Why some parasites are widespread and abundant while others are local and rare?

Abundances and distributions of species are usually associated. This implies that as a species declines in abundance so does the number of sites it occupies. Conversely, when there is an increase in a species' range size, it is usually followed by an increase in population size (Gaston et al. 2000). This ecological phenomenon, also known as the abundance-occupancy relationship (AOR), is well documented in several species of animals and plants (Gaston et al. 2000) but has been little investigated in parasites. In this issue of Molecular Ecology, Drovetski et al. (2014) investigated the AOR in avian haemosporidians (vector-borne blood parasites) using data from four well-sampled bird communities. In support of the AOR, the research group found that the abundance of parasite cytochrome b lineages (a commonly used proxy for species identification within this group of parasites) was positively linked with the abundance of susceptible avian host species and that the most abundant haemospordian lineages were those with larger ranges. Drovetski et al. (2014) also found evidence for both hypotheses proposed to explain the AOR in parasites: the trade-off hypothesis (TOH) and the niche-breadth hypothesis (NBH). Interestingly, the main predictor of the AOR was the number of susceptible hosts (i.e. number of infected birds) and not the number of host species the parasites were able to exploit.