Invariant size-frequency distributions along a latitudinal gradient in marine bivalves

Physiological variation in insects: large-scale patterns and their implications

In this paper we demonstrate how broad scale comparative physiology has an important role to play in informing a variety of assumptions made in macroecology. We do so by examining large-scale geographic variation in insect development, thermal tolerance and metabolic rate. From these studies, and those from the literature on insect water loss and thermoregulation, we show that there is often a bias to the geographic extent of available empirical data. Studies of cold hardiness are most usually undertaken at high latitudes, while investigations of upper thermal tolerances and water loss are most common in warm arid regions. Likewise, we demonstrate that much variation in insect physiological tolerances is partitioned at higher taxonomic levels, which has important implications for comparative physiology. Intriguingly, data on the full range of variables we review are available for only three species. We also show that, despite its importance, body size is regularly reported in only some kinds of investigations (metabolic rate, water loss rate), whereas in others (upper lethal temperature, cold hardiness, development) this variable is often ignored. In short, although large-scale comparative physiology can contribute considerable understanding to both physiology and ecology, there is much that remains to be done.

Testing the accuracy of methods for reconstructing ancestral states of continuous characters

Many methods are available for estimating ancestral values of continuous characteristics, but little is known about how well these methods perform. Here we compare six methods: linear parsimony, squared-change parsimony, one-parameter maximum likelihood (Brownian motion), two-parameter maximum likelihood (Ornstein-Uhlenbeck process), and independent comparisons with and without branch-length information. We apply these methods to data from 20 morphospecies of Pleistocene planktic Foraminifera in order to estimate ancestral size and shape variables, and compare these estimates with measurements on fossils close to the phylogenetic position of 13 ancestors. No method produced accurate estimates for any variable: estimates were consistently less good as predictors of the observed values than were the averages of the observed values. The two-parameter maximum-likelihood model consistently produces the most accurate size estimates overall. Estimation of ancestral sizes is confounded by an evolutionary trend towards increasing size. Shape showed no trend but was still estimated very poorly: we consider possible reasons. We discuss the implications of our results for the use of estimates of ancestral characteristics.

Invariant scaling relations across tree-dominated communities

Organizing principles are needed to link organismal, community and ecosystem attributes across spatial and temporal scales. Here we extend allometric theory-how attributes of organisms change with variation in their size-and test its predictions against worldwide data sets for forest communities by quantifying the relationships among tree size-frequency distributions, standing biomass, species number and number of individuals per unit area. As predicted, except for the highest latitudes, the number of individuals scales as the -2 power of basal stem diameter or as the -3/4 power of above-ground biomass. Also as predicted, this scaling relationship varies little with species diversity, total standing biomass, latitude and geographic sampling area. A simulation model in which individuals allocate biomass to leaf, stem and reproduction, and compete for space and light obtains features identical to those of a community. In tandem with allometric theory, our results indicate that many macroecological features of communities may emerge from a few allometric principles operating at the level of the individual.