Linking abundance, occupancy and spatial structure: an empirical test of aneutral model in an open-forest woody plant community in eastern Australia

Aims We implemented a neutral model of a positive relationship between abun dance and distribution (occupancy) to examine how spatial structure influen ces abundance-occupancy relationships. The spatially explicit neutral model distributes individuals of species randomly and independently of one anoth er in space to produce a positive abundance-occupancy relationship. Using e mpirical data, we tested whether abundance-occupancy relationships diverged significantly from the theoretical neutral model, and determined whether s ignificant divergences emerged through intraspecific aggregation or over-di spersion of individuals. Location Field work was conducted in open-forest vegetation of the Black Mo untain region in south-eastern Australia. Methods At eight floristically similar sites in open-forest vegetation, we established a 20 x 20 m census plot and spatially mapped all individuals of each woody species. The abundance and distribution of each species was det ermined at each site at three spatial scales within the census plot. Observ ed abundance-occupancy relationships were compared with the spatially expli cit neutral model using linear regression techniques. Monte-Carlo methods u sing a two dimensional Poisson process were then used to classify the spati al structure of species as random, aggregated or over-dispersed. Results We found consistent evidence among the eight sites for abundance-oc cupancy relationships to diverge significantly from the neutral model at th e three spatial scales within each community. The direction that the slopes of relationships diverged from the neutral model provided consistent evide nce that aggregation of individuals within species was responsible for modi fying the form of abundance-occupancy associations in this vegetation, a fe ature most evident with increasing scale. Main conclusions Aggregation is not a mechanism that causes positive abunda nce-occupancy relationships, tinder the neutral model of a positive abundan ce-occupancy relationship, aggregation should be viewed as a mechanism whic h modifies a pre-existing relationship, rather than causing a relationship which would not have otherwise existed. In other words, in the absence of a ggregation a positive abundance-occupancy relationship would still exist.