Understory Arthropod Diversity in a Mixed Dryland Ecosystem, Hawai'i

Tropical drylands are among the most threatened ecosystems worldwide, and fewer than 10% of tropical dry forests in Hawai'i remain intact. Terrestrial arthropods comprise most of the endemic fauna on many islands, including Hawai'i, where they provide ecosystem services such as pollination, decomposition, and a reliable prey base for predators. The objective of our study was to measure understory arthropod diversity and relative abundances in two common habitat types within dryland Hawai'i - a non-native grassland, dominated by Cenchrus setaceus (fountain grass) with few native shrubs, and a native woodland, dominated by Metrosideros polymorpha ('ōhi'a). We used generalized additive models to examine patterns of understory arthropod dynamics measured monthly or bimonthly in relation to the two habitat types and local climatic variables. We found that temperature and habitat type were significant predictors of the abundance of certain arthropod families and orders. A greater abundance of spiders was in the woodland than grassland habitat, and more beetles and ants (particularly the Argentine ant, Linepithema humile) were in the grassland than woodland habitat. Temperature significantly predicted overall family-level richness and diversity of arthropods as well as Araneae, Heteroptera, Hymenoptera, and Lepidoptera abundance. Precipitation significantly predicted Heteroptera abundance. Significant positive associations were found among some arthropod groups, including several arachnid groups and families of Hemiptera. Documentation of insect and other arthropod community dynamics improves our understanding of ecological community function, supporting management of island communities comprised of both native and non-native species.