Abstract
There is growing concern that current pollinator decline will affect the reproduction of plant species, potentially driving a decline in plant population densities. We experimentally tested whether a reduction in flower visitation caused a reduction in fertilization rate in several species, and whether any reduction in fecundity of species depends on their degree of reproductive dependence on pollinators and their attractiveness for pollinators. We recorded visitation rate, fertilization rate, seed weight, flower size and density of nineteen insect-pollinated perennial herbs inside thirty 2 x 2 m dome-shaped cages covered with fishnet (experimental plots) and in thirty control plots in a Norwegian hay meadow. We used a bagging experiment to estimate the ability of the study species to produce seeds in the absence of pollinators. The visitation rate for fifteen of nineteen study species was lower inside cages than outside and only three of the fifteen species showed significantly reduced fertilization rates in the experimental plots. The magnitude of reduction in fertilization rate was positively related to the degree of pollinator dependence, but not to their attractiveness for pollinators or to the reduction in visitation rate. Seed weight was not affected by the experiment. The lack of an overall effect of reduced pollinator visitation on fertilization rate suggests that some species may be robust to a pollinator decline that could increase pollen limitation on plant reproduction. Our results suggest that species with greater pollinator dependence are more vulnerable to pollinator loss.
Highlights
The majority of plant species are dependent on animal pollination (Burd 1994; Dauber et al 2010) and many of them experience pollen limitation on their reproductive success (Knight et al 2005)
From literature we know that most of the fifteen species, which we studied in our study site, are probably able to produce seeds to a greater or lesser degree without pollinator visitation, with the exception of Centaurea jacea, Primula veris, Trifolium pratense and most likely Lathyrus linifolius (Table 1)
The overall visitation rate was significantly lower in the experimental plot compared to the control plots (Table 3) for Centaurea jacea (χ21 = 13.7, P = 0.000), Hieracium cymosum (χ21 = 4.0, P = 0.045), Knautia arvensis (χ21 = 15.9, P = 0.000) and for Leucanthemum vulgare (χ21 = 14.8, P = 0.000)
Summary
The majority of plant species are dependent on animal pollination (Burd 1994; Dauber et al 2010) and many of them experience pollen limitation on their reproductive success (Knight et al 2005). Pollinator decline is caused by many factors, such as degradation of natural and semi-natural habitats, climate change, and changes in plant and pollinator distributions (Aguilar et al 2006; Hegland et al 2009; Kearns et al 1998; Knight et al 2005). These threats may all disrupt plant-pollinator interactions, leading to pollinator and pollen limitation, with potential large effects on plant population dynamics and plant community composition (Ashman et al 2004). There is a lack of experimental studies of the effect of community-wide reductions in pollinator visitation rate and their effects on plant fecundity through pollen limitation (but see Lázaro et al submitted)
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