affect ecosystem functioning via nutrient fluxes and stoichiometry (Benelli et al, 2019).…”

“…Differences in functional traits and effects have been linked to phylogeny across various organisms (Allgeier et al, 2021), including mussels (Atkinson, van Ee, & Pfeiffer, 2020), and are based on the hypothesis that evolutionary processes drive trait diversification and thus enhance the functional trait space of a community (Srivastava et al, 2012). Through niche complementarity or synergistic interactions, species richness may enhance ecosystem function and stability (Hopper et al, 2023;Loreau & Mazancourt, 2013). Therefore, a better understanding of functional trait diversity is key to improving our ability to understand and predict the biodiversity effects on ecosystem stability.…”

Secondary production and biomass in mussel assemblages relate to species richness and stream size but not life history

“…The results of the present study highlight the generality of the SAR and the importance of assemblage density effects in BEF studies using observational data. Efforts to disentangle the importance of species richness from those of density and abundance in natural assemblages are ongoing and involve a range of mathematical and statistical approaches (Genung et al, 2020;Hopper et al, 2023;van der Plas, 2019;Winfree et al, 2015). We also highlight the need for a more thorough understanding of how the interplay between species' functional response and effect traits strengthen or weaken their impacts on ecosystem function.…”

“…In addition, mussel assemblages often show a strong longitudinal SAR with increasing stream size (Haag, 2012;Haag & Warren, 1998;Ortmann, 1913;Watters, 1992). BEF relationships have also been documented in mussel assemblages at various scales for functions such as aquatic-to-terrestrial resource subsidies, primary production, and nutrient recycling (Allen et al, 2012;Hopper et al, 2023;Vaughn, 2010;Vaughn et al, 2007). A similar relationship may exist between mussel species richness and secondary production due to positive species interactions that promote the filtration or assimilation of suspended particles from the water column.…”