Latitudinal gradients in individual specialization
The increase in the number of species with decreasing latitude is a striking pattern of global biodiversity. An important feature of the study on this pattern up to now is the focus on species as the fundamental unity of interest, neglecting potential within-species ecological diversity. Here, we took a new perspective on this topic by testing for the existence of a latitudinal gradient of within-species niche diversification. We reviewed the literature in search of case studies in which the variation in the niche attributes (prey taxa consumed, foraging behaviour, and microhabitat use) of individuals belonging to the same population (individual specialization) was quantified. We used ordinal linear regression to test the relationship between individual specialization and latitude.
We found 117 populations belonging to 62 animal species spanning a latitudinal gradient from 54o S to 63o N. We found a negative relationship between the degree of individual specialization and latitude, indicating that lower latitude populations contain more ecologically diverse assemblages of individuals. Our results add a new level of complexity to our understanding of global patterns of biodiversity, indicating that not only are there more species at lower latitudes, but also individuals within their populations are more ecologically diverse. Within-population niche variation creates frequency dependence conducive to disruptive selection and sympatric speciation. Our results, therefore, suggest a new mechanism underlying higher diversification rates and therefore higher species diversity in the tropics.