Bird Visitation, Seed-Set, and Germination Rates in Two Species of Lobelia on Mount Kenya

Abstract

Seed-set in natural plant populations may be limited by pollinators, resources, or seed predators. Bierzychudek (1981) found pollinator numerical limitation of seed-set to be common in natural populations. However, many plant species are capable of self-pollination when pollinators are absent (Proctor and Yeo 1973); in others natural levels of outcrossing are sufficient for full seed-set, and seed-set appears to be resource limited (Primack and Lloyd 1980, Udovic 1981, Udovic and Aker 198i). In most pollination studies of natural plant populations, numerical seed-set is the only measure of reproductive success, although recent studies have shown that other factors such as seed size, germination rates, and seedling success may be affected by the degree of outcrossing (Price and Waser 1979, Schoen 1982). Such effects have long been known for crop plants (Crumpacker 1967). This paper reports an attempt to determine the importance of pollinators for the seed production of two species of Lobelia on Mount Kenya. Two species of rosette plants in the genus Lobelia (family Lobeliaceae) occur in the alpine zone of Mount Kenya. L. telekii is primarily semelparous, and L. keniensis is primarily iteroparous. These two species are thought to be bird pollinated (Hedberg 1964). The morphology of the reproductive structures is such that birds, but no native insects, normally come into contact with the pollen source (T. P. Young, personal observation). The cylindrical inflorescences project above the surrounding vegetation, and the purple flowers secrete copious nectar. L. telekii has narrow, hairy bracts and is visited primarily by territorial individuals of the Scarlet-tufted Malachite Sunbird (Nectarinia johnstonii). L. keniensis has broad glabrous bracts, and is visited by a variety of birds, including N. johnstonii, the Mountain Chat (Pinarchroa sordida), and the Slender-billed Chestnut-winged Starling (Onagnathus tenuirostris). The tall terminal inflorescences mature progressively upwards from the base. The five petals are united and open between the upper two petals to form a lower-lipped flower (Fig. 1). The style is enclosed by a tube formed of the five united filaments and anthers. This tube terminates in a brush that acts as a block to pollen release. Dehisced pollen collects inside the distal end of the filament tube. When the brush is pushed back by the forehead of a visiting bird, pollen is released. I have observed white pollen on the foreheads of P. sordida and 0. tenuirostris visiting L. keniensis flowers. The male state of the flowers lasts up to 2 wk. The style lengthens during this time, and is finally exserted from the filament tube. The stigmas separate and become receptive several days later, and begin to dry up after 2-4 wk. Seeds develop in a pair of inferior ovaries and are wind dispersed over a period of many weeks. The time from the first appearance of the flowers to final dispersal of seeds is =12-18 mo, depending