Post-fire succession niches facilitate co-occurrence of divergent flower forms

Pollinator-mediated floral divergence is common, but the ecological mechanisms allowing divergent forms to coexist are often unclear. In South Africa, Lapeirousia anceps corolla tube length coevolves with the proboscis lengths of its local pollinators, long-proboscid flies. At most sites, proboscis and corolla length are closely matched, but there are occasional bimodal populations, where short- and long-tubed plants coexist. To understand the selective environment that may lead to bimodality and coexistence, we studied a bimodal population over a 21-year period, during which several fires occurred. Early in succession, pollinators and long-tubed plants were more abundant, and they produced more seeds than shorter-tubed plants. Later in succession, shorter-tubed plants produced relatively more seeds and flowered more frequently. In these plants, self-pollination contributed more to seed set, which explains their success in the absence of pollinators. Patterns of selection on tube length shifted in response to pollinator visitation rates, suggesting that the coexistence of short- and long-tubed plants might be facilitated by differences in post-fire successional niches. This illustrates how temporal heterogeneity caused by fire could promote co-occurrence of divergent forms. If so, ongoing changes in natural fire regimes may affect floral diversity.