Abundance, Condition and Size of a Foundation Species Vary with Altered Soil Conditions, Remnant Type and Potential Competitors

Abstract

Native biodiversity often depends on remnant vegetation for survival in agricultural landscapes. However, the size and shape of remnant patches can affect their conservation values through edge effects and isolation of ecologically important species such as foundation species. Foundation species interact with and profoundly affect other species, leading to cascading community effects if they are lost from an ecosystem. We used spinifex grass (Triodia scariosa) to investigate the patterns, processes and consequences of remnant shape and habitat degradation, including nutrient enrichment and soil compaction, for a foundation plant species in agricultural landscapes. First, we quantified differences in biotic and soil variables between narrow linear remnants and larger, more equilateral nature reserves. We then assessed whether these differences explained variation in spinifex abundance and condition. Finally, we tested whether edge effects influenced spinifex abundance. We surveyed 4230 quadrats across 282 sites in south-eastern Australia and measured the size and condition of 11,514 spinifex clumps. Linear remnants were characterised by different plant communities, elevated nutrient levels, more compact soil and higher abundance of non-spinifex grasses compared to nature reserves. Spinifex was twice as abundant in reserves than linear remnants and was negatively associated with soil compaction and soil nutrients. Spinifex condition was negatively associated with soil nitrogen and non-spinifex grasses. Linear remnants had proportionally fewer small spinifex clumps compared to reserves, suggesting lower recruitment in linear remnants. We found minimal evidence of edge effects on environmental variables, and no clear edge effect on spinifex abundance. Given the large contribution of linear remnants to native vegetation cover, measures to minimise nutrient enrichment and disturbance in these areas may help arrest potential ongoing declines in this foundation plant species in agricultural landscapes.