Aim: To assess whether the reduced nutritional resources available for pollinators due to plant community simplification along an elevational plant-diversity gradient changes pollinator niche breadth and richness. Additionally, we evaluated how body size and proboscis length of pollinators shifted along the gradient, and whether these changes were related to pollinator niche breadth.
Location: An elevational gradient (2,350-3,520 m a.s.l.) on the oceanic high-mountain strato-volcano of El Teide (Tenerife, Canary Islands).
Taxon: Flowering plant and pollinator species.
Methods: We compared quantitative plant–pollinator networks along the plant-diversity gradient. We calculated a set of niche-based topological metrics that capture the degree of specialization, niche breadth and niche overlap. Furthermore, we obtained β-diversity measures and the proportion of replacement and richness components.
Results: There was an overall decline in species richness of pollinators with increasing elevation. This decline was mainly driven by the loss of species along the elevational gradient, which conformed a nested subset pattern. The whole network showed less specialization, greater connectance and lower modularity towards the summit. At high elevations, pollinators were more generalized and less selective in their flower choice, showing a greater trophic niche breadth compared to pollinators at lower elevations. Mean body size of pollinators increased with elevation, and species body size and proboscis length were positively associated with the number of plant species visited.
Main conclusions. Overall, results indicated that the elevational gradient filters pollinator species, probably according to their thermal tolerance and ability to exploit a wide range of trophic resources. The finding that pollinators become more generalized and opportunistic at higher elevations is a novel result, which may have implications for new research into how ecological networks vary over environmental gradients. From an applied perspective, our results highlight the importance of considering the spatial variation of species assemblages when aiming to construct functionally reliable interaction networks along environmental gradients.
LaraRomero_JBI_Pollination_Networks
Study site
The study was carried out along an elevational gradient on El Teide strato-volcano (3,718 m), within El Teide National Park (Canary Islands; 28º16’15’’N 16º38’21’’O). The area is influenced by a typical high-mountain climate with great thermal oscillations throughout the year (differences of c. 10ºC between maximum and minimum monthly average temperatures). The sites receive an annual precipitation of c. 370 mm, most of which falls during winter. Four sites at different elevations were selected on the South-East faces of the strato-volcano: Montaña Rajada (2,350 m), Montaña Blanca (2,730 m), Refugio de Altavista (3,300 m), and La Rambleta (3,520 m). Dry open sclerophyllous scrubland occurs above the tree line (ca. 2,000 m), with vegetation cover decreasing with elevation. Eleven entomophilous plant species were found along the gradient, all of them endemic to the Canary Islands.
For monitoring of plant–pollinator interactions, we established four plots of approx. 1 Ha distributed along an elevational gradient at the study site. Flower-visiting animals were observed on the plant species from 15th May to 15th August, for two consecutive years (2014 and 2015), coinciding with the flowering of all plant species in the different elevational communities. The censuses were carried out between 9:00 and 19:00 h, avoiding the beginning and the end of the day, when there were usually low temperatures and little insect activity in this high mountain environment, and also avoiding windy days. All plant species were censused 7-8 hours per locality and year, throughout the flowering phenology of each plant species, to maximize the possibility of detecting different floral visitors. Each census lasted 15 minutes, during which the observer remained facing the plant, recording all insects contacting the flowers as well as, whenever possible, the number of flowers contacted per individual. The individual plants to be censused of each species were arbitrarily chosen at each elevation. We recorded a plant-pollinator interaction when an insect maintained contact with the reproductive organs of a flower for more than 1 second. Thus, all flower-visiting insects (hereafter, named pollinators) that feed on flowers were recorded, regardless of the efficacy of their visit. Insects were either identified in the field or collected for later identification in the laboratory. Insect collection was not performed simultaneously with sampling of plant-pollinator interactions, to avoid affecting the data on interaction frequency. During the entire study period, a total of 868 observation hours were spent during 57 observation days. README file contains abbreviations used for species.
