Abstract
Despite the extensive knowledge of pollen limitation in angiosperms, its assessment within tropical forests is still limited. Especially lacking are large scale comparisons of species within this biome – one that is highly diverse but also becoming increasingly threatened. In fact, many tropical plant species depend upon pollinators for reproduction but evaluation of the impact of this dependence via different levels of pollination specialization has yet to be made at the biome scale. We assessed the occurrence and magnitude of pollen limitation for species in the Brazilian Atlantic forest and tested the association of pollination specialization, breeding system, and life habit with pollination efficiency. We compiled data from studies published between 1985 and 2012. We calculated species' effect size (d) from data on fruit set after hand cross-pollination and natural pollination and conducted standard and phylogenetically independent meta-analysis. Overall pollen limitation was moderate, with magnitude of 0.50, and 95% confidence interval [0.37, 0.62] for 126 species. Pollen limitation was observed in 39% of species. Pollination specialization was the factor that best explained the occurrence of pollen limitation. Specifically, phenotypic and ecological specialists (plants with zygomorphic flowers and pollinated by one species of pollinator, respectively) had higher pollen limitation than generalist plants (actinomorphic flowers and pollination by two or more species). Functional generalists (plants pollinated by three or more functional groups) were not pollen limited. On the other hand, breeding system and life habit were not associated to pollen limitation. Pollen limitation was observed in the Atlantic forest and its magnitude was comparable to that for angiosperms as a whole. The finding that pollination specialization was the strongest predictor of pollen limitation suggests that specialist plants in this biome may be most prone to the reproductive failure as a result of pollinator loss.
Highlights
Several synthetic and quantitative reviews have assessed the frequency and degree to which flowering plant reproductive performance is reduced by inadequate receipt of pollen on the stigma, i.e., pollen limitation of reproduction (PL) [1,2,3,4]
This may be due to the fact that more tropical species are animal-pollinated than temperate species (94% versus 78% [5]), or that plants in tropical forests tend to be outcrossing and have low density of adults – both phenomena that may place a premium on efficient cross-pollination to achieve maximal reproductive success
We used fruit set as the response variable because it is the most frequently reported measure in the pollination experiments located by our search and fruit set can be a proxy of PL [36,37], but note that it comes with caveats
Summary
Several synthetic and quantitative reviews have assessed the frequency and degree to which flowering plant reproductive performance (seed or fruit set) is reduced by inadequate receipt of pollen on the stigma, i.e., pollen limitation of reproduction (PL) [1,2,3,4]. Drawing on large data sets of angiosperms (e.g., 306 species [4]), these studies have established that PL is widespread While these syntheses have included a broad range of biomes (e.g., forests, grasslands, deserts), only a small percentage (15% of studies [3]) of the studies reviewed were conducted on species in the tropics. One review suggests that tropical species may be more prone to PL than temperate ones (e.g., the subset of selfincompatible species [2]). Will be informative as the putative causal traits can be assessed in the context of the shared evolutionary history of the tropical flora, and their ecological interactions with fauna
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