Species diversity estimates vary with sampling effort and scale

Let's imagine that you are sampling your backyard for insect species. It makes sense that the more samples you collect, the more species you are likely to find. However, eventually you reach a point in your sampling effort at which any additional sampling will reveal so few new species that you could stop sampling and still have a good idea of the species richness of your backyard.

FIGURE 16.9 When Are All the Species Sampled? Speciesaccumulationcurvescanhelp us determine when most or all of the species in a community have been observed. In this hypothetical example, the number of new species observed in each sample decreases after about half the individuals in the samples have accumulated. View larger image

Jennifer Hughes and colleagues (2001) used species accumulation curves to ask how communities differ in the relationship between species richness and sampling effort. Are there some very diverse communities in which we are unable to estimate species richness accurately despite intensive sampling? Hughes and colleagues calculated species accumulation curves for five different communities: a temperate forest plant community in Michigan, a tropical bird community in Costa Rica, a tropical moth community in Costa Rica, a bacterial community from the human mouth, and a bacterial community from tropical soils in the eastern Amazon (FIGURE 16.10).

FIGURE 16.10 Communities Differ in Their Species Accumulation Curves Hughesand colleagues found that communities of five different types varied greatly in the sampling effort that would be needed to estimate their species richness. The data sets were standardized by calculating for each data point the proportions of the total number of individuals and species that

had been sampled up to that point.

Based on the graph, which of these communities would require more sampling to adequately estimate their species richness? Which would require very little additional sampling?

(After J. B. Hughes et al. 2001. Appl Environ Microbiol 67: 4399-4406.) View larger image

A comparison of species accumulation curves not only provides valuable insight into the differences in species richness among communities, but also demonstrates the influence of the spatial scale at which sampling is carried out. For example, if we were to sample the richness of bacteria in tropical soils at the same scale at which we sampled Costa Rican moths, the bacterial richness would be immense in comparison. But such comparisons do suggest that our ability to sample all the bacteria in the human mouth is roughly equivalent to our ability to sample all the moth species in a few hundred square kilometers of tropical forest. The work of Hughes et al. also reminds us how little we know about the community structural characteristics of rarely sampled assemblages, such as microbial communities.