Population energy use scales positively with body size in natural aquatic microcosms

Abstract

Aim We test the 'energetic equivalence rule' (EER) -the idea that the amount of energy used by a population per unit area per unit time is independent of body mass -in meio-invertebrate communities from a series of natural, multitrophic aquatic 'rock pool' microcosms. Our study represents the first rigorous test of the EER at local scales of observation in a community of naturally coexisting species.Location Discovery Bay, Jamaica.Method We estimated population energy use (PEU) for every occurrence of every species of meio-invertebrate fauna found in each of 29 microcosms (233 observations of 31 species) using estimates of population density obtained in January 2005 in combination with published metabolism-mass relations for closely related taxa. ResultsIn the rock pool system as a whole, population density decreased ( ancova : b = -0.38 (-0.55 to -0.19), r 2 = 0.19, P < 0.001) and PEU increased with body mass ( ancova : b = 0.55 (0.36-0.73), r 2 = 0.28, P < 0.001). Main conclusionsThe positive PEU-body mass relation found here suggests that larger organisms are energetically dominant and points to the importance of sizestructured competition in these systems. Our results contrast those obtained in the few other previously published tests of the EER and challenge the idea that all species use similar amounts of energy regardless of their size.

Size-Energy Relationships in Ecological Communities

“…Previous studies of size-energy hypotheses, which have all used this method, have produced variable results, with most community-scale studies indicating that energy use peaks at intermediate body sizes [13], [17], [20], [26], [30] or increases with body size [11], [16], [21], [22], [56]. These results have been invoked to explain a number of broad ecological patterns, including the island rule (the common pattern of dwarfism and gigantism on isolated islands; [17]), Cope's Rule (the common evolutionary cycle in which large-bodied organisms emerge and become extinct more rapidly than small-bodied species; [16]), and the distribution of colony sizes in eusocial species [22].…”

“…First, all previous studies have indirectly used size-density relationships, rather than size-energy relationships, to test size-energy hypotheses (e.g., [11], [21], [22]). This method (hereafter, the ‘indirect method’) has spread because in practice it is easier to measure the density scaling relationship (i.e., the exponent a ) than the energy scaling relationship ( c ) in a community.…”

“…However, recent study suggests that metabolic scaling exponents are highly variable [25], and so a specific exponent cannot be assumed for all systems (i.e., b is not constant across all systems, and thus a is insufficient to determine c ). This approach also suffers from a lack of data on species-specific metabolic scaling exponents and is subject to the hidden non-linearity, propagation of error variances, and inherent imprecision of multiplying allometric relationships [21], [26]. This practice is also circular, since in the analysis, body size serves both as the predictor variable ( M ) and as a means to calculate the response variable ( PEU ); results therefore will tend to be biased to suggest a positive correlation.…”

Energetic inequivalence in eusocial insect colonies

“…Population size and body mass are usually negatively related, and the exponent a is frequently close to the negative of b, lending support to the EER [1][2][3][4]. In other cases a is less than 2b, suggesting that population-level metabolic rate increases with individual body mass [5][6][7][8]. In addition, the relationship between size and abundance is not always a scaling law, causing a peak in population-level metabolic rate at intermediate body sizes [9].…”

World Wide Food Webs: Power to Feed Ecologists

“…Life-history experiments in laboratory, and extensive studies in space and time, even as investigations at geological scales, are now widespread (e.g., Ernest et al 2009;Jones et al 2009). In this way, valuable metabolism-mass and biovolume equations became available in digital internet-accessible forms, together with an increasing amount of (cor)relationships investigated at broad scales of complexity (Hayward et al 2009;Jones et al 2009;). …”