Pollen limitation in hummingbird-pollinated plants

Pollen limitation (PL) occurs when plant reproductive success is reduced because flowers receive less pollen and/or lower quality pollen than is needed for full seed or fruit set. While pollen quantity limitation has been characterized as the consequence of plants receiving too few pollen grains to fertilize all of their ovules, quality limitation depends on the difference in survival of embryos sired by naturally delivered pollen vs. by pollen of maximal quality. Estimates of PL have been made for many angiosperm species, but most studies have measured PL in one or a few species. Few studies have assessed PL for numerous species within a single community.

The tropics are a particularly important location for community-based studies of PL because tropical species are hypothesized to be at risk of PL and the tropics support high biodiversity and pronounced levels of endemic species. PL tends to increase with species richness. Greater PL in high diversity sites is proposed to result from greater interspecific competition for pollinators, or greater heterospecific pollen transfer. Endemic species may be particularly at risk of PL because of smaller population sizes, reduced density and/or stronger habitat specificity than more widespread species. Despite these implications, only a small percentage (15 %) of the studies of PL conducted to date have been performed on tropical species, although tropical species represent more than half of animal-pollinated plants.

Several plant and pollinator traits make hummingbird-pollinated plants unique. Not only do bird-pollinated species have floral features associated with bird perception and foraging (e.g. diurnal anthesis, bright colours, lack of perceptible floral scent, nectar as reward) or that restrict visitor access (e.g. tubular narrow corollas in hummingbird-flowers), but they are also significant components of Neotropical forest communities. A new paper in Annals of Botany examines 21 species of hummingbird-pollinated plants in a tropical montane rain forest and finds that just over half the species show PL for at least one response variable related to either quantity or quality aspects of reproduction. However, common predictors of PL, phylogenetic relatedness, self-incompatibility, autogamy, plant density and pollinator specialization level, do not adequately explain variation in PL within this community. This result reinforces recent studies that highlight the need to use other measures for PL for single-species or community-wide studies, especially those that that do not confound post-pollination effects and can separate quantity and quality components.