Abstract
Management of small plant populations requires an understanding of their reproductive ecology, particularly in terms of sensitivity to Allee effects. To address this issue, we explored how components of pollen transfer and pollination success of individual plants varied among 36 populations of the self-compatible moth-pollinated orchid Satyrium longicauda in South Africa. Mean fruit set, seed production, proportion of flowers with pollen deposited or removed and proportion of removed pollen that reached stigmas (approx. 8% in this species) were not significantly related to population size (range: 1-450 flowering individuals), density or isolation. Plants in small populations did, however, have significantly higher levels of pollinator-mediated self-pollination (determined using colour-labelled pollen) than those in larger populations. Our results suggest that small populations of this orchid species are resilient to Allee effects in terms of overall pollination success, although the higher levels of pollinator-mediated self-pollination in small populations may lead to inbreeding depression and long-term erosion of genetic diversity.
Highlights
Plants in small or sparse populations often experience lower reproductive output than their conspecifics in large populations (e.g. Lamont et al 1993; Agren 1996; Kery et al 2000; Agren et al 2008)
We investigated the relationships between population size and components of pollination success in Satyrium longicauda, a widespread South African terrestrial orchid pollinated by hawkmoths (Harder & Johnson 2005; Jersakova & Johnson 2007)
While pollen transfer efficiency (PTE) was apparently unaffected by population size, self-pollination was negatively related to population size in analysis of covariance (ANCOVA) models both that did (F1,9Z8.61, pZ0.017; table 1) and did not take the effects of population density and isolation into account (F1,12Z12.6, pZ0.004; figure 1; statistically significant after sequential Bonferroni correction)
Summary
Plants in small or sparse populations often experience lower reproductive output than their conspecifics in large populations (e.g. Lamont et al 1993; Agren 1996; Kery et al 2000; Agren et al 2008). Some studies have shown that lower seed set of plants in small populations is due to pollen limitation (Agren 1996; Ward & Johnson 2005). This is usually an ecological consequence of inadequate pollinator visitation affecting pollen quantity, but there is evidence that the quality of pollen (e.g. compatibility type) received by stigmas can be lower in small populations (Busch & Schoen 2008). Other evidence for a role for genetic factors in Allee effects includes correlations between inbreeding coefficients and low seed production in small populations (reviewed by Leimu et al 2006). Increased homozygosity in small populations can be due to loss of alleles through drift, founder effects, biparental inbreeding or higher rates of self-fertilization (cf. Klinkhamer & de Jong 1990; Ellstrand & Elam 1993; Young et al 1996)
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