The role of native and nonnative insects in the reproductive ecology of critically endangered Lakela’s mint, Dicerandra immaculata var. immaculata (Lamiaceae)
The reproductive ecology of rare plants can influence their persistence and conservation and this is especially true for short-lived plants that regenerate only through seeds because population dynamics are closely linked to seed dynamics. Seed production can be limited by a number of factors, such as the abundance and behavior of pollinators. Lakela’s mint, Dicerandra immaculata Lakela var. immaculata (Lamiaceae), is a critically endangered short-lived perennial that persists only in Florida scrub habitat on the Atlantic Coastal Ridge and little is known about its biology and ecology. We sought to answer the following questions about the reproductive ecology of Lakela’s mint: 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 relative amount of sunlight, size of the plant, or floral density; and 3) does the visitation rate to flowers within a plant differ among the major groups of pollinators. We used hand pollination experiments in a common garden to test the mating system. We also observed plants of varying floral density across shaded and sunlit habitat in natural field sites to identify pollinators and quantify the number of flowers they visited within a plant.
Lakela’s mint produced the most seeds when it was outcrossed (either by hand or insect transfer of pollen), a moderate amount of seeds when pollen was hand-transferred to stigmas within the same plant, and few seeds when anthers were removed (to test for asexual reproduction) or bagged with no manipulations for the duration of flowering (to test for spontaneous self-pollination). Over 93% of the pollinators were nonnative honeybees. Native pollinators were primarily bumblebees, lepidopterans, and dipterans. 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. Honeybees also visited nearly three times more flowers than native pollinators. Overall, these results indicate: 1) that Lakela’s mint is dependent on insects for pollination and the highest number of seeds are produced when plants are outcrossed; 2) Lakela’s mint may be pollinated primarily by nonnative honeybees which are preferentially attracted only to some plants in the population; and 3) nonnative honeybees may be promoting more self-pollination than native pollinators. The efficiency of honeybees as pollinators as well as their influence on spatial distribution and population genetics of Lakela’s mint should be explored.