Plant-Hummingbird Interactions: Effects of Island Size and Degree of Specialization on Pollination

Abstract

(1) Dispersal of pollen by hummingbirds in two plant species, Mandevilla hirsuta (Apocynaceae) and Justicia secunda (Acanthaceae), was investigated on Trinidad, a large continental island, and Tobago, a smaller island with fewer species of hummingbirds. (2) Frequencies of hummingbird visits to flowers of each species (and, in Justicia, to flowers at different densities) on each island were quantified over a 13-month period. (3) During February 1978, pollen dispersal on each island was monitored with fluorescent powders, and fruit-set of Mandevilla was recorded. (4) In Justicia, which has a relatively open corolla accessible to many hummingbirds, frequencies of bird visits were similar on both islands. Patterns of total pollen dispersal, relating to labelled pollen deposited on any flower part, were also similar on both islands. Patterns of pollen deposition on reproductive parts were affected by feeding behaviour of the hummingbirds. (5) On Trinidad, Justicia pollen was dispersed less far from flowers defended by territorial hummingbirds than from undefended flowers. (6) Mandevilla, which has a long corolla accessible to only a few hummingbird species, received consistent visits from two hummingbirds 'constant' to that species at one Trinidad site. (7) On Tobago, where these 'constant' hummingbird species were absent, visits to Mandevilla were sporadic and were made only by species that fed on many plant species, and were not 'constant' on Mandevilla. (8) Accordingly, pollen dispersal of Mandevilla on Trinidad was extensive, with dye from marked flowers reaching most other flowers in the population on a given day, whereas on Tobago dye was dispersed only very short distances. (9) Fruit-set in Mandevilla on Tobago was significantly lower than on Trinidad. (10) These results support the prediction that a generalized plant such as Justicia would be affected less by an island-induced reduction in pollinatordiversity than would a specialized plant such as Mandevilla.