The effect of local plant density on pollinator behavior and the breeding system of Persoonia bargoensis (Proteaceae)

Local plant density can vary dramatically within populations and may modify both the quantity and effectiveness of pollinator activity and thus the reproductive output of plants. We tested the effects of local plant density and plant size on pollinator activity on the endangered plant Persoonia bargoensis in two remnant populations in Australia. Pollinator visitation was weakly but positively correlated with local plant density in both populations (R2 ¼ 0:25, P < 0:001; R2 ¼ 0:06, P ¼ 0:024) and with plant size in one population (R2 ¼ 0:24, P < 0:001). Within-plant movement of fluorescent dyes (added to anthers as a pollen mimic) was inversely related to local flower density (R2 ¼ 0:689, P ¼ 0:041). To assess self-compatibility in P. bargoensis, we measured the presence of pollen tubes in flowers that were hand pollinated with self or outcross pollen, which indicated that plants were self-compatible with a weak preference for outcross pollen. The autogamy (mechanical self-pollination) treatment indicated that pollinators are required for pollination. The open treatment showed low and highly variable pollination rates for a rare species (only 20% of 120 had pollen tubes), suggesting that plants receive variable pollinator service. Reliance on pollinators and a preference for outcross pollen implies that the observed pollinator behavior would reduce the reproductive output of P. bargoensis at low local densities.