Pollen limitation causes an Allee effect in a wind pollinated invasive grass.
Davis, Heather1, Taylor, Caz2, Civille, Janie1, Lambrinos, John*,1, Strong, Donald1, 1 University of California, Davis, CA, USA2 Simon Fraser University, Vancouver, BC, Canada
ABSTRACT- Pollen availability is usually assumed not to limit reproduction in wind pollinated plants. However, reproductive failure or depression is known to affect the rate of spatial spread of exotic species. Pollen limitation in wind pollinated invaders could explain why some introductions fail to persist or why there is a lagtime between introduction and rapid population growth. We performed a manipulative pollen addition and exclusion study to investigate the role of pollen limitation in an invasive perennial estuarine grass, Spartina alterniflora. We found an eight fold reduction in seed set caused by pollen impoverishment at the low density leading edge of an invasion compared to high density plants. Additionally, we found nine fold more pollen on stigmas of high density plants than on low density plants. Pollen deposition rates on stigmas and pollen traps along a windward to leeward gradient demonstrated that pollen loads did not increase downwind of low density plants, but did increase downwind of high density plants and dropped off precipitously across a gap that lacked pollen donors. Furthermore, we found the pollen loads on stigmas was determined by pollen availability in the air. The delay of appreciable numbers of seed persists for decades until vegetative growth coalesces plants into continuous meadows. Spatially explicit stochastic simulation and spatially implicit deterministic modeling structured by density classes has shown that this weak Allee effect slows the rate of spread of this invasion.
Key words: invasive species, pollen limitation, Allee effect, Spartina alterniflora
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