Austral Ecology (2003) 28, 227–236 Patterns of invertebrate biodiversity across a natural edge J. MARK DANGERFIELD,* ANTHONY J. PIK, DAVID BRITTON, ANDREW HOLMES, MICHAEL GILLINGS, IAN OLIVER, DAVID BRISCOE AND ANDREW J. BEATTIE Key Centre for Biodiversity and Bioresources, Macquarie University, North Ryde, Sydney, NSW 2109, Australia (Email: mdangerf@rna.bio.mq.edu.au) Abstract Most ecologists are comfortable with the notion of habitats as recognizable entities and also with situations where the junction between two adjacent habitats forms a discrete edge. Such edges form naturally because of sharp changes in important edaphic, geomorphological, climatic or chemical properties to which plants, in particular, respond. Less clear is the effect of such edges on assemblages of mobile organisms, especially invertebrates that operate at relatively small spatial scales. The objective of the present study was to sample invertebrate composition across a natural edge between a well-developed riparian habitat on fluvial sands and a saltbush habitat developed on a stony gibber plain in a semi-arid region of New South Wales, Australia. A total of 150 pitfall traps on five 1-km-long transects that straddled the edge produced more than 13 000 adult specimens from 21 ordinal invertebrate taxa. A total of 10 446 beetle, ant, wasp, fly and springtail specimens were further sorted into 426 morphospecies. Comparisons and estimates of trends in abundance and richness were made, along with computation of multivariate dissimilarity and permutation statistics, to determine if the land system edge was coincident with changes in invertebrate abundance and composition. These analyses were unable to detect disjunctions in diversity coincident with the edge. The data suggest that many taxa are either present consistently in both habitats or are mostly found in one habitat but ‘leak’ several hundred metres across into the other. Few taxa were unique to either habitat. The result is that assemblage composition for invertebrates changes gradually over distances of up to 400 m either side of the edge and that the distance to a recognizable change in composition is taxon dependent. Even sharp habitat edges, as defined by discrete changes in soils and plants, are not edges but broad transition zones for many invertebrate taxa. There are several implications of these results, especially for landscape ecology. Key words: Australia, boundary, ecotone, invertebrate assemblage, morphospecies, semi-arid, spatial scale. INTRODUCTION No two samples of biological diversity are the same. The composition of biological entities is simply too variable in space to make repetition likely. It is also very difficult to obtain sampling precision because most collection methods have inherent variation and bias in their application or cover only a small proportion of a habitat or an organismal assemblage. Despite this, both empirical and theoretical approaches have emerged to describe and predict patterns of biodiversity (Gaston et al. 1995). Some researchers, notably those who study plants and vertebrates, have found that determin- ants of environmental space are reasonable predictors of the occurrence of particular species (Busby 1991) or structural types (Mackey 1993). In this sense the environment might represent a sufficient template to define the local composition of organisms. Most agree, however, that if assemblages or communities exist then they are, to a greater or lesser extent, brought together and maintained by biological interactions that are either self-reinforcing (Perry 1995) or hierarchical (Allen & Starr 1982). Similarly, a discontinuity in abundance of organisms may not be an inherent property of the landscape, but may emerge only from the interplay of species interactions with landscape structure (With & Cirst 1995). The debate over the legitimacy of these emergent constructs has been long and fraught (Palmer & White 1994) and dominated by empirical examples from plant ecology (Hoagland & Collins 1997). Another conceptual approach to understanding bio- diversity has been to consider ecosystems as a series of patches that vary in size and distribution through the landscape (Farnsworth & Ellison 1996). Organisms respond to this heterogeneity in numerous ways and across many scales (Gosz 1993). Similarly, no two organismal groupings would be expected to respond to a significant environmental disjunction in the same way and therefore discontinuity in species abundance should reflect breaks in the grain of this environmental heterogeneity (Shipley & Keddy 1987; Hoagland & Collins 1997). Breaks in the physical environment are often obvious. Changes in slope, soil or drainage may pro- duce very distinct vegetation patterns with sharp edges. *Corresponding author. Accepted for publication August 2002.