Invertebrates of an urban old growth forest are different from forest restoration and garden communities

Areas of indigenous forest in urban and rural areas are often the last remaining examples of lowland ecosystems that were once extensive before human settlement. Conserving the indigenous invertebrate species in these remnants requires knowledge of how many taxa are functionally isolated and how many are capable of dispersing to, and persisting in, forest restoration sites and the surrounding matrix. Invertebrate communities (Coleoptera, Diptera and Lepidoptera) in Riccarton Bush, a 7.8 ha old growth forest remnant New Zealand were compared with suburban gardens and Wigram Retention Basin, a 10-year old 2 ha forest restoration site in Christchurch, New Zealand. Insects were collected with Malaise traps and all Lepidoptera. Coleoptera, and Diptera: Sciaroidea (fungus gnats) were sorted and identified. We also compared our survey with previous Malaise trap surveys of Christchurch restoration sites (Travis and Styx wetlands). Insect composition in the remnant forest was well differentiated from the other habitats: indigenous taxa detected only in the remnant comprised of 27 species of Coleoptera (60% of total), 12 species of fungus gnats (27% of total), and 22 species of Lepidoptera (21% of total). Species accumulation curves suggested that most species not detected in gardens were truly absent. The restoration habitat we surveyed was more similar in species richness and composition to surrounding gardens than it was to the remnant forest. The three restoration habitat sampling sites only shared five species exclusively with the remnant forest sites, suggesting that either these restoration sites provide unsuitable habitat, or have yet to be reached by the species living in the remnant forest. Previous studies suggest that indigenous invertebrate communities can gradually colonise habitat restoration sites, but the timespans over which this occurs are long and the process poorly understood. Our study highlights the importance of preserving remnants for conserving invertebrate biodiversity and the challenges of mitigating biodiversity loss through forest restoration.