Abstract
The relationship between average body masses (M) of individuals within species and densities (N) of populations of different species and the mechanisms and consequences of this relationship have been extensively studied. Most published work has focused on collections of data for populations of species from a single broad taxon or trophic level (such as birds or herbivorous mammals), rather than on the populations of all species occurring together in a local food web, a very different ecological context. We here provide a systematic analysis of relationships between M and N in community food webs (hereafter simply webs), using newly collected, taxonomically detailed data from 166 European and North American pelagic, soil, riparian, benthic, inquiline, and estuarine webs.
We investigated three topics. First, we compared log(N)‐versus‐log(M) scatter plots for webs and the slope b1 of the ordinary‐least‐squares (OLS) regression line log(N) =b1 log(M) +a1 to the predictions of two theories (Section V.A). The energetic equivalence hypothesis (EEH) was not originally intended for populations within webs and is used here as a null‐model. The second theory, which extends the EEH to webs by recognizing the inefficiency of the transfer of energy from resources to consumers (a trophic transfer correction, or TTC), was originally proposed for webs aggregated to trophic levels. The EEH predicts approximate linearity of the log(N)‐versus‐log(M) relationship, with slope −3/4 for all webs. The relationship was approximately linear for most but not all webs studied here. However, for webs that were approximately linear, the slope was not typically −3/4, as slopes varied widely from web to web. Predictions of the EEH with TTC were also largely falsified by our data. The EEH with TTC again predicts linearity with b1 <−3/4 always, meaning that populations of larger taxa in a web absorb less energy from the environment than populations of smaller taxa. In the majority of the linear webs of this study, on the contrary, b1>−3/4, indicating that populations of larger taxa absorb more energy than populations of smaller ones. Slopes b1> −3/4 can occur without violating the conservation of energy, even in webs that are energetically isolated above trophic level 0 (discussed later).
Second, for each web, we compared log–log scatter plots of the M and N values of the populations of each taxon with three alternate linear statistical models (Section V.B). Trophic relationships determined which taxa entered the analysis but played no further role except for the Tuesday Lake and Ythan Estuary webs. The assumptions of the model log(N) =b1 log(M) +a1+ ɛ1 (including linearity of the expectation) were widely but not universally supported by our data. We tested and confirmed a hypothesis of Cohen and Carpenter (2005) that the model log(N) =b1 log(M) +a1+ ɛ1 describes web scatter plots better, in general, than the model log(M) =b2 log(N) +a2+ ɛ2. The former model is also better than the model of symmetric linear regression.
Third, since not all of our log–log scatter plots formed approximately linear patterns, we explored causes of nonlinearity and examined alternative models (Section V.C). We showed that uneven lumping of species to web nodes can cause log(N)‐versus‐log(M) scatter plots to appear nonlinear. Attributes of the association between N and M depended on the type of ecosystem from which data were gathered. For instance, webs from the soil of organic farms were much less likely to exhibit linear log(N)‐versus‐log(M) relationships than webs from other systems. Webs with a larger range of measured log(M) values were more likely to appear linear. Our data rejected the hypothesis that data occupy a polygonal region in log(N)‐versus‐log(M) space.