The effect of density-dependent insect visits, flowering phenology, and plant size on seed set of the endangered vernal pool plant Pogogyne abramsii (Lamiaceae) in natural compared to created vernal pools

Abstract

Pollinator activity, flowering phenology, plant size, and seed set of an endangered annual plant, Pogogyne abramsii, were compared in natural and created vernal pools on Del Mar Mesa, San Diego County, California, USA. The purpose was to test the hypothesis that an artificial habitat would have fewer or less effective pollinator visits than natural vernal pools and that, as a result, the plants would have lower fecundity The effect of differences in flowering phenology and plant size among pool types on pollinator visitation and seed set was also evaluated. The most frequent insect visitors to P. abramsii flowers were the Eurasian honey bee, Apis mellifera, two anthophorid bees, Exomalopsis nitens and E. torticornis, and three species of bee flies, of which Bombylius facialis was by far the most abundant. All species of insect visitors displayed density-dependent foraging on P. abramsii, with greater numbers of visits per unit area where flower density was greater. Nevertheless, visits per flower were negatively correlated with flower density. Flowering phenology of P. abramsii was delayed in most created compared to natural vernal pools, which affected the frequency by species of insect visitors to created compared with natural vernal pools. Peak flower densities were significantly lower in created than in natural vernal pools. The created pools had more visits per flower. Plants were smaller in created pools compared to natural pools, and seed production was positively correlated to plant size. Seed set in P. abramsii was greater than one per plant in both created and natural pools but was significantly lower in created vernal pools. Most of this difference could be attributed to the larger size of plants in the natural pools. We conclude that pollinator limitation should not preclude the sparser populations of smaller plants in the created pools from having a positive growth rate. Subsequent establishment and persistence of populations in a majority of created basins on this site confirms this assertion.