The effect of species geographical distribution estimation methods on richness and phylogenetic diversity estimates

Diversity assessments are widely used in various fields of knowledge and rely on good estimates of species distribution. There are several methods available to estimate species distribution and the effect of using them is not clearly understood. In this research, we assess the effect of species distributions derived from four geographical distribution estimation methods on derived species richness and phylogenetic diversity (PD). We used the following four most common approaches to determine species geographical distributions: (1) range-wide occurrences are records of presence from databases and museum collections; (2) marginal occurrences are generally expert drawn distributional maps; (3) species distribution modeling (SDM) combines geographic records and environmental data to predict species occurrence; and (4) a combined approach that constrains the statistical modeling predictions by the marginal occurrence distributions. Using these approaches, we estimated richness and PD and their correspondent geographic hotspots for three different analysis resolutions using non-overlapping square windows of 2 degrees, 4 degrees, and 6 degrees across Australia. We also assessed the differences of the resultant geographical distributions for three different spatial resolutions. We found markedly different hotspots when using range-wide occurrences and statistical modeling approaches. Range-wide occurrences resulted in low values of species richness and PD and statistical modeling resulted in high values. The combined and marginal occurrences approaches both had intermediate values, with the combined approach showing a finer level of detail than the marginal occurrences. There is a tendency for species range sizes between methods to converge with decreasing spatial resolution. Even for a relatively well-sampled group such as the Australian marsupials, the range-wide occurrences approach is likely to underestimate the presence of species. Conversely, because SDMs usually do not account for dispersal abilities or biotic interactions, it is likely that species geographical distributions using this approach are overestimated. Depending on the method used, the resultant estimates of diversity may be completely different. Caution is needed when choosing the method to estimate species distribution.