An endemic plant and the plant-insect visitor network of a dune ecosystem

Network theory increasingly is used to quantify and evaluate mutualistic interactions, such as those among plants and their flower-visiting insects or pollinators. Some plant species have been shown to be important in community structure using network metrics; however, the roles of plant taxa, particularly rare species, are not well understood. Pitcher's thistle (Cirsium pitcheri), a threatened endemic of Great Lakes shorelines, flowers late-June to early-August, when other floral resources may be less abundant or unavailable. We performed 10 min insect visitor observations on all insect pollinated plants in 44–10 m by 10 m plots at Sturgeon Bay, northern lower MI, USA, during C. pitcheri flowering and recorded plant species, number of open flowers, species of insect visiting, and number of visits by insects. Pitcher's thistle received 18.2% of all 600 recorded visits, 61.1% more than the next most visited plant. Pitcher's thistle also received visits from 22 of the 59 different insect species in the network, twice as many as the next most visited plant species. Species-level network analysis metrics showed that Pitcher's thistle was most generalized, with greatest species strength, betweenness, and connectance scores of any other plant taxon, demonstrating network topological importance. Pitcher's thistle received significantly more insect visits relative to its abundance that did any other plant species. Therefore, conservation of C. pitcheri and of other rare taxa, particularly in xeric and low diversity systems, can be significant beyond species-level management and may extend to conservation of the plant-insect community.