Introduction

The production of offspring has long been thought to be predominantly limited by the availability of resources rather than mating opportunities, a view based on ‘Bateman's principle’ espoused by Bateman in 1948. Applied to vascular plants, this principle predicts that fruit production is limited by maternal resources rather than pollen transfer. However, in the past 20 years limitation of seed production by pollination has been reported in numerous studies, and reviews suggest that more than 50% of plants studied show increased fruit production following experimental pollen supplementation (Burd 1994). Of course, short-term pollen supplementation studies fail to capture the lifetime success of the whole plant, leading some critics to maintain that lifetime reproductive output remains resource-limited. Nevertheless, population-wide declines in reproductive success, at least in the short term, owing to reduced pollen availability or ineffectual pollination have now been recorded among a range of plant species and geographic locations.

The cause of the declining efficiency in pollination has often been traced to changes in the spatial distribution of plants in the population, which in turn has affected the abundance of pollinators or led to changes in their foraging behaviour. Plant spatial distribution has become increasingly relevant following rapid changes in land use and landscape structure driven by anthropogenic activities. Logging and land clearance for agriculture and development has caused degradation of forest habitats either through the partial removal of economically important species, or by wholesale clearance and effective fragmentation and isolation of remnant forest patches.