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

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 >7.5 g. 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.

Size‐dependent movement explains why bigger is better in fragmented landscapes

“…For instance, the ingestion rate and speed of movement of an individual are correlated with its body size (Peters, 1983). Also, shifts in body size structure of communities have been shown to affect ecosystem functioning (Fritschie & Olden, 2016;Yvon-Durocher & Allen, 2012). Hence, body size can be considered a super trait (Brose et al, 2017;Fritschie & Olden, 2016) relating to both ecological effects and responses and therefore constraining ecological and evolutionary dynamics (Applebaum, Pan, Hedgecock, & Manahan, 2014;Llandres et al, 2015).…”

Decoupled responses of soil bacteria and their invertebrate consumer to warming, but not freeze–thaw cycles, in the Antarctic Dry Valleys

“…Such a research gap points to an underrated consequence of climate change, as shifts in body size distribution even within a single species have the potential to affect ecosystem functioning (Dossena et al . ; Fritschie & Olden ).…”

“…Whether smaller soil invertebrates such as nematodes would also experience a similar body size shift in response to FTCs remains unknown. Such a research gap points to an underrated consequence of climate change, as shifts in body size distribution even within a single species have the potential to affect ecosystem functioning (Dossena et al 2012;Fritschie & Olden 2016).…”

Body size variation in aquatic consumers causes pervasive community effects, independent of mean body size

“…A number of studies have demonstrated that variably sized (or size‐structured) populations produce different ecological consequences than do homogeneous populations (e.g., Asquith & Vonesh, ; Fritschie & Olden, ; Kishida, Mizuta, & Nishimura, ; Peacor & Werner, ; Rudolf & Rasmussen, ; Yamaguchi & Kishida, ). Such studies have tended to focus on distinct age classes, such as cohorts from different breeding events (e.g., Peacor & Werner, ).…”

“…Ecologists typically focus on average (mean) values of traits within species when characterizing functional trait diversity (Cianciaruso, Batalha, Gaston, & Petchey, 2009;Fritschie & Olden, 2016). However, within a single species, individuals exhibit significant variability in ecologically relevant phenotypic traits.…”