Pollinator behaviour, not increased resources, boosts seed set on forest edges in a New Zealand Loranthaceous mistletoe

Abstract

Abstract The endemic New Zealand mistletoe Peraxilla tetrapetala (Loranthaceae) has previously been shown to be pollinator limited, and to experience higher fruit set in increasingly fragmented habitats, but the mechanism for higher fruit set on edges has not been studied. Edges could affect plant resources (through higher light) and/or pollinator behaviour. This study measures the degree to which pollination and resource limitation affect female reproductive success along a gradient of increasing exposure to the forest edge. Forty‐two plants in pairs at varying distances from the forest edge were selected from within two Nothofagus forest fragments in the 1999/2000 season; and on one plant in each pair, all ripe undamaged flowers were hand‐pollinated. Light levels and the degree of edge exposure were measured for each plant, and path analysis was used to determine the relationship of these variables to proportion fruit set. In agreement with previous studies, unmanipulated plants were found to be strongly pollen limited, especially in shady locations. However, on plants receiving hand‐pollination, fruit set was uniformly high and was unaffected by edge exposure or light levels. Direct observation of flower visitation rates by birds in the 2000/01 season showed that edge flowers were visited twice as often as interior flowers. These results show that the higher fruit set on edges is due to visitation patterns of pollinators, and not due to greater resource availability on edges. This improved pollination on edges may help to buffer this declining plant against local extinction.