Reproduction of Echinacea angustifolia in fragmented prairie is pollen‐limited but not pollinator‐limited

Abstract

Pollen limitation of plant reproduction occurs in many plant species, particularly those in fragmented habitat; however, causes of pollen limitation are often unknown. We investigated the relationship between pollen limitation and pollinator visitation in the purple coneflower, Echinacea angustifolia (Asteraceae), which grows in the extremely fragmented tall grass prairie of North America. Previous investigations showed that pollen limitation of E. angustifolia increases with plant isolation and decreases with population size. We observed insect visitation to E. angustifolia over two flowering seasons and estimated pollen limitation of observed plants, using seed set as a proxy measure in 2004 and persistence of receptive style rows in 2005. We analyzed spatial patterns of bee visitation and pollination at two spatial scales: individual isolation, as measured by the distance to their kth nearest flowering neighbors (k = 1 - 15), and population size. Our results indicate that E. angustifolia is pollinated by over 26 species of native bees, with 70-75% of visits by halictid bees. Surprisingly, in both years, bee visitation increased with isolation of individual plants and did not vary significantly with population size. As expected, plant isolation increased pollen limitation and lowered seed set. There was no effect of population size on seed set in 2004, and pollen limitation decreased nonsignificantly with population size in 2005. We conclude that pollen receipt limits reproduction in E. angustifolia, but pollinator visitation does not. Remarkably, isolated plants simultaneously have increased rates of pollinator visitation by pollinators and decreased reproduction. We discuss alternative explanations of pollen limitation that are consistent with this apparent discrepancy, including a decline in the availability of compatible conspecific pollen with increased plant isolation.

Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass‐flowering species

“…The higher visitation rates in low‐density patches did not attenuate the potentially deleterious consequences of low mate availability for pollen transfer, as found in other species (Duncan et al. 2004b; Campbell and Husband 2007; Wagenius and Lyon 2010). Similar correlations between pollen loads and plant density have been reported for Limnanthes douglasii rosea (Briscoe Runquist 2013) and for experimental plots of Piriqueta caroliniana, in which the floral constancy of the pollinators may account for the lack of effect of other co‐flowering species on pollen loads and reproductive success at low plant densities (Feldman 2008).…”

Reproductive success of Cabralea canjerana (Meliaceae) in Atlantic forest fragments, Brazil

“…In contrast, other studies found no changes in the reproductive output of plants in fragmented habitats (e.g., Aldrich & Hamrick, 1998;Dick, 2001;Lopes, & Buzato, 2007;Aguirre, & Dirzo, 2008). Fragment size may affect plant reproductive output also without any change in pollinator frequency or richness (Ghazoul, 2005;Wagenius, & Lyon, 2010). Plant fertility may be affected by habitat fragmentation indirectly through abiotic changes (Kwak, Velterop, & Andel, 1998), for example, in the light intensity and the availability of water and nutrients in the soil (De Jong, & Klinkhamer, 1989;Saunders, Hobbs, & Margules, 1991).…”

Effects of landscape composition and configuration on pollination in a native herb: a field experiment

“…Since we controlled for the abundance of conspecific plants in each study location, and because only one site had naturally occurring conspecifics, we conclude that the increased variation in pollen limitation and seed set is most likely explained by distancemediated changes in pollinator occurrence (c.f. Wagenius and Lyon 2010;Delmas et al 2015). In this respect, our experimental stations can be compared with small, isolated plant populations exposed to an increased probability of pollination failure (Aguilar et al 2006;Pauw 2007), which may have negative consequences for long-term population persistence (Broadhurst and Young 2006).…”

“…Finally, pollination of wild plants can be affected by other flowering plants at a more local scale (Knight et al 2005;Jakobsson et al 2009;Wagenius and Lyon 2010;Delmas et al 2015). Co-flowering insect-pollinated plants can compete for pollinators or facilitate the pollination of other plants, depending on abundance (Dietzsch et al 2011) and floral traits determining quality and accessibility of floral rewards (Mitchel et al 2009;Carvalheiro et al 2014).…”