Species living at latitudes with broad seasonal variations in temperature are expected to achieve larger geographic ranges than species living at latitudes with low seasonality, generating a positive relationship between range size and latitude (Rapoport’s rule). However, this expectation fails to take into account the strong nonlinear distribution of temperature variation across latitudes in both terrestrial and marine environments, and its potential effect on geographic range size. Low latitudes show much narrower spatial variation in mean temperature than do high latitudes, an effect that can counteract the relationship between range size and latitude posited by Rapoport’s rule. Therefore, we model the relationship between range size and latitude so that both (1) temperature seasonality and (2) spatial variation in mean temperature determine species range limits. We test predictions of such a model using marine bivalve species (N=5,653 species) along three shelf-depth transects at 5º spatial resolution (Western Pacific, Eastern Pacific, and Western Atlantic).
Results/Conclusions
Observed distribution patterns of marine bivalve species match the model predictions. First, latitudinal ranges increase towards low latitudes (in contrast to Rapoport’s rule). This is because the habitat area characterized by a given temperature decreases with latitude. Second, species with narrow thermal ranges at lower latitudes attain broader latitudinal ranges than species with broad thermal ranges at higher latitudes. Third, range limits cluster at latitudes with the steepest gradients in temperature. Although tropical species evidently do have narrower temperature tolerances than high-latitude species (as held by Rapoport’s rule and other hypotheses), the nearly constant temperatures over wide areas of the tropics allow latitudinal ranges to be decoupled from thermal ranges. Large areas at low latitudes encompass similar temperatures, creating greater scope for range expansion, and thus helping to generate a tropical diversity peak and a significantly positive diversity-temperature relationship. This research was supported by the National Science Foundation, NASA, and the Slovak Research and Development Agency (APVV- 0644-10).