The nutrients excreted by fish in their "pee" may be critical to the health of coastal ecosystems. But knowing whether generalizations can be made about how to predict these nutrient levels in various ecosystems has vexed researchers - until now.
In a paper published in Proceedings of the National Academy of Sciences, North Carolina State University associate professor Craig Layman and colleagues show that ecologists can better predict the rates of how these chemical nutrients are transferred by fish if they know the various fish species living in an ecosystem, along with the body size of the fish.
"The findings suggest that body size and taxonomic identity are the most important factors to predict the amount of nutrients that fish recycle to the environment," Layman said. "Body size is important because the bigger you are the more you excrete. Taxonomy is important because different species of fish eat different food items and have different body structures."
The researchers studied fish and invertebrate species in The Bahamas. The robust size of the data set - some 900 individual fish or invertebrates in a diverse marine community - provided heft to the study, says Jake Allgeier, the lead author of the paper and a researcher at the University of Washington.
"We examined everything from sea cucumbers to moray eels - 102 species in all - and using these data were able to test what best predicts how animals recycle nitrogen and phosphorus," Allgeier said. "These findings can be applied to draw general conclusions across other ecosystems. But we also need to acknowledge that certain species can have unique effects on these ecological processes."
These research findings, along with a growing body of evidence from Allgeier and other researchers, carry much significance for coastal management initiatives, Layman added. "Coral restoration is becoming a widespread management practice and fish-based nutrient supply has been shown to be important for coral health - and thus the success of restoration efforts," he said.
Researchers from the University of Washington and the University of Georgia co-authored the paper. Funding was provided by the National Science Foundation and the Environmental Protection Agency.
Note: An abstract of the paper follows.
"Metabolic theory and taxonomic identity predict nutrient recycling in a diverse food web"
Authors: Jacob Allgeier and Daniel Schindler, University of Washington; Seth Wenger and Amy Rosemond, University of Georgia; and Craig Layman, North Carolina State University
Published: April 13, 2015, in Proceedings of the National Academy of Sciences
Abstract: Reconciling the degree to which ecological processes are generalizable among taxa and ecosystems, or contingent on the identity of interacting species, remains a critical challenge in ecology. Ecological stoichiometry (EST) and metabolic theory of ecology (MTE) are theoretical approaches used to evaluate how consumers mediate nutrient dynamics and energy flow through ecosystems. Recent theoretical work has explored the utility of these theories, but empirical tests in species-rich ecological communities remain scarce. Here we use an unprecedented dataset collected from fishes and dominant invertebrates (n = 900) in a diverse subtropical coastal marine community (50 families, 72 genera, 102 species; body mass range: 0.04-2,597 g) to test the utility of EST and MTE in predicting excretion rates of nitrogen (EN), phosphorus (EP), and their ratio (ENP). Body mass explained a large amount of the variation in EN and EP but not ENP. Strong evidence in support of the MTE 3/4 allometric scaling coefficient was found for EP, and for EN only after accounting for variation in excretion rates among taxa. In all cases, including taxonomy in models substantially improved model performance, highlighting the importance of species identity for this ecosystem function. Body nutrient content and trophic position explained little of the variation in EN, EP, or ENP, indicating limited applicability of basic predictors of EST. These results highlight the overriding importance of MTE for predicting nutrient flow through organisms, but emphasize that these relationships still fall short of explaining the unique effects certain species can have on ecological processes.