- University of Toronto
Summary
Temporal changes in the composition of the pollinator fauna of the tristylous aquatic plant Pontederia cordata were documented during the flowering seasons of 1982 and 1983. A total of 18 species of insects, mainly bumblebees, and a single species of hummingbird were observed visiting flowers during the 2-month flowering period. The abundance of pollinators within each season varied greatly, with highest densities corresponding to peak flowering in both years. Although overall abundance was similar each year, the diversity of species was greater in 1983 because of the addition of short-tongued bumblebees and the longer residency periods of other species. It is suggested that low summer rainfall in 1983 decreased flowering levels in terrestrial plant communities of the region, resulting in several bee species shifting their feeding onto P. cordata. The study demonstrates that changes in the structure of a plant's pollinator fauna occur over time and that despite the possession of a specialized pollination mechanism, tristylous P. cordata is visited by a wide range of pollinator species.
Methodology
All work reported here was conducted at Pothole Lake, Leeds and Grenville County, Ontario. Pothole Lake is connected to the Rideau Lake System situated about 50 km north of Kingston, Ontario. The lake is approximately 800 m long and varies from 50 to 300 m in width. Pontederia cordata is the most abundant emergent plant in the littoral zone and grows along the shoreline and in shallow areas in the middle of the lake. Further details of the study site are provided in Wolfe (1985).
The flowering phenology of the Pothole Lake population was quantified in 1983. Two transects, each 1 m wide x 10 m long separated by about 7 m, were laid out from the shoreline to the population edge. Transects were censused every 2 days during the entire flowering season and the commencement of flowering was recorded for a total of 379 inflorescences.
The abundance and species richness of visitors to P. cordata flowers were determined at regular intervals (about 5 days) throughout the 1982 and 1983 flowering seasons. A record was made of all insects and birds that entered a 2 x 2 m quadrat of high inflorescence density and visited at least one flower of P. cordata. Except for the queens of two bumblebee species, which were difficult to distinguish (see Results), all visitors were identified while foraging. Observations were conducted for 30 min of each hour from flower opening (09:00) until 15:00 or 16:00 when flowers closed. Data were used to determine seasonal patterns of abundance and species richness.