Pollen limitation causes an Allee effect in a wind-pollinated invasive grass ( Spartina alterniflora )

Abstract

It is usually assumed that pollen availability does not limit reproduction in wind-pollinated plants. Little evidence either supporting or contradicting this assumption exists, despite the importance of seed production to population persistence and growth. We investigated the role of pollen limitation in an invasive estuarine grass (Spartina alterniflora), with a manipulative pollen supplementation and exclusion experiment in areas of high population density and at the low-density leading edge of the invasion. We also quantified pollen deposition rates on stigmas and pollen traps along a windward to leeward gradient. We found pollen impoverishment at the low-density leading edge of a large invasion, causing an 8-fold reduction in seed set. We found 9-fold more pollen on stigmas of high-density plants than on those of lowdensity plants. Pollen deposition rates on stigmas and traps 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. The delay of appreciable numbers of seed caused by pollen limitation persists for decades until vegetative growth coalesces plants into continuous meadows, and this Allee effect has slowed the rate of spread of the invasion.

Sex allocation, pollen limitation and masting in whitebark pine

“…Density-dependent pollen limitation has also been shown in several temperate wind-pollinated species, both herbaceous and woody (e.g. Knapp, Goedde & Rice 2001;Davis et al 2004;Hesse & Pannell 2011;Koenig et al 2012), in addition to this current study (Fig. 2a).…”

Mating patterns and rates of biological invasion

“…We need hard evidence that rates of population growth change over time, along with experiments to reveal the mechanism behind that change. In this issue of PNAS, Davis et al (7) demonstrate that a lack of sexual partners results in a lag in the spread of an important weedy grass invading mudflats on the northwest coast of North America.…”

“…Studies of reproductive biology placed in a demographic and ecological context, like that of Davis et al (7), can provide us with insights into the important biological drivers of major landscape change. …”

Pollination ecology of Isoglossa woodii, a long‐lived, synchronously monocarpic herb from coastal forests in South Africa

“…up to 120 years in some bamboos; Janzen 1976), because an entire generational cohort shares a single opportunity to reproduce successfully, and because there is little or no intraindividual variation in flowering intervals. Supra-annual flowering (= temporal aggregation) often also involves spatial aggregation and even monodominance (Kelly & Sork 2002), particularly in wind-pollinated plants where the benefits of cross-pollination may only be realised at short distances (Koenig & Ashley 2003;Davis et al 2004;Ghazoul 2005).…”