Relative contribution of reproductive attributes to density-dependent effects on fruit-set

How much fruit can a plant set? Pollinator dependence and pollinator behaviour play a role, as does plant density.

Reproductive success of biotically pollinated plant species usually declines with population density. However, the dynamics of breeding system and pollination mechanism in a particular demographic set up is also crucial in determining success. In a recent study published in AoB PLANTS, Singh et al. examined the relative contribution of these factors on fruit-set in a threatened tree species Anogeissus sericea var. nummularia.

Insect pollinators of Anogeissus sericea var. nummularia.
Insect pollinators of Anogeissus sericea var. nummularia. Image credit: Singh et al.

Although flowers in the species are of generalist type, the narrow stigmatic surface appears to impose a requirement for a specialist pollinator. Pollination in the tree species is therefore mediated only by the flies. As a result, the trees exhibit partial selfing and suffer from strong inbreeding depression at the early life-history stages of the selfed progeny. There were significant differences between the densely and sparsely populated plots in terms of inflorescence visits per tree, and the number of trees covered in a bout by the pollinators. Moreover, tree density showed a strong positive correlation with fruit-set. The study shows that besides the breeding system of the species, the extent of pollinator dependence and its foraging behaviour significantly contributes to the net outcome of density-dependent effects on fruit-set. What does this mean for conservation management of A. sericea? The results of this study suggest the reintroduction of unrelated individuals would be a crucial consideration along with the planting density.

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