A trade-off between stress adaptation and competition for pollinators in a glade system: Implications for endemic plant species

Background/Question/Methods Variation in the distributions of species has intrigued scientists for centuries. One mechanism that may play a significant role is a trade-off between competition and adaptation to stressful, abiotic conditions. In response to xeric environments, plants often have similar suites of traits that are thought to be adaptive and are sometimes termed the “stress resistance syndrome”. While advantageous in more xeric habitats, stress adaptations often come at a cost, such as decreased competitive ability. Historically, competition between plant species has been conceptualized in the context of abiotic resources. A more recent emphasis on biotic factors indicates that many plant species compete strongly for pollinators, and pollinators are important for the reproductive success and persistence of many native plant species. However, little is known about the pollination biology of endemic glade species. A trade-off has been shown to exist between growth and reproduction, which could be exacerbated in predictably stressful environments. Field and herbarium measurements suggest that endemic species have traits associated with more specialized pollination systems, specifically longer floral spur length and tube depth, and with more restricted reproduction, specifically lower relative reproductive potential, again unlike their widespread congeners.The proposed objectives of this study were to elucidate the degree of specialization, overlap in the pollination biology, and stress adaptation of these endemic species in comparison to their widespread congeners via field and greenhouse experimentation.

Results/Conclusions We found that glade endemics have more specialized pollination systems (i.e., fewer visitors, lower visitation rates, and lower specialization scores) and traits associated with adaptation to stressful environments (i.e., lower total leaf area and shorter stature) in comparison to their widespread congeners. However, the pollination biology of endemic and widespread species do overlap, and could result in competition for pollinator services. Future work should explicitly explore the relative competitive abilities by introducing the congeneric pairs to shared pollinators. These results suggest a trade-off between stress and reproduction that could contribute to the limited biogeographical extent of these endemic glade species. Therefore, plant-pollinator interactions should be incorporated in the conservation strategies and ecological niche models used to predict their responses to climate change.