Ecological effects of increasing time since invasion by the exotic African olive (Olea europaea ssp. cuspidata) on leaf-litter invertebrate assemblages.

Invasive African olive, Olea europaea ssp. cuspidata (Wall. ex G.Don) Cif., forms increasingly dense stands between initial and mature stages of invasion, leading to a progressive decline in native plant diversity. Here, we examined the response of leaf-litter invertebrates to increasing time since olive invasion. We compared invertebrate assemblages among early-stage olive (0–7 years since invasion, scattered olive shrubs and seedlings in native woodland), mature olive (>15 years, uniform olive stands dominated by multi-trunked trees) and uninvaded native grassy woodland habitats (both mature stands and edges) in a critically endangered ecological community of south-eastern Australia. Invertebrate species richness was significantly reduced in mature olive compared with early-stage olive and mature native woodland habitats. Species richness did not differ significantly between early-stage olive habitat and mature native woodland, demonstrating resistance in species richness to initial invasion. Invertebrate species composition of native woodlands differed significantly from both mature olive and early-stage olive habitats, demonstrating a lack of resistance in species composition to initial olive invasion. Compositional differences were principally driven by reduced abundances within Coleoptera, Hymenoptera and Polyxenida in mature olive habitat compared with mature native woodland. These changes were significantly correlated with an increase in bare ground, plant canopy cover and litter depth, and higher moisture and lower temperature within leaf litter, in mature olive habitat. Our findings show that negative ecological impacts of invasive African olive extend beyond plants to leaf-litter invertebrate assemblages and that significant impacts on invertebrate species assemblage composition occur early in the invasion process.