The organization of flowering times of the insect-pollinated flora of south-facing bluffs in interior Alaska: Evaluation of the reproductive-interference hypothesis

Abstract

South-facing bluffs in interior Alaska are island-like habitats that support an insect-pollinated flora. I divided this flora into three guilds based upon the length of the floral tube: long-tube, short-tube, and open flowers. I posed the question: Are the flowering times of these guilds organized by reproductive interference (i.e., negative interactions for pollination)? To evaluate this question, I used both null-model and experimental approaches. I developed a method that characterizes the dispersion of flowering times as a Dispersion Index (DI). When DI is reasonably precise and has been derived based upon properly defined assumptions, it is appropriate for characterizing the dispersion of species along any resource or character axis. The flowering times of all guilds were significantly uniform in dispersion, a result consistent with the reproductive-interference hypothesis. The phenological patterns of the long- and short-tube guilds were best explained by the local extinction of some populations that experienced reproductive interference. I posed the hypothesis that reproductive interference is responsible for the phenological relationship between two sequentially blooming open-flower species. I retarded the blooming of potted individuals of the earlier species, Saxifraga reflexa, so that flowering coincided with that of the later species, Saxifraga tricuspidata. These plants were placed on a bluff in sites with and without the naturally occurring congener. I measured the seed set of these plants and found no evidence that S. reflexa experienced negative effects from flowering synchronously with, and occurring in close proximity to S. tricuspidata. Neither the presence, nor the absence of reproductive interference on south-facing bluffs was verified by my experimental studies. More experimental work is needed to resolve this issue. On the basis of my null-model studies, I conclude that reproductive interference has been an important factor in the organization of flowering times of the insect-pollinated flora of south-facing bluffs in interior Alaska.