Adjacent land-use intensification facilitates plant invasions into indigenous shrubland fragments

Indigenous vegetation fragments in agricultural landscapes are vulnerable to creeping edge effects and stochastic extinctions on top of the effects of historic land use and disturbance which have already resulted in significant changes to baselines. Agricultural intensification can potentially increase these threats through spillover of nutrients, water, and weeds, especially in dryland ecosystems which are naturally low in nitrogen and soil moisture. We use plot-based vegetation data and soil measurements of stable isotopes of nitrogen to test whether adjacent agricultural intensification increases plant invasions into dryland shrubland fragments in Canterbury, New Zealand. Nitrogen spillover was only associated with edges adjacent to intensive agriculture. Animal effluent was the most likely source. Edges adjacent to intensive agriculture had higher dominance by exotic species, higher exotic graminoid cover, and depressed native bryophyte cover immediately adjacent to the agricultural boundary. Changes in exotic cover were due to weedy species that dominate in areas of high disturbance and nutrients rather than pasture species moving over the fenced boundary. Spillover created more abrupt environmental and vegetation gradients at the edge but didn’t change the extent of the edge, which typically transitioned to the fragment core at about 40–50 m from the fragment boundary. Hence, the core vegetation remained little affected by adjacent intensification. Spatial offsets to manage fertiliser and irrigation spillover will help prevent further degradation of edge communities adjacent to intensive agriculture. However, the longer term threat to the ecological integrity of the core area of these spatially isolated fragments is likely to be random extinction and vegetation succession. The loss of spatial linkages between vegetation patches and the mosaic of vegetation at different developmental stages means that many of the species that once made up the regional species pool will be lost from this landscape without intervention.