From the matrix to roadsides and beyond: The role of isolated paddock trees as dispersal points for invasion

Abstract

Aim: Vertebrate seed dispersers play an important role in determining the spatial distributions of many plants, yet few studies have examined patterns of dispersal in fragmented rural landscapes. This study investigated the factors that influence the occurrence and density of roadside populations of two vertebrate-dispersed trees (Brachychiton populneus and Schinus molle) in roadside environments. Location: Temperate woodland in southern NSW, Australia. Methods: Generalized linear models (GLMs) were used to investigate the factors that influence the occurrence and density of the study species in roadsides. Akaike's information criterion (AIC) was used to determine the relative importance of biophysical, disturbance and landscape variables in predictive models for the two species. Results: We identified two significant determinants of the occurrence of B. populneus and S. molle populations in roadsides: (1) the presence of large 'perch' trees of non-target species (predominately Eucalyptus spp.) in roadsides and (2) distance to nearest paddock tree of each target species. These factors were important for both species, but combined to form a stronger predictive model for S. molle than for B. populneus. Factors influencing population density were less clear, although populations of both species tended to be denser on narrow roads. Main conclusions: Large perch trees were an important predictor of the presence of roadside populations of both species. These results provide criteria to enable roadside managers to either promote native B. populneus, or prevent incursions of exotic S. molle, as desired. Examination of the dispersal pathways for B. populneus and S. molle has revealed that in fragmented agricultural landscapes, isolated paddock trees are able to disperse to nearby roadside environments via frugivore activity. As a result, road networks may provide 'conduits' for the movement and potential invasion of vertebrate-dispersed plant species. © 2013 John Wiley & Sons Ltd.