Influence of plant traits, density, and the environment on pollinators and reproduction of Dicerandra immaculata (Lamiaceae)

Background/Question/Methods

The reproductive ecology of rare plants can influence their persistence and spatial distribution and seed production can be limited by a number of intrinsic and extrinsic factors. Dicerandra immaculata Lakela var. immaculata (Lamiaceae) is a critically endangered short-lived perennial plant that persists in Florida scrub habitat. Little is known about the reproductive ecology of D. immaculata var. immaculata, so we used an experimental garden and three populations of D. immaculata var. immaculatato identify how plant traits, density, and the environment influenced pollinators and seed production. Specifically, we asked: 1) what is the mating system and are insects necessary for reproduction; 2) which insect species are pollinators and is pollinator abundance influenced by the amount of sunlight, size of the plant, or floral density; 3) does the visitation rate to flowers within a plant differ among the major groups of pollinators and is it influenced by environmental factors; and 4) is seed output influenced by the relative amount of sunlight, size of the plant, or floral density?

Results/Conclusions

We found that insects were not necessary for seed production, but insect-pollinated flowers produced more seeds than flowers reproducing autonomously or asexually. The honey bee Apis mellifera L. was the most frequent pollinator observed at all locations and during both years of this study. A. mellifera was the only nonnative pollinator that we observed and its behavior differed from native pollinators. A. mellifera visited more flowers within plants than native pollinators, which may promote a higher rate of self-pollination and influence the genetic structure of D. immaculata var. immaculata populations. Pollinator abundance was influenced by environmental factors and tended to be higher in areas in full sun or with a high density of flowering plants, although individual pollinator species or groups varied from these trends. Whereas more flowers were pollinated and more total seeds were produced in sunlit areas compared to shaded areas, plants in the sun produced only half as many intact seeds as plants in the shade. We found no evidence that differential selection of plants by pollinators influenced seed output, so factors that affect the fate of seeds, germination, or early survival of plants are likely the factors influencing the spatial distribution of D. immaculata var. immaculata.