Plant isolation reduces outcross pollen receipt in a partially self-compatible herb

1. Few studies of pollination success in fragmented systems measure stigmatic pollen load, and those that do often find it unrelated to plant or population density, size or isolation. Reduced reproductive output, however, is commonly reported, probably because incompatible pollen is contributing substantially to pollen loads of isolated flowers. 2. We used manipulated floral arrays of a bee-pollinated species (Dianella revoluta) to investigate isolation effects on deposition of outcross pollen, while precluding self-pollen transfer. 3. Outcross pollen receipt declined significantly over short distances up to approximately 50 m but even the most isolated flowers received some pollen grains. In contrast, heterospecific pollen did not decline, indicating that the outcross-pollen decline was not due to reduced pollinator visitation. Increased distance of experimental arrays from a nature reserve did not reduce the probability of pollen receipt. 4. Many flowers were damaged by flower feeding beetles in the genus Arsipoda, which would be likely to substantially reduce the efficiency with which flowers are converted to fruits. The probability of flower damage from these predators was significantly lower in arrays that were more distant from the nature reserve. 5. This study indicates that reduced plant density and increased isolation from a source of outcrossed pollen can lead to a substantial decline in the probability of outcrossing, even when pollinator visitation is maintained at a high level. Depending on the mating system, this process will lead to reduced seed set or increased inbreeding for plants in fragmented habitats, even when pollinator abundance and behaviour are unaffected.