Geographic Gradients

Linnaeus, upon travelling north from southern Sweden, was struck by the differences between that colder land and his homeland. The differences in the composition of species begged explanation. Ever since (and probably before), naturalists, ecologists, and biogeographers have sought to understand why species and the communities they make up differ from place to place. Why does body size, both within and among species, vary geographically? Where are there more species and why? The ease of posing such questions has attracted generations of biologists – each lured by the possibility of finding elegant answers to superficially simple problems. But to date, consensus explanations remain elusive. And so how should we move forward? Major headway, we suspect, will come not from the search for general explanations of ecological gradients, but instead through the careful study of a few focal taxa and the factors that influence their distribution along gradients. The most obvious focal taxa are likely to be vertebrate groups (e.g. birds and mammals). But because most animal species are insects, having even a single well-studied insect taxon whose ecological gradients are well understood would be disproportionately valuable. To date, the best candidate for such intensive focus is ants. They are relatively easy to sample (see Box 3.1), well known, ecologically important and have been the subject of gradient studies for nearly 50 years. In this chapter, we reviewour current understandingof gradients in antdiversity, body size, range size, and other life-history traits. We examine the underlying causes of these gradients and discuss their consequences for some of the processes ants mediate, such as predation and seed dispersal. We start with the patterns themselves by reviewing latitudinal and elevational gradients in ant diversity at multiple spatial and taxonomic scales. Then, we move on to two other common gradients in ants – gradients in body size and range size. For each gradient, we explore some of the potential underlying mechanisms.