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

Abstract

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 handpollination, 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.Keywords bird pollination; edge effects; fragmentation; Loranthaceae; mistletoe; Peraxilla; pollinator limitation; resource limitation 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 †Author for correspondence.

Progress in understanding pollination systems in New Zealand

“…In New Zealand, strong pollen limitation has been found in populations of several bird-pollinated species Kelly et al 2000Kelly et al , 2004Montgomery et al 2001Montgomery et al ,2003Molloy 2004). Recent work has attempted to identify the ecological correlates of pollen limitation within and between populations in these species.…”

“…Hand-pollinations confirm that interior plants are limited by pollen supply and not by the resources needed to mature the fruit. Moreover, observations of the plants showed that visitation rates to plants by birds are greater on edge plants than in interior plants (Montgomery et al 2003). It appears, therefore, that these plants may be tolerant of some degree of forest fragmentation and take advantage of the increased edginess that it creates.…”

Mistletoes: Pathology, Systematics, Ecology, and Management

“…1A and B). Elaborate pollination and seed dispersal mechanisms involving birds have evolved in some of these loranth species (69,79,104,109,121,144,176). For example, birds pry open the fused corollas to reach their nectar reward, upon which the pollen "explodes" onto the bird's head (109,121).…”

Is the pollen‐limited mistletoe Peraxilla tetrapetala (Loranthaceae) also seed limited?

“…Unless buds are opened by animals, the flower has a very low probability of producing a seed. Consequently, P. tetrapetala fruit set is frequently pollen limited; this has been shown for Craigieburn for 1993(Robertson et al 1999Kelly et al 2004), but for only 2 years in the mid 1990s at Ohau (Robertson et al 1999;Montgomery et al 2003;Robertson et al 2005). Therefore, we quantified pollen limitation at Ohau and Craigieburn during the course of the seed-limitation experiment, as it is important to measure both at the same time (Ashman et al 2004).…”