Behavior and other forms of phenotypic plasticity potentially enable individuals to deal with novel situations. This implies that establishment of a population in a new environment is aided by plastic responses, as first suggested by Baldwin (1896). In the early 1980s, a small population of dark-eyed juncos from a temperate, montane environment became established in a Mediterranean climate in coastal San Diego. The breeding season of coastal juncos is more than twice as long as that of the ancestral population, and they fledge approximately twice as many young. We investigated the adaptive significance of the longer breeding season and its consequences for population persistence. Within the coastal population, individuals with longer breeding seasons have higher offspring production and recruitment, with no measured detrimental effects such as higher mortality or lower reproductive success the following year. Population size has remained approximately constant during the 6 years of study (1998-2003). The increase in reproductive effort in the coastal population contributes substantially to the persistence of this population because there is no evidence of density-dependent recruitment, which would otherwise negate the effects of increased fledgling production. These results provide the first quantitative support of Baldwin's proposition that plasticity can be crucial for population persistence during the early stages of colonization.

If two successive trait measurements have a less-than-perfect correlation, individuals or populations will, on average, tend to be closer to the mean on the second measurement (the so-called regression effect). Thus, there is a negative correlation between an individual's state at time 1 and the change in state from time 1 to time 2. In addition, whenever groups differ in their initial mean values, the expected change in the mean value from time 1 to time 2 will differ among the groups. For example, birds feeding nestlings lose weight, but initially heavier birds lose more weight than lighter birds, a result expected from the regression effect. In sexual selection, males who remain unmated in the first year are, on average, less attractive than mated males. The regression effect predicts that these males will increase their attractiveness in the second year more than mated males. In well-designed experiments, changes in the experimental and control groups would be compared. In observational studies, however, no such comparison is available, and expected differential effects must be accounted for before they can be attributed to external causes. We describe methods to correct for the regression effect and assess alternative causal explanations.

The ecology of the component species of an adaptive radiation is likely to be influenced by the form of the founding ancestor to the radiation, its timing, and rates of speciation and extinction. These historical features complement environmental selection pressures. They imply that, if the history of the species' radiations are very different, ecological communities are unlikely to be completely convergent even when placed in identical environments. We compare the adaptive radiation of the Dendroica warblers of North America with that of the Phylloscopus warblers of Asia. We consider the ecology of the species in two localities where species' diversity is very high (New Hampshire, U.S.A., and Kashmir, India, respectively) and contrast the history of the two radiations on the basis of a molecular (mitochondrial cytochrome b) phylogeny. By comparison with the Phylloscopus, the Dendroica are on average larger and morphologically more similar to one another. Although there is some similarity between the Dendroica and Phylloscopus communities, they differ in foraging behavior and in associations of morphology with ecological variables. The Dendroica likely reflect an early Pliocene radiation and are two to four times younger than the Phylloscopus. They probably had a colorful sexually dichromatic ancestor, implicating sexual selection in the production of the many ecologically similar species. The Phylloscopus are much older and probably had a drab, monomorphic ancestor. Given the difference in ages of the two radiations, it is plausible that the close species' packing of the Dendroica warblers is a transient phenomenon. If this is the case, community structure evolves on the timescales of millions of years. Differences in ancestry and timing of the species' radiations can be * E-mail: tprice@ucsd.edu. related to the different biogeography of the two regions. This implies that the historical imprint on adaptive radiations could be predicted on the basis of the attributes of ancestors and biogeographical context.

A biologically explicit simulation model of resource competition between two species of seed-eating heteromyid rodent indicates that stable coexistence is possible on a homogeneous resource if harvested food is stored and consumers steal each other's caches. Here we explore the coexistence mechanisms involved by analyzing how consumer phenotypes and presence of a noncaching consumer affect the competitive outcome. Without cache exchange, the winning consumer is better at harvesting seeds and produces more offspring per gram of stored food. With cache exchange, coexistence is promoted by interspecific trade-offs between harvest ability, metabolic efficiency, and ability to pilfer defended caches of heterospecifics or scavenge undefended caches of dead conspecifics or heterospecifics. Cache exchange via pilferage can equalize competitor fitnesses but has little stabilizing effect and leads to stable coexistence only in the presence of a noncaching consumer. In contrast, scavenging is both equalizing and stabilizing and promotes coexistence without a third consumer. Because body size affects a heteromyid rodent's metabolic rate, seed harvest rate, caching strategy, and ability to steal caches, interspecific differences in body size should produce the trade-offs necessary for coexistence. The observation that coexisting heteromyids differ in body size therefore indicates that cache exchange may promote diversity in heteromyid communities.

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