Body size and shape responses to warming and resource competition

Hanrong Tan,Andrew G Hirst,Pavel Kratina,David Atkinson

doi:10.1111/1365-2435.13789

Abstract

Abstract Body size is a fundamental trait that impacts many aspects of species biology and ecology. It is, in turn, influenced by a suite of environmental factors, and often decreases with warming. Although environmental conditions can also impact body shape, which is another functional trait that influences locomotion, resource acquisition and potentially physiological processes, such responses are poorly understood and rarely quantified. We experimentally tested the independent and combined effects of environmental temperature, resource level and interspecific competition on the body shape and size responses of two model protist species. We also tested the degree to which these individual‐level phenotypic responses are associated with population densities and species coexistence. Body shape was strongly influenced by resource competition, whereas body size changes were mainly driven by environmental temperature. In both species, lower resource levels resulted in body shape elongation, suggesting that relatively more elongate individuals with potentially higher swimming speed were advantaged in the resource scarce environment. However, competition had contrasting influence on the body shape of the two species. Competition decreased the population densities of Blepharisma japonicum, which exhibited relative body shape elongation, similar to the response at low resource levels. In contrast, competition increased the population densities of Paramecium aurelia, which exhibited reduced elongation similar to body shape response at high resource levels. Hence, body shape responses could be indicative of changes in resource availability, aiding our understanding of competitive hierarchies and species interactions. Coexistence was observed in all treatment combinations, likely because body size of both species decreased similarly under warming, potentially maintaining constant per capita competitive intensity. These findings, along with recent research on phytoplankton, diverse pelagic invertebrates, and birds highlight the importance of body shape and morphology across different taxonomic groups. Hence, we call for body size and shape to be considered in concert when investigating ecological consequences of climate warming. A free Plain Language Summary can be found within the Supporting Information of this article.

Highlights

Climate warming is characterised by a rapid rise in global mean temperatures (IPCC, 2018) and an increase in the frequency and severity of extreme temperature events (Easterling et al, 2000; Meehl, 2004), which have profound impacts on global biota (Vasseur et al, 2014; Woodward et al, 2016)
Understanding how the environment modulates functional traits of organisms can improve our knowledge of how global changes alter biotic interactions, species coexistence and community dynamics (McGill et al, 2006)
Our simple factorial experiment showed that the model protist species have divergent body shape responses to interspecific resource competition (Figures 1 and 4), but similar body size responses to warming (Figures 2 and 4)

Summary

| INTRODUCTION

Climate warming is characterised by a rapid rise in global mean temperatures (IPCC, 2018) and an increase in the frequency and severity of extreme temperature events (Easterling et al, 2000; Meehl, 2004), which have profound impacts on global biota (Vasseur et al, 2014; Woodward et al, 2016). Reduction in body sizes has emerged as a third universal ecological response to mean increase in global temperatures (Daufresne et al, 2009; Evans et al, 2020; Sheridan & Bickford, 2011), and potentially to extreme temperature events (Fischer et al, 2014) Such body size reductions could have profound ecological repercussions because this fundamental trait (Peters, 1983) underlies metabolic rates (Brown et al, 2004), fecundity (Arendt, 2007), species interactions and community dynamics (Brose et al, 2006; Rall et al, 2012). Our results show that body size changes are closely related to whole-organism metabolic demands, whereas body-shape changes are governed mainly by resource acquisition

| MATERIALS AND METHODS

Findings

| DISCUSSION