SKEWED SEX RATIOS, POLLEN LIMITATION, AND REPRODUCTIVE FAILURE IN THE DIOECIOUS SEAGRASS PHYLLOSPADIX.

Abstract

Dioecious mating systems in angiosperms, in which individuals are unisexual, require that pollen disperses between male and female individuals and may cause dioecious species to be vulnerable to reduced reproductive success via pollen limitation. This potential cost has rarely been documented, in part because of a dearth of studies investigating abiotic pollen dispersal, which tends to be associated with dioecy. I document widespread variation in reproductive success in an ecologically important, habitat-forming group, the seagrasses (marine angiosperms), using two species of the dioecious genus Phyllospadix. At multiple sites in the state of Washington, USA, I demonstrate pervasive male rarity (all sites <24% male by area). Male abundance is shown to predict female reproductive success at two scales. Within sites, the ratio of maturing seeds to total ovules declines rapidly with increasing distance to the nearest male, while among sites, mean seed : ovule ratios scale positively with male abundance. At some sites, less than 1 in 100 ovules mature into seeds. A field experiment conducted at a high pollen availability site shows that manipulating pollen availability could produce the range of seed sets observed across sites, but pollen limitation was not definitively demonstrated. Overall my results are consistent with pollen limitation in Phyllospadix and comprise the first strong evidence of this phenomenon in seagrass populations. In addition, seed production rates predicted local seedling recruitment, demonstrating a measurable demographic consequence of low pollen availability.