Expanding islands of speciation

Abstract

empirical scrutiny-one of the more controversial problems in theoretical movement ecology. The authors' data set of more than 10 7 measurements, compiled from electronic tags on 55 individual fish-sharks, tuna, bill-fish and a sunfish-is an order of magnitude larger than the last reported data set 7. They find strong evidence of Lévy flights, but, as predicted theor etically, these flights are not universal. Lévy flights are expected in places where prey is scarce (such as the open ocean), whereas a Brownian strategy is more likely to occur where prey is abundant (as in marine regions where the mixing of water bodies produces high densities of phytoplankton, zoo plankton and organisms higher in the food chain). The observed 2 pattern of switching between search modes is not entirely consistent with these expectations. But it is nonetheless plausible, as seen for instance in the data on a blue shark that moved from the prey-rich waters of the western English Channel to the oceanic environment of the Bay of Biscay. Figure 1 | Comparison of a Lévy flight with a Brownian random walk. a, Walks of identical total length of 1,000 unit steps are shown drawn to scale. Lévy flights (or walks) have ultra-long steps, which are absent from Brownian walks. b, A close-up of the Brownian walk, in which the walker returns many times to previously visited locations (a phenomenon known as 'oversampling'). By contrast, the Lévy walker occasionally takes long jumps to new territory. This reduction in oversampling is part of the theoretical basis for interest in the Lévy-flight foraging hypothesis, which predicts that Lévy flights offer higher search efficiencies in environments where prey is scarce. Humphries et al. 2 show that marine predators often move in patterns that are consistent with this hypothesis. (Plots modified from ref. 10.) The fact that some organisms perform Lévy flights has deep implications that transcend those for marine ecosystems, and it raises many questions. Did humans disperse from Africa superdiffusively rather than diffusively? Does pollen from genetically modified crops spread superdiffusively? What are the con sequences if influenza epidemics spread superdiffusively? In a reaction-diffusion context, superdiffusion leads to significantly increased overall reaction rates, because the reacting species-which may be chemical or biological-meet each other more often. What more can be learned about such interactions? These questions and many more await investigation. ■ Gandhimohan M. Viswanathan is at