Long-term ecology of euglossine orchid-bees (Apidae: Euglossini) in Panama.

Abundance patterns during 6-7 years and orchid visitation were determined for 51 species of the 57 local euglossine bees. Male bees were counted at 3 chemical attractants presented in the same manner each month. Sites were separated by 75 km but included wet Atlantic forest at 500 m elevation, moist forest at 180 m near Barro Colorado Island, and cloud forest at 900 m near the Pacific ocean. 1. From 15 to 30 euglossine species of 4 genera were active in each month and site; monthly species number and general bee abundance were positively correlated. Many species had 3 annual abundance peaks (range 1-4) and were active throughout the year, but peak annual abundances rarely occurred during late wet or early dry seasons. In contrast, Eufriesea generally were present as adults only 1-2 months in a year. 2. Euglossine populations were exceptionally stable. Species at each site were more stable than any known insect population, and stability and abundance were positively associated. However, year-to-year population stability and the degree of seasonality were not correlated. Among the three sites, the more diverse (species rich) bee assemblages displayed lower stability; these were the wetter and upland sites. 3. The most abundant bees visited more orchid species. Eg. and El. each visited and average of 4 orchid species (range 0-13); Ex. and Ef. visited 0-3. Stable populations did not visit more or fewer orchid species than did unstable populations. 4. Less than 68% of species at each site visited orchid flowers; less than a few dozen of the 100-800 bees counted in a day carried orchid pollinaria. Over 20% of the euglossine species never were seen with pollinaria at any site and probably seldom visit orchids in central Panama. 5. Most bee species visited 1 or no fragrance orchids in a given habitat. Orchids tended to utilize common pollinators that seldom included more than 1 species, and they utilized stable or unstable, seasonal or aseasonal bees. However, the most stable and abundant bee, Eg. imperialis, rarely pollinated orchids; fewer than 10 of ca. 20000 bees carried pollinaria. 6. Orchids may interact primarily with discrete seasonal bee population peaks-probably the emerging adults. Although specialized orchid preferences are implicated for species that visit few or no local orchids but pollinate other species and carry pollinaria in other areas, euglossine bees do not need orchids to survive or reproduce.