Using scale–area curves to quantify the distribution, abundance and range expansion potential of an invasive species

Aim Invasive species distribution and abundance data are essential for management decisions on mitigating impacts but is seldom available. Here, we use scale-area curves to assess the distribution, abundance and consequent management implications of an invasive plant (Acacia longifolia) within selected occupancy grid cells, spread across regional ranges and representing the full national extent. We determine whether occupancy patterns are explained by climatic suitability or range structure and identify areas where A. longifolia can still be regarded as an important invasive based on contiguous occupancy. Location South Africa including the Fynbos, Thicket, Savanna and Grassland biomes. Methods The quarter degree occupancy of A. longifolia was used to select core, edge, and climatically unsuitable grid cells within different regions of the national range. Cells were surveyed across a linear resolution from 25 km to 2.5 m allowing the first multi-scales description of an invasive species' space-filling properties. Patterns from grid cells in turn were viewed regionally to describe regional variation in spatial structure. Results In regions with contiguous areas of favourable habitat, scale-area curves indicated greater occupancy in core than edge areas, whereas patterns were reversed when suitable areas were more fragmented. Also, at times climatically suitable areas were unoccupied, while unsuitable areas were occupied. Within cells, occupancy was well explained by the presence of fynbos vegetation types, while nationally, contiguous occupancy was almost exclusive to the Fynbos Biome. Main conclusions Scale-area curves can advance the understanding of biological invasions and invasive plant distributions. Here, we detected potential areas of invasive concern, plus differences in abundance and distribution patterns, and associated correlates, at landscape and national scales. As there was no general relationship between range position or climatic suitability and A. longifolia's spatial structure, we propose habitat suitability as an alternative explanation which, in turn, suggests limited range expansion potential.