Complementarity in nitrogen uptake by seaweeds occurs via simultaneous effects of diversity on multiple ecosystem functions.
Bracken, Matthew*,1, Stachowicz, John1, 1 University of California, Davis, Davis, CA
ABSTRACT- Because the diversity of organisms can profoundly impact the transformation and flux of energy and materials in an ecosystem, current biodiversity declines will likely influence biogeochemical processes in a variety of ecosystems. While empirical and theoretical studies suggest mechanistic links between diversity and ecosystem functioning, most of this research has been conducted in grasslands, making it difficult to predict the consequences of biodiversity loss in other ecosystems. In this study, we evaluated the mechanisms linking marine seaweed diversity and the uptake of nitrate and ammonium from the water column, which collectively represent the flux of inorganic nitrogen into intertidal ecosystems. Specifically, we compared the abilities of eight macroalgal species in monoculture and polyculture to take up nitrogen provided as either nitrate, ammonium, or nitrate + ammonium. We then compared the measured nitrogen uptake coefficients of the eight-species polycultures with those predicted by the component monocultures. This provided a statistical test for non-transgressive overyielding, which occurs when a polyculture's impact on ecosystem functioning is greater than the average effects of its component monocultures. We found that in diverse assemblages, uptake of either nitrate or ammonium alone was determined by statistical averaging of the component species' performances in monoculture (the selection effect), but when both nitrogen forms were present, uptake rates were 21% greater than predicted by the monocultures. This nitrogen niche complementarity occurred because different algal species were more effective at using different forms of nitrogen. Our findings suggest that selection, which is often considered a statistical artifact, can provide a valid and under-appreciated link between diversity and ecosystem functioning when multiple ecosystem functions are simultaneously considered.
Key words: biodiversity, ecosystem functioning, macroalgae, rocky intertidal
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