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

Reproductive success of a plant species can be affected by the distribution pattern of its conspecifics in a small population. Besides the low mate availability, the dynamics of breeding system and pollination mechanism may also contribute to low fruit-set in such populations. We examined the relative contribution of these reproductive attributes on fruit-set across the contrasting distribution pattern (denser vs. sparser plots) in two isolated natural populations of a near-threatened tree species, Anogeissus sericea var. nummularia. 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 mediated only by the flies. The trees exhibit partial selfing and suffer from strong inbreeding depression at the early life-history stages of the selfed progeny. We recorded significant difference between the denser and sparser 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. Besides the requirement of having proximity among the conspecifics to facilitate pollinator movement, pollen quality also seemed to be a crucial attribute in the reproductive success of the tree species. It is inferred that the mating pattern and fecundity of plants in small and isolated populations are significantly influenced by the extent of sexual incompatibility and magnitude of their dependence on pollinators.

Nectar robbing positively influences the reproductive success of Tecomella undulata (Bignoniaceae).

The net consequence of nectar robbing on reproductive success of plants is usually negative and the positive effect is rarely produced. We evaluated the influence of nectar robbing on the behaviour of pollinators and the reproductive success of Tecomella undulata (Bignoniaceae) in a natural population. Experimental pollinations showed that the trees were strictly self-incompatible. The three types of floral colour morphs of the tree viz. red, orange and yellow, lacked compatibility barriers. The pollinators (Pycnonotus cafer and Pycnonotus leucotis) and the robber (Nectarinia asiatica) showed equal preference for all the morphs, as they visited each morph with nearly equal frequency and flower-handling time. The sunbirds caused up to 60% nectar robbing, mostly (99%) by piercing through the corolla tube. Although nectar is replenished at regular intervals, insufficient amount of nectar compelled the pollinators to visit additional trees in bloom. Data of manual nectar robbing from the entire tree showed that the pollinators covered lower number of flowers per tree (5 flowers/tree) and more trees per bout (7 trees/bout) than the unrobbed ones (19 flowers/tree and 2 trees bout). The robbed trees set a significantly greater amount of fruits than the unrobbed trees. However, the number of seeds in a fruit did not differ significantly. The study shows that plant-pollinator-robber interaction may benefit the self-incompatible plant species under conditions that increases the visits of pollinators among the compatible conspecifics in a population.