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

Aims: We implemented a neutral model of a positive relationship between abundance and distribution (occupancy) to examine how spatial structure influences abundance-occupancy relationships. The spatially explicit neutral model distributes individuals of species randomly and independently of one another in space to produce a positive abundance-occupancy relationship. Using empirical data, we tested whether abundance-occupancy relationships diverged significantly from the theoretical neutral model, and determined whether significant divergences emerged through intraspecific aggregation or over-dispersion of individuals. Location: Field work was conducted in open-forest vegetation of the Black Mountain region in south-eastern Australia. Methods: At eight floristically similar sites in open-forest vegetation, we established a 20 × 20 m census plot and spatially mapped all individuals of each woody species. The abundance and distribution of each species was determined at each site at three spatial scales within the census plot. Observed abundance-occupancy relationships were compared with the spatially explicit neutral model using linear regression techniques. Monte-Carlo methods using a two dimensional Poisson process were then used to classify the spatial structure of species as random, aggregated or over-dispersed. Results: We found consistent evidence among the eight sites for abundance-occupancy relationships to diverge significantly from the neutral model at the three spatial scales within each community. The direction that the slopes of relationships diverged from the neutral model provided consistent evidence that aggregation of individuals within species was responsible for modifying the form of abundance-occupancy associations in this vegetation, a feature most evident with increasing scale. Main conclusions: Aggregation is not a mechanism that causes positive abundance-occupancy relationships. Under the neutral model of a positive abundance-occupancy relationship, aggregation should be viewed as a mechanism which modifies a pre-existing relationship, rather than causing a relationship which would not have otherwise existed. In other words, in the absence of aggregation a positive abundance-occupancy relationship would still exist.