Island Ecology: Reduced Hummingbird Diversity and the Pollination Biology of Plants, Trinidad and Tobago, West Indies

Abstract

To examine effects of pollinator diversity on pollination biology of plants, we studied early successional, hummingbird—pollinated plants for 13 mo on Trinidad and Tobago. A large, land—bridge island just off the Venezuelan coast, Trinidad has 16 hummingbird species. Smaller and more isolated, Tobago has a subset of five of these. Study sites in the Arima Valley of Trinidad had 17 species of plants adapted for hummingbird pollination; 12 of these, plus one other, existed on comparable Tobago sites. From hypotheses of island ecology and plant—pollinator coevolution, we developed four predictions; results supported the first three: (1) Overall, among the eight plant species observed at least 4 d (up to 28 d) on each island, Tobago flowers received significantly fewer visits than conspecific Trinidad flowers from hummingbirds and the Bananaquit Coereba flaveola, the only other avian visitor. A two—way ANOVA showed that the treatment (island) effect significantly exceeded even the tremendous month—to—month, plant—to—plant variation in visits received within each plant population. (2) Pollen loads carried by hummingbirds and Bananaquits on Tobago tended to be mixed, whereas birds on Trinidad often carried single—species loads. (3) In two of three plant species studied intensively, Tobago flowers secreted greater volumes of nectar than Trinidad flowers. The third species showed a slight but significant increase as well. An indirect measure of nectar standing crop, the time a hummingbird spent probing a flower, suggested that Tobago plants as a group secreted higher nectar volumes than Trinidad conspecifics. We found no significant differences in sugar concentration, however. (4) Contrary to predictions that flowering seasons should overlap less on Tobago than on Trinidad, there was no discernible difference in flowering seasons between Trinidad and Tobago conspecifics. Evidently, pollination service on Tobago is currently more erratic than on Trinidad, and has been sufficiently erratic in the past to select for increased nectar production but not so erratic as to override other, stabilizing, selective forces on flowering times.