Alien Insects Dominate the Plant-Pollinator Network of a Hawaiian Coastal Ecosystem

Little is known regarding pollination webs involving island coastal plants and pollinators, and roles that nonnative flower visitors may play in these interaction networks. Plant-pollinator observations made in March 2008 and 2009 were used to describe the pollination network for Ka'ena Point, one of Hawai'i's best-conserved coastal communities. The network includes 15 native plant species, two native bee species, and 26 nonnative insect taxa, forming 119 interactions. Network connectance is 29.4% and weighted nestedness is 17.9, which are similar to values of other dry-habitat, island networks. The network's structure has a core of generalized pollinators plus several more-specialized pollinators. Nearly all plant species interact with two or more generalist pollinators and a variable number of specialists. Small, nonnative bees (Lasioglossum, Ceratina), wasps (Proconura), and flies (mostly Tachinidae) were responsible for 72.7% of flower visits, and they visit five plant species not visited by native bees. The two native visitors were the bees Hylaeus anthracinus and H. longiceps (both proposed as endangered). Hylaeus spp. (especially females) provided 19.8% of flower visits, foraging at high visitation rates and on many species, including the endangered Scaevola coriacea and Sesbania tomentosa. In Hawai'i's coastal habitat, nonnative insects form novel interactions with native species and may maintain an ecosystem's function following loss of most of the original native pollinators. However, their high visitation rates suggest that the two remaining native Hylaeus species are potentially important pollinators for many of the native plants on which they rely for nectar and pollen resources.