Do humans homogenize or differentiate biotas? It depends

A common concern of globalization is that humans are homogenizing the planet. This is captured in such colourful descriptions of our future planet as: the geography of nowhere, planet of weeds, the new Pangea and the Homogecene (reviewed in Olden et al., 2005; Olden, 2006). This notion of a homogenizing biosphere is, however, seductively simplistic. As one begins to examine them in more detail, the effects of human activities on the biosphere are seen to be much more complicated than these metaphorical concepts imply. For example, although biological homogenization (BH) is commonly associated with the introduction of widespread (invasive) species, it is also increased by the extinction of local (endemic) native species (McKinney & Lockwood, 1999; Olden, 2006). Furthermore, the degree of biological homogenization depends on how we measure it. Genetic homogenization is the mixing into native gene pools of exotic genes (Olden & Rooney, 2006). Functional (or ecological) homogenization is the increase in similarity of ecological roles or traits between communities (McKinney & Lockwood, 1999; Olden & Rooney, 2006), such as the increase in generalist species (and the decline in specialist species) that occurs with increasing anthropogenic disturbance (Devictor et al., 2008). BH is most commonly measured in terms of ‘taxonomic homogenization’ (Olden & Rooney, 2006); that is, by the relative gain in the number of shared species between two areas as measured by pairwise similarity indices (e.g. Jaccard’s). This metric can be extended to construct cumulative species–area curves and other methods to examine the loss of beta diversity (La Sorte et al., 2008). Yet another complication is that most studies of taxonomic homogenization have access only to species presence–absence data, whereas a fuller assessment of homogenization would include the abundance of shared and unshared species. Fortunately, the omission of abundance data may not change the basic results in most cases (Cassey et al., 2008). Aside from types of measurement, several other complexities arise in assessing human impacts on reshuffling biological communities. Two of the most important relate to scale, both temporal and spatial (Olden, 2006). Regarding temporal scale, anthropogenic homogenization almost certainly increases with time, as long as human disturbances (e.g. farming, urbanization) continue to intensify (McKinney, 2006). This has been a major obstacle for many homogenization studies, which typically compare existing communities, with little or no data on past (and future) changes in community composition. For example, the omission of data on the number of native species that have gone extinct from recent human activities would probably lead to an underestimation of previous homogenization if extinction is biased towards rare endemic species (probably not shared between the communities compared). Future homogenization will be underestimated where data on currently threatened species, likely to go extinct, are not included. Furthermore, many introduced species are just beginning to spread and will ultimately have a more homogenizing effect than their current distribution indicates. Regarding spatial scale, comparisons of communities separated by large distances show that species introductions consistently have a homogenizing impact (Qian & Ricklefs, 2006). This is because communities separated by large distances share few, if any, native species, so that even a few shared introduced species will increase community similarity. In contrast, communities separated by short distances often show reduced similarity (differentiation) from species introductions, because introduced species are shared to a lesser extent than native species owing to a high spatial autocorrelation of native communities (La Sorte et al., 2008). Fortunately, as interest in homogenization grows, researchers are employing increasingly sophisticated methods to study it. A good example is a recent study by Spear & Chown (2008), who examined the effects of ungulate introductions on biotic similarity across four spatial scales, at three resolutions within South Africa and among 41 nations. In addition to comparing various spatial scales, the study included the effects of temporal factors such as historical native species extirpations and future native species extirpations (based on hypothetical extirpations of currently threatened and endangered species). Furthermore, by examining ungulates, the study benefited by focusing on a group (mammals) that has generally been omitted from the homogenization literature. The large majority of studies thus far have focused on plants, fishes and birds (Olden, 2006). The Spear and Chown study thus provides a unique and exceptionally reliable insight into the effects of our propensity simultaneously to eradicate local species and import species from many different areas. It is therefore very significant that their findings generally reinforce previous findings based on other data sets involving different taxa. Notably, ungulate species introduced (translocated) from less distant localities (other areas of South Africa) tend to play a much larger role in homogenizing local faunas than do species introduced from more distant areas. Indeed, the latter (extraregional) species often have the opposite effect of reducing similarity among South African ungulate assemblages. This pattern of differentiation by extra-regional introductions and homogenization by intraregional introductions has also been found in plants and freshwater fishes (McKinney, 2005). The role of spatial scale is further demonstrated, with perhaps the best analysis of Journal of Biogeography (J. Biogeogr.) (2008) 35, 1960–1961