Seasonal change mediates the shift between resource and pollen limitation in Hedysarum scoparium(Fabaceae)

Theory suggests that with sufficient environmental variation, pollen limitation might be observed at some places or times, and resource limitation at others, but there are no empirical data about the effect of seasonal change on the variation of pollen limitation and resource limitation within a flowering season. In this study, we examined pollen and resource limi- tation by comparing fruit set and seed production in natural- and hand-pollinated Hedysarum scoparium flowers in the middle reaches of the Hexi Corridor region, China, in 2010. We also described a role for the first substantial autumn rainfall in mediating a shift between pollen and resource limitation in H. scoparium, but did not analyze this experimentally Our results indicated that H. scoparium was resource limited at peak flowering during the summer, and was pollen limited at peak flowering during the autumn. The seasonal change （summer to autumn） mediated the shift between pollen and resource limitations in H. scoparium. The shift timing depended on the date of the first autumn rainfall in 2010. Changes in the first substantial rainfall in autumn may affect fruiting of H. scoparium, thus affecting population persistence of this species and development/structure of the local ecosystem if such conditions persist.

Effects of nectar-robbing on plant reproduction and evolution

The relationship between plant and pollinator is considered as the mutualism because plant benefits from the pollinator’s transport of male gametes and pollinator benefits from plant’s reward.Nectar robbers are frequently described as cheaters in the plant-pollinator mutualism,because it is assumed that they obtain a reward(nectar)without providing a service(pollination).Nectar robbers are birds,insects,or other flower visitors that remove nectar from flowers through a hole pierced or bitten in the corolla.Nectar robbing repre-sents a complex relationship between animals and plants.Whether plants benefit from the relationship is always a con-troversial issue in earlier studies.This paper is a review of the recent literatures on nectar robbing and attempts to acquire an expanded understanding of the ecological and evolutionary roles that robbers play.Understanding the effects of nectar robbers on the plants that they visited and other flower visi-tors is especially important when one considers the high rates of robbing that a plant population may experience and the high percentage of all flower visitors that nectar robbers make to some species.There are two standpoints in explaining why animals forage on flowers and steal nectar in an illegitimate behavior.One is that animals can only get food in illegitimate way because of the mismatch of the morphologies of animals’mouthparts and floral structure.The other point of view argues that nectar robbing is a relatively more efficient,thus more energy-saving way for animals to get nectar from flowers.This is probably associated with the difficulty of changing attitudes that have been held for a long time.In the case of positive effect,the bodies of nectar robbers frequently touch the sex organs of plants during their visiting to the flowers and causing pollination.The neutral effect,nectar robbers’behavior may destruct the corollas of flowers,but they neither touch the sex organs nor destroy the ovules.Their behavior does not affect the fruit sets or seed sets of the hosting plant.Besides the direct impacts on plants,nectar robbers may also have an indirect effect on the behavior of the legitimate pollinators.Under some circumstances,the change in pollinator behavior could result in improved repro-ductive fitness of plants through increased pollen flow and out-crossing.