Why is species richness often higher in more densely populated regions?

Over the last two decades, a number of studies in different locations have identified a positive correlation between human population size or density and the species richness of various taxonomic groups across space (e.g. Hunter & Yonzon, 1993; Chown et al., 2003; Gaston & Evans, 2004; Fjeldså & Burgess, 2008). This correlation is surprising given the undoubted negative impact of human development on the persistence of many species. At closer inspection, the spatial congruence between people and species richness is generally weaker when the size of the sampling unit (grain size) is small (Luck, 2007; Pautasso, 2007). For example, strong positive correlations are often reported for studies across broad extents (e.g. continents or countries) using a sampling grain of 0.5–11 grid cells, but these correlations tend to weaken or become negative when grain size is much smaller (e.g. 10km). The latter makes intuitive sense, as increasing urbanization can reduce species richness at local scales (McKinney, 2008), but the broad-scale positive correlation is nevertheless unexpected and warrants greater attention. While some have speculated about what drives this broad-scale correlation, there have been few comprehensive tests of potential drivers (although see, e.g. Hugo & van Rensburg, 2008; Steck & Pautasso, 2008; Luck et al., 2010). Barbosa et al. (2010) make an important contribution to this research field in three ways. First, they use Carabid beetles as their focal taxonomic group. The human population density (HPD)–species richness correlation has generally been poorly studied among invertebrates. Second, they examine relationships at multiple spatial scales (see also Chown et al., 2003). Not surprisingly they record a positive correlation between HPD and Carabid richness across Italian regions (the largest sampling grain size), no correlation across provinces, and a negative correlation across 10 10 km grid cells. Finally, and most importantly, they examine how their results change when accounting for variation in sampling effort across survey sites. As Barbosa et al. (2010) notes, variation in sampling effort has largely been ignored by researchers working on the HPD–species richness correlation, although it could reasonably be described as ‘the elephant in the room’. Indeed, the positive correlation between HPD and species richness could simply be an artefact of greater sampling in areas with more people. Sampling effort is often positively correlated with both HPD and species richness, confounding attempts to identify the existence of potential environmental drivers of the HPD–richness correlation. Others have raised this issue (e.g. Balmford et al., 2001; Luck et al., 2004), but generally not dealt with it in an explicit way. Yet, accounting for variation in sampling effort is a crucial first step in determining if the spatial congruence between human populations and species richness is driven by underlying environmental factors or simply a result of uneven sampling effort. If the former, the conservation implications are substantial. If the latter, we need to proceed more cautiously and attempt to increase sampling effort in areas of low HPD to get a better estimate of true species richness. After accounting for variation in sampling effort and area, Barbosa et al. (2010) find that the positive relationship between Carabid species richness and HPD across regions largely disappears. However, and most surprisingly, they find that the negative correlation between HPD and richness across 10 10 km grid cells reverts to a positive correlation (although the partial r is very small). This result appears to be driven by widespread species, which show a positive association with people, whereas endemic species are negatively correlated with human density. The results of Barbosa et al. (2010) at the largest sampling grain contrast with the findings of Luck et al. (2010), who examined the congruence between HPD and bird species richness across Australia at a grain size of 11 grid cells. Luck et al. (2010) found that the positive correlation between HPD and species richness remained even after controlling for variation in sampling effort. Moreover, they identified net primary productivity as a possible key driver of this spatial congruence, and found that the strength of the