Habitat determinants of the taxonomic and functional diversity of Hymenoptera in native forest

Abstract

Habitat loss and fragmentation are two of the most pressing threats facing biodiversity. Yet to conserve insect diversity, habitat conservation is often the only option. For this to be successful, knowledge of how species interact with their environment and utilize resources or structural components of habitat is necessary. Globally, Hymenoptera (e.g. ants, bees, social wasps, parasitoids) are critical for ecosystem functioning. Nonetheless, they remain underrepresented in conservation planning. This thesis studied Hymenoptera communities within native forest, in the Waitakere Ranges, New Zealand, specifically communities of parasitoids (superfamily Ichneumonoidea), and spider predators (Pompilidae). Spatial and temporal patterns in taxonomic and functional diversity and community composition, and how these both related to habitat structure, were analysed. Parasitoid communities are highly diverse and temporally variable. Five key aspects of habitat structure important to parasitoids were identified. These were the vegetation community, successional stage, landscape heterogeneity, plant diversity, and coarse woody debris. Taxonomic and functional diversity were incongruent but yielded complementary information on parasitoid habitat resource requirements and preferences. Community level–habitat relationships, species-specific habitat responses, and co-occurrence were also analysed for Pompilidae. In addition, phenotypic plasticity in body size and forewing melanism was also examined, to determine how phenotypes vary in different environmental conditions. Coarse woody debris and plant diversity were the most important variables for structuring the diversity and community composition of pompilids. Pompilidae show random co-occurrence patterns, for which niche-based arguments, related to nesting and searching behaviour, provide rationale. As body size decreases, forewing melanism increases. This indicates that forewing melanism may be necessary to overcome thermoregulatory deficits associated with small body size. Correlations between forewing melanism and environmental factors did not yield consistent pattern, and melanism fluctuates in response to local environmental conditions differently in each species. This is the first community ecology study on Hymenoptera from the Waitakere Ranges, and one of only a few from New Zealand. Thus, it provides essential baseline data to compare future collections. In sum, the preservation of habitat mosaics, with adequate resource availability, is needed to provide adequate Hymenoptera conservation in New Zealand’s native forests.