Positive effects of forest edges on plant reproduction: literature review and a case study of bee visitation to flowers of Peraxilla tetrapetala (Loranthaceae)

Positive effects of fragmentation on plant reproduction are uncommon; in a literature review we found significant negative effects on fruit or seed set for 50 plant species, compared to 26 species showing no effect, and only nine affected positively. One of these is the declining New Zealand mistletoe Peraxilla tetrapetala (Loranthaceae), and here we investigate the mechanism of this positive effect. P. tetrapetala requires visits from native bird or bee pollinators to produce fruit. Fruit set was consistently pollen limited at several South Island sites because of a shortage of pollinators, but within a site at Lake Ohau, plants on forest edges had higher fruit set than those in the forest interior. Previous work showed that this difference was not caused by a shortage of resources in interior plants, but was associated with higher bird visitation rates to flowers on edges. In this study, we tested whether native bees also show a preference for edge flowers. At two sites (Ohau and Craigieburn) edge mistletoes had higher visitation by native bees (Hylaeus agilis and Leioproctus sp.) and higher fruit set. Some, but not all, of the higher visitation to edge flowers was explained by a preference amongst bees for flowers in direct sunshine. Therefore, P. tetrapetala experiences higher fruit set on edges because both of its main groups of pollinators (endemic birds and bees) visit edge flowers more often. The other eight published cases of positive effects of fragmentation on fruit set also all reported increased visitation rates by pollinators.