Traditionally, heterogeneity levels have been associated with refuge availability and, consequently, with security areas for prey. Hence, there is assumed to be a positive correlation between the spatial distribution of potential victims and habitat heterogeneity levels. Among fish, the magnitude of predator-prey interaction has been associated with their size ratio, which includes body size as a central variable for individual predation risk. To date, the importance of body size and its interaction with spatial heterogeneity in relation to fish spatial distribution has been scarcely considered, being this the main objective of the present study. Based on a semiexperimental design, defined by the level of spatial heterogeneity, censuses in 24 artificial pools were conducted for 2 years at the rocky intertidal zone. This study focused on carnivorous fish assemblages residing off the rocky shore of central Chile. The results indicated a counterintuitive response of fish to spatial heterogeneity, with a higher abundance of small fish in open areas. Apparently, this differential distribution could be related to the size ratio between prey and predator (size-dependent susceptibility to predation), which is a hypothesis supported by decreases in the co-occurrence of fish with increasing size similarity.
The climatic variability hypothesis (CVH) states that species are geographically more widespread at higher latitudes because individuals have a broader range of physiological tolerance or phenotypic flexibility as latitude and climatic variability increase. However, it remains unclear to what extent climatic variability or latitude, acting on the phenotype, account for any observed geographical gradient in mean range size. In this study, we analyzed the physiological flexibility within the CVH framework by using an intraspecific population experimental approach. We tested for a positive relationship between digestive-tract flexibility (i.e., morphology and enzyme activities) and latitude and climatic and natural diet variability in populations of rufous-collared sparrows (Zonotrichia capensis) captured in desert (27°S), Mediterranean (33°S), and cold-temperate (41°S) sites in Chile. In accordance with the CVH, we observed a positive relationship between the magnitude of digestive-tract flexibility and environmental variability but not latitude. The greatest digestive flexibility was observed in birds at middle latitudes, which experience the most environmental variability (a Mediterranean climate), whereas individuals from the most stable climates (desert and cold-temperate) exhibited little or no digestive-tract flexibility in response to experimental diets. Our findings support the idea that latitudinal gradients in geographical ranges may be strongly affected by the action of regional features, which makes it difficult to find general patterns in the distribution of species.
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