Interspecific pollen loss by hummingbirds visiting flower mixtures: effects of floral architecture

Ecology, March, 1996 by Carolina Murcia

MATERIALS AND METHODS

Study site and species

The short-flowered guild of hummingbird-pollinated plants at Monteverde includes many understory species from a range of families, plus several canopy epiphytes mostly in the Ericaceae (Murray et al. 1987). Hummingbirds of one species, Lampornis calolaema, effect nearly all pollination in the understory species and also frequently visit the epiphytes, resulting in many pollen grains of Ericaceae on stigmas of understory flowers (Feinsinger et al. 1986, 1991, Murray et al. 1987; W. H. Busby, unpublished data).

The understory shrub or treelet Palicourea lasiorrachis Oersted (Rubiaceae), which is distylous and self-incompatible, relies almost exclusively on Lampornis for pollination (details in Feinsinger and Busby 1987, Feinsinger et al. 1988, 1991). During its flowering season, which peaks once or twice between March and June, Palicourea shares Lampornis with up to 15 other plant species (Murray et al. 1987, Feinsinger et al. 1991). In March 1989, the most abundantly flowering of these were the shrub or treelet Cephaelis elata Sw. [=Psychotria elata (Sw.) C. Taylor & W. Burger, ined.], also Rubiaceae; the shrub Hansteinia blepharorachis (Leonard) Durkee and the herb Dicliptera iopus Lindau, both Acanthaceae; the shrub Besleria triflora (Oersted) Hanst., Gesneriaceae; and the epiphytic Satyria warszewiczii Klotsch, Ericaceae. Plant names follow Haber (1991).

All experiments required fresh, unvisited flowers from the six plant species. We used nylon mesh bags [TABULAR DATA FOR TABLE 1 OMITTED] to exclude hummingbirds and other flower visitors from inflorescences in the field, and collected fresh flowers each dawn.

Traits of flowers

We obtained measurements and pollen grain counts from a sample of 10 flowers per species (per style morph, in Palicourea and Cephaelis), collected from at least three individual plants, except where abundant data were available from previous studies. We measured to the nearest 0.5 mm the length of corolla tube, pistil, and stamen to anther midpoint (to anther tip in Satyria, which has poricidally dehiscent anthers). Using a compound microscope equipped with Hoffman optics and a micrometer, we estimated stigmatic surface areas for each species (for each style morph in the two Rubiaceae). We measured pollen grains as spheres (treating each Satyria tetrad as a sphere) or as ovoids (in Dicliptera) and counted, or estimated for species with [greater than]5000 grains, total pollen grains produced per virgin flower. From these data, we estimated for each species the number of pollen grains sufficient to cover a short-styled (thrum) Palicourea stigma one layer deep.

Effect of intervening flowers on Palicourea pollen transfer

We netted three male Lampornis, maintained them temporarily in 1 - [m.sup.3] cages in the laboratory, and trained them to visit flowers offered one at a time by hand (see Feinsinger and Busby 1987, Feinsinger et al. 1988, Feinsinger and Tiebout 1991, Podolsky 1992, 1993). For each experimental trial, a bird was released in the cage after having its bill, crown, and throat wiped clean, then was given a series of five Palicourea pollen donor flowers followed by 20 Palicourea pollen recipient flowers, with or without an intervening series of five flowers belonging to one of the other five species. Pollen donors were always the long-styled (pin) morph of Palicourea and pollen recipients the short-styled (thrum) morph [see explanation in Feinsinger et al. (1988), Feinsinger and Tiebout (1991)]. For those intervening species that also had floral dimorphisms, we mimicked field conditions as closely as possible by using mixtures of the morphs (Table 1). We re-did any trial in which the hummingbird had wiped its bill against a perch, a behavior commonly observed in the field as well as the laboratory.