Influence of community structure on the spatial distribution and pollination of critically endangered Dicerandra immaculata var. immaculata (Lamiaceae)

Background/Question/Methods

Community structure at local scales is a major factor controlling population and community dynamics of plant species. Lakela’s mint, Dicerandra immaculata Lakela var. immaculata (Lamiaceae), is a critically endangered short-lived perennial that is known only from scrub habitat along a 4.8 km length of the Atlantic Coastal Ridge in Indian River and St. Lucie Counties, FL. Very little is known about the biology and ecology of this species. Using sites where populations of D. immaculata were wild and/or introduced we sought to answer the following questions: 1) how do habitat characteristics at locations supporting D. immaculata vary from random locations within the same habitat; 2) which insect species pollinate D. immaculata; and 3) how do habitat characteristics influence pollination of D. immaculata? We measured 12 habitat characteristics at each of 160 locations supporting D. immaculata within seven field sites and compared these characteristics to habitat at 160 random locations within the same sites. We also observed 102 plants of varying floral density across shaded and sunlit habitat for 5 minutes each to identify all insect pollinators and quantify the number of flowers they visited within a plant.

Results/Conclusions

Lakela’s mint tended to grow in locations with open gaps at ground and canopy-level. Over 93% of the pollinators were nonnative honeybees. Native pollinators were primarily bumblebees, lepidopterans, and true flies. Honeybees were more likely to visit plants in sunny habitat and those with large floral displays, whereas native pollinators were influenced only by size of the floral display. The number of flowers visited by a honeybee was not influenced by size of the floral display or sunlight, but they visited nearly three times more flowers than native pollinators. The number of calyxes on a plant, which is an indication of how many flowers were fertilized, was positively correlated with size of the plant. Overall, these results indicate: 1) access to sunlight is likely essential for survival of D. immaculata; 2) pollinators and pollination are not limiting in shaded habitat (and therefore do not drive the spatial distribution of D. immaculata); 3) larger plants experience greater pollination; and 4) D. immaculata may be pollinated primarily by nonnative honeybees. The efficiency of honeybees as pollinators as well as their influence on population genetics of D. immaculata (honeybees may promote more selfing) should be explored.