The reproductive biology and conservation of the endangered Hawaiian legume, Sesbania tomentosa, with emphasis on its pollination system

The endangered Hawaiian pea Sesbania tomentosa is pollinated by endemic bees of the genus Hylaeus. This mutualism was studied at Ka'ena Point, Oahu and observed at other locations throughout the Hawaiian Islands. While S. tomentosa exhibited a low level of autogamy, fruit set was significantly increased in the presence of Hylaeus bees. Sesbania tomentosa is pollen-limited, with significantly more fruit and seed being produced from experimental hand pollinations.;Anthropogenic habitat fragmentation has resulted in the isolation of individuals and small groups of S. tomentosa. These isolated plants had significantly reduced pollinator visitation and fruit-set relative to a large population found at Ka'ena Point. Both seed germination and pollen fertility were high in both the isolated plants as well as those within the large population, suggesting that inbreeding depression was not a factor in the viability of these plants.;Alien honey bees, ants, and paper wasps do not pollinate S. tomentosa , yet they were seasonal consumers of its nectar. Honey bees foraged for nectar during the early spring, when S. tomentosa was at or near its flowering peak, but shifted their foraging to a more abundant native plant when it came into flower. When present at S. tomentosa , honey bees consumed large quantities of nectar, totally depleting many flowers. While ants were present at S. tomentosa throughout the year, they became significant consumers of nectar only when flowering declined (summer). The presence of either of these visitors reduced nectar foraging by Hylaeus, which, in the absence of ants or honey bees, spent significantly more time foraging for nectar and greatly reduced the nectar standing crop. Honey bees appeared to exclude Hylaeus through exploitation competition, depleting nectar to levels where it was not accessible by the native Hylaeus. The lower nectar consumption by ants as well as their aggressiveness toward other foragers, suggest that ants had a greater impact through interference competition. Neither the presence of honey bees or ants appeared to affect pollen foraging by the native Hylaeus. Nectar was only readily available to Hylaeus during winter, when flowers were abundant and before honey bee recruitment.