Understanding the processes and mechanisms that determine the species richness (how many species) and relative species abundance (number of individuals of each species) of an assemblage or community of organisms has long been a central aim of ecological research (May 1975; Pielou 1975; Tokeshi 1990; Williamson & Gaston 2005; Mcgill et al. 2007). Species abundance distributions (SADs) represent a detailed summary of a community, incorporating information on both species richness and abundance. In general, SADs show that a few species are very abundant and the majority of species are less common or rare (Mcgill et al. 2007). The universality of this observation across taxa and habitats suggests that it is one of ecology’s true universal laws (Mcgill et al. 2007). However, the universality of SADs has been inferred predominantly from studies on conspicuous macroorganisms and has rarely been examined in microbial communities and it is not certain whether microbial taxa follow this ‘universal’ pattern.
Arbuscular mycorrhizal (AM) fungi (Phylum: Glomeromycota) are a widespread and functionally important group of soil microbes with poorly resolved community ecology (Rosendahl 2008). AM fungi are obligate plant-root endosymbionts that colonize approximately two-thirds of terrestrial plant species, acquire all their carbon from the host plant and deliver to the plant a range of benefits, notably increased phosphorus uptake; they thus have profound effects on plant community dynamics and diversity (Fitter 2005; Rosendahl 2008). However, despite the importance of AM fungi within terrestrial ecosystems, we do not know whether AM fungal communities have a similar structure to that observed in higher taxa or whether similar processes are responsible for creating that structure. This lack of data on community-level processes affecting AM fungi reflects the difficulties of studying these organisms. For example, AM fungal species cannot be identified morphologically in roots beyond the genus level (Merryweather & Fitter 1998) and most isolates from natural habitats are not amenable to culture (Helgason et al. 2002). Modern DNA-based techniques have now allowed the quantification of AM fungal communities from a range of natural habitats (for a review see Opik et al. 2006). However, most studies have simply catalogued species from different habitats, and little attempt has been made to ascertain whether general patterns of community structure exist in AM fungal communities.
In this study, we examine SADs in AM fungal communities in order to investigate whether a common process structures natural AM fungal communities. First, we characterized a natural AM fungal community from a single locality and fitted the observed SADs to three commonly used theoretical models; the broken-stick, the lognormal and the geometric series. Secondly, using 32 data sets collected from 25 published studies, we examined whether the observed fit of AM fungal SADs to theoretical models is common across studies and habitats. Finally, we examined patterns of dominance in AM fungal communities to determine whether the dominant taxa in each data set are widespread generalists or locally abundant specialists.