Seasonal Changes in the Numbers and Foraging Activities of Two Jamaican Exomalopsis Species (Hymenoptera, Anthophoridae)

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Exomalopsis globosa (F) was more numerous during the winter, whereas E. similis Cresson was more so during the spring. These differences were explained from comparison of the bees' daily foraging activities and flight ranges with the seasonal and daily rainfall patterns. The types of pollen collected by the two species during the year did not account for the seasonal changes in the numbers of bees. Exomalopsis globosa (F) fue mas numerosa durante el invierno, mientras que E. similis Cresson lo fue tambien la primavera. Estes diferencas fueron explicadas por la comparacion de la actividad por las abejas en el forrajear diario y por sus radios de vuelo con los patrones diarios y estadonales de la lluvia. Los tipos de polen recojidos de las dos especies no explican los cambios estacionales en los numeros de abejas. Two MAJOR CONTRASTS exist between the flowering of tropical and temperate plants. Both regions are subject to seasonality, but the wetness and dryness of the seasons in the tropics do not generally have such drastic effects on the grow,th of plants as the warmth and coldness of the temperate seasons have (MacArthur 1972). The marked seasonality in temperate regions has maintained low diversities of plant species there and consequent high local densities of some of them. However, in the moist tropics, where the effect of seasonality is small, the diversity of plant species is very high and the densities of even the commonest species are low (Dobzhansky 1950). The second con,trast between the two regions is, like the first, maintained by the different effects of seasonality in them. The flowering of plants in temperate regions is largely restricted to the spring and summer, and the flowering periods of many species are only a few weeks in duration. However, in the moist tropics, many plants, especially the herbs, flower throughout the year (Adams 1972). These spatial and temporal concentrations of flowers in temperate regions have affected the activities of bees there. Such large numbers of flowers may be produced at the same time within foraging range that the bees can choose to feed at flowers of only a few of the available species. Species of bees in the same genus, for example in Bombus (Brian 1951), Andrena (Chambers 1968), and Osmia (Raw 1974), nesting at the same time of year and in the same locality have been found to collect pollen from largely different types of plants. In extreme situations bees have become oligolectic, in which each species of bee collects pollen from one or a few related taxa of plants (Linsley et al. 1963 and 1964, Cruden 1972). In contrast to the situation in temperate regions, the moist tropics have low densities of flowers of many species that are available to bees throughout the year and this led Michener (1954) to suggest that very few species were oligolectic there. However, so far, the food preferences of troipical bees have not been investigated quantitatively. The species within several Jamaican genera commonly feed at the same types of plants, so the interaction of two Exomalopsis species was investigated. E. globosa (F) and E. similis Cresson are probably the most common indigenous species of bees in Jamaica. Adults of both are active throughout the year and in many localities they have been seen to visit much the same types of flowers. Their nests are built in the ground, and in the study area the entrances of the nests of the two species were only a few metres apart. Adults of the two species are almost identical in size and were thought to have similar flight ranges and hence similar potential foraging areas in the locality studied (but see discussion). Up to 19 adult females of E. globosa co-operated in building and provisioning a nest as did up to 12 of E. similis. Most of the bees from a nest collected pollen, and many of these foragers had well-developed ovaries. Castes of egg-laying and non-egg-laying bees could not be distinguished by their size, age or ovarian development in either species (Raw, in press).