The more recent concept of monitoring restoration through ecosystem functioning has received support from scientists, but little practical study. New research by Hanna et al. (2012) studies the effects of invasive wasp species on plant pollination and fruiting in Hawaii.
Metrosideros polymorpha is an endemic flowering tree, historically pollinated by honeycreeper birds and bees. Honeycreepeer populations became threatened by predation from invasive spoecies, habitat degradation, competition and disease, causing population declines and extinctions. Following the decline of honeycreepers, and invasion of wasp species, Vespula pensylvanica, and introduction of the western honey bee, Apis mellifera, the non-native species began feeding on the nectar stores of M. polymorpha. V. pensylvanica is a non-pollinating insect; it reduces nectar availability to other wasps by defending a flower, where it stays, drinking the nectar and chasing away or hunting and eating pollinating bees. This reduces the plants fruiting success, as the wasp reduces the nectar supplies M. polymorpha produces to attract pollinators, and directly repels other insects, preventing pollination. A. mellifera, however, is known to pollinate some plants, though only those whose flower shape enables pollination transfer from the small honey bee.
Hanna et al. experimentally removed V. pensylvanica from selected sites, and recorded the effects on pollinaters and fruiting success. Visitation by pollinating insects increased, likely as predation risk decreased and nectar more available, which led to increased fruiting success. Both A. mellifera and endemic Hylaeus began visiting the flowers more often, but Hanna et al. believe they had differing effects on fruiting success. As A. mellifera visited more frequently, it would appear that they are more responsible for the increased fruiting success than Hylaeus, whose visitation rates increased less. A. mellifera could be acting as replacement pollinators following the drop in honeycreeper numbers, helping to sustain plant fruiting. Though this effect is postive for M. polymorpha, an increase in A. mellifera could increase competition with other pollinators, and may not be an effective pollinator of all plant species from which it takes nectar, especially ones which attract a less diverse range of pollinators.
This study emphasises the need to assess how species alter ecosystem functioning when forming management plans, and highlights the variability of impacts of invasive species. Hanna et al. suggest the next steps taken should be to assess wider effects of A. mellifera, and how V. penslyvanica affects pollination at different times of the season.