Evaluation of Character Displacement Among Plants in Two Tropical Pollination Guilds

Abstract

In cloud forest at Monteverde, Costa Rica, two guilds of bird—pollinated plants exist; on guild pollinated by long—billed hummingbirds, primarily the Green Hermit (Phaethornis guy), and one guild pollinated by short—billed hummingbirds, primarily the Purple—throated Mountain—gem (Lampornis calolaema). Plants were assigned to guilds based on hummingbird visit patterns documented during >4000 plant—hours of field observations, and on identities of pollen grains collected from 600 mist—netted hummingbirds. Other studies indicated that pollination in these plants is often insufficient for maximum seed set. Each guild was examined for character displacement expected within a stable assemblage of plants structured by competition for pollination. (1) By comparing observed flowering phenologies with those obtained through a randomization procedure, we determined whether each species' phenology minimized overlap with the remainder of its guild. (2) We also examined complementarity between phenological displacement and morphological displacement in reproductive structures. Neither guild exhibited pronounced character displacement. (1) In most cases, flowering phenologies were indistinguishable from those generated at random; the few statistically significant departures mostly indicated aggregation, rather than displacement, of flowering seasons. (2) In most cases, morphological similarity was independent of phenological similarity. The only statistically significant result among the studied species was a positive correlation, among long—flowered species only, between rarity and uniqueness of flowering season. We do not conclude that this absence of expected pattern indicates that competition never occurs or that competition is an inconsequential ecological event. Rather, we attribute absence of pattern to the following aspects of biological variability, two of which we have demonstrated in other studies. (1) Within any one year, density—dependent competition for pollination is sporadic, and is not clearly related to flowering season or morphological similarity. (2) The nature of interspecific interactions varies among years, as neither the relative intensities of flowering nor the flowering seasons themselves are consistent from year to year. (3) The nature of interspecific interactions varies with changes in species composition, which occur over short distances. (4) The assemblage of species is probably not stable over long time spans; the species have Gleasonian ecologies that change distribution and abundance faster than natural selection or diffuse competition can screen out improper phenotypes or species, respectively.