Insect rate-temperature relationships: environmental variation and the metabolic theory of ecology

Much of the recent discussion concerning the form and underlying mechanistic basis of metabolic rate-temperature and development rate-temperature relationships has been precipitated by the development of the metabolic theory of ecology (MTE). Empirical tests of the theory's fundamental equation are an essential component of establishing its validity. Here, we test the temperature component of the fundamental equation of the MTE as it applies to metabolic rate and development rate, using insects as model organisms. Specifically, we test (i) whether mean activation energies, E, approximate the 0.65 eV value proposed by the proponents of the MTE and whether the range of values is tightly constrained between 0.6 and 0.7 eV, as they have argued; (ii) whether phylogenetic signal is apparent in the rate-temperature relationships; (iii) whether the slopes of the rate-temperature relationships show consistent, directional variation associated with environmental variables; and (iv) whether intra- and interspecific rate-temperature relationships differ significantly. Because the majority of activation energy values fell outside the predicted range and rate-temperature relationships showed consistent directional variation correlated with large-scale climatic variation, we conclude that data from insects provide only limited support for the MTE. In consequence, we consider alternative explanations for variation in rate-temperature relationships.