Absence of distance decay in the similarity of plots at small extent in an urban brownfield

Similarity in species composition among different areas plays an essential task in biodiversity management and conservation since it allows the identification of those environmental gradients that functionally operate in determining variation in species composition across spatial scale. The decay of compositional similarity with increasing spatial or environmental distance derives from: 1) the presence of spatial constraints which create a physical separation among habitats, or 2) the decrease in environmental similarity with increasing distance. Even if the distance decay of compositional similarity represents a well known pattern characterising all types of biological communities, few attempts were made to examine this pattern at small spatial scales with respect to both grain and extent. Aim of this work was to test whether the distance decay of similarity 1) can be observed at a local scale in situations where environmental conditions are relatively homogeneous and ecological barriers are absent, and 2) is dependent on the grain size at which plant community data are recorded. We selected two urban brownfields located at Bremen university campus, Germany, of 40 m × 20 m each, systematically divided in nested plots with an increasing spatial scale of 0.25 m2, 1 m2, 4 m2 and 16 m2. Both plant species composition and soil variables were recorded in each cell. Linear and logarithmic least squares regression models were applied in order to examine the decay of similarity due to spatial distance (calculated as the Euclidean distance among pairs of plots) and environmental distance (calculated as the Euclidean distance among PCA-transformed soil variables). A general lack of distance decay was observed, irrespective of the type of distance (spatial or environmental) or the grain size. We argue that this is probably due to a random variation both of the important environmental parameters and of the local distribution patterns of individual species, the latter mainly caused by the high dispersal abilities of the majority of species occurring in the brownfields.