Energy In-Equivalence in Australian Marsupials: Evidence for Disruption of the Continent’s Mammal Assemblage, or Are Rules Meant to Be Broken?

Adam J Munn,Dennis W H Müller,Marcus Clauss,Craig Dunne

doi:10.1371/journal.pone.0057449

Adam J Munn, Dennis W H Müller + Show 2 more

Open Access

https://doi.org/10.1371/journal.pone.0057449

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Journal: PloS one	Publication Date: Feb 27, 2013
Citations: 38	License type: CC BY 4.0

Affiliation: University of Wollongong, University of Zurich

Abstract

The energy equivalence rule (EER) is a macroecological hypothesis that posits that total population energy use (PEU) should be independent of species body mass, because population densities and energy metabolisms scale with body mass in a directly inverse manner. However, evidence supporting the EER is equivocal, and the use of basal metabolic rate (BMR) in such studies has been questioned; ecologically-relevant indices like field metabolic rate (FMR) are probably more appropriate. In this regard, Australian marsupials present a novel test for the EER because, unlike eutherians, marsupial BMRs and FMRs scale differently with body mass. Based on either FMR or BMR, Australian marsupial PEU did not obey an EER, and scaled positively with body mass based on ordinary least squares (OLS) regressions. Importantly, the scaling of marsupial population density with body mass had a slope of −0.37, significantly shallower than the expected slope of −0.75, and not directly inverse of body-mass scaling exponents for BMR (0.72) or FMR (0.62). The findings suggest that the EER may not be a causal, universal rule, or that for reasons not yet clear, it is not operating for Australia’s unique native fauna.

Highlights

Identifying mechanistic associations between organism body size, resource use and whole-ecosystem processes is central to predicting how different species and their ecosystems might respond to environmental challenges, or to other factors affecting body size and resource use (e.g. [1])
The Equivalence Rule (EER) was derived from the observation that individual energy requirements apparently scale with animal body mass raised to a power close to 0.75, whereas the scaling of animal population densities scale with body mass raised to a power close to 20.75
We explored the scaling of marsupial population densities, basal metabolic rate (BMR), field metabolic rate (FMR) and population energy use (PEU) against body mass using ordinary least squares (OLS) regressions on log10-transformmed data

Summary

Introduction

Identifying mechanistic associations between organism body size (mass), resource use and whole-ecosystem processes is central to predicting how different species and their ecosystems might respond to environmental challenges, or to other factors affecting body size and resource use (e.g. [1]). The EER is a type of size-density relationship that states that total population energy-fluxes by different species should be equivalent, regardless of their respective body masses [2,3,5,6]. The EER states that whole-population energy fluxes (kJ per unit area) should be the same for differently-sized organisms, because total population energy use (PEU) equals energy turnover (or basal metabolic rate; BMR) multiplied by its density; i.e. Leaving aside debate concerning the ‘correct’ scaling exponent for BMR ( [8,15,16,17]; see McNab’s MISTCHEF model for the scaling of metabolism in mammals [18]), an EER should possess ecological relevance provided the pertinent scaling exponents for energy metabolism and density are inversely related [5]

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