Disentangling the influences of mean body size and size structure on ecosystem functioning: an example of nutrient recycling by a non-native crayfish.

Keith J Fritschie,Julian D Olden

doi:10.1002/ece3.1852

Abstract

Body size is a fundamental functional trait that can be used to forecast individuals' responses to environmental change and their contribution to ecosystem functioning. However, information on the mean and variation of size distributions often confound one another when relating body size to aggregate functioning. Given that size-based metrics are used as indicators of ecosystem status, it is important to identify the specific aspects of size distributions that mediate ecosystem functioning. Our goal was to simultaneously account for the mean, variance, and shape of size distributions when relating body size to aggregate ecosystem functioning. We take advantage of habitat-specific differences in size distributions to estimate nutrient recycling by a non-native crayfish using mean-field and variance-incorporating approaches. Crayfishes often substantially influence ecosystem functioning through their omnivorous role in aquatic food webs. As predicted from Jensen's inequality, considering only the mean body size of crayfish overestimated aggregate effects on ecosystem functioning. This bias declined with mean body size such that mean-field and variance-incorporating estimates of ecosystem functioning were similar for samples at mean body sizes >7.5g. At low mean body size, mean-field bias in ecosystem functioning mismatch predictions from Jensen's inequality, likely because of the increasing skewness of the size distribution. Our findings support the prediction that variance around the mean can alter the relationship between body size and ecosystem functioning, especially at low mean body size. However, methods to account for mean-field bias performed poorly in samples with highly skewed distributions, indicating that changes in the shape of the distribution, in addition to the variance, may confound mean-based estimates of ecosystem functioning. Given that many biological functions scale allometrically, explicitly defining and experimentally or statistically isolating the effects of the mean, variance, and shape of size distributions is necessary to begin generalizing relationships between animal body size and ecosystem functioning.

Highlights

Trait-based ecology holds considerable promise for forecasting the effects of global change on ecosystem functioning by scaling up the actions of individuals independent of their taxonomy (Luck et al 2012)
Ecology and Evolution published by John Wiley & Sons Ltd
After controlling for total biomass, we found that true aggregate ammonium recycling was highest among habitats with low mean body size and high coefficient of variation (CV) – that is, banks vs. the mid-channel (1.28 Æ 0.51)

Summary

Introduction

Trait-based ecology holds considerable promise for forecasting the effects of global change on ecosystem functioning by scaling up the actions of individuals independent of their taxonomy (Luck et al 2012). Body size has long been considered a useful trait in determining species’ responses to the environment, their ecological relationships, and their roles in ecosystem processes (Peters 1983; Woodward et al 2005). The metabolic theory of ecology provides a mechanistic basis for the scaling relationships between body size and many system properties (Brown et al 2007), suggesting that body size may represent a universal trait to predict the effects of biological change on ecosystem functioning (Seguin et al 2014).

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