Effects of plant abundance on reproductive success in the biennial Sabatia angularis (Gentianaceae): spatial scale matters

Abstract

Summary 1.Small and low-density populations often suffer significant reductions in reproduction, as exemplified by studies on rare and threatened species. While this phenomenon is less studied in common species, if general, it should not be restricted to those in peril. We addressed this assertion by examining the effects of plant abundance, measured at population-level (population size, density) and local-level (local neighbourhood size, LNS) spatial scales, on fruit set, seed set and subsequent reproductive success (RS) across 19 natural populations of the widespread biennial Sabatia angularis (Gentianaceae). We also examined stigmatic pollen loads across a subset of populations to address whether changes in reproduction were related to reduced pollination. 2. Effects of plant abundance on reproduction were evident at both spatial scales. At the population level, population size -but not density -affected fruit set significantly. This effect was non-linear. Fruit set in populations larger than 15 individuals was relatively constant, but dropped abruptly in populations below that size. This relationship did not prevail between population size and RS. Instead, RS declined significantly with population density. 3. At the local level, increases in LNS between 1-m and 4-m radii from study plants increased fruit set significantly. In contrast, increases within 1 m significantly reduced seed set. Both of these effects prevailed at different spatial scales for RS; increases in 1-m LNS reduced RS, but increases beyond that distance and within 4 m increased RS. 4. Although non-significant, trends in the data are consistent with facilitative interactions for pollination influencing local-level effects on fruit set. However, negative interactions, presumably because of competition for resources, are more important for determining seed set at local scales. Both interactions influence RS, but the effect of competition appears to dominate at the population level. 5. Synthesis . This study demonstrates that although significant reductions in fruit set in small S. angularis populations imply that common species are not impervious to reproductive disadvantages, small populations need not experience reduced RS. Furthermore, low-density populations actually enjoy increased RS. As such, this study highlights the complexity of ecological interactions affecting reproduction and the importance of incorporating multiple spatial scales when examining population dynamics.

Floral colour versus phylogeny in structuring subalpine flowering communities

“…are more specialized), while purple flowers are more generalized in alpine environments (Lazaro et al 2008). With rare species being more purple/blue, our results may then indicate that these rare, purple-flowered species benefit from sharing generalist pollinators with common purple species, despite experiencing high amounts of heterospecific pollen (Sargent & Otto 2006;Spigler & Chang 2008). While recent studies have indicated a role of mutualisms in phylogenetically structured extinction cascades (Rezende et al 2007), common species may act as magnets for pollinators of rare species; a pattern that should result in stabilizing forces maintaining diversity within communities.…”

“…Rare species will receive more interspecific pollen owing to the relative abundance of coflowering species and therefore may only persist in a community if they exhibit increased divergence in floral traits than do common species (Ishihama et al 2006). Recent models (Sargent & Otto 2006) and field studies (Gumbert et al 1999;Spigler & Chang 2008) indicate that rare species are under more pressure to have divergent floral traits and to evolve specialist pollination systems than are common species. Conversely, plantpollinator network studies have documented that rare species are pollinated by generalist or super-generalist pollinators (Bascompte et al 2006;Vazquez et al 2007;Petanidou et al 2008).…”

“…Furthermore, whether floral characters are important determinants of competition or facilitation of pollination may be affected by the local abundance of a population (Spigler & Chang 2008). The abundance or density of a species within a community directly determines the amount of conspecific pollen that is available for pollination (Spigler & Chang 2008). Rare species will receive more interspecific pollen owing to the relative abundance of coflowering species and therefore may only persist in a community if they exhibit increased divergence in floral traits than do common species (Ishihama et al 2006).…”

“…If increasing the visitation rate outweighs the costs of any interference from interspecific pollen transfer (Morales & Traveset 2008), then floral traits within communities should be, on average, more similar than chance would predict. Furthermore, whether floral characters are important determinants of competition or facilitation of pollination may be affected by the local abundance of a population (Spigler & Chang 2008). The abundance or density of a species within a community directly determines the amount of conspecific pollen that is available for pollination (Spigler & Chang 2008).…”

Effects of floral neighborhood on seed set and degree of outbreeding in a high-alpine cushion plant

“…Whereas many studies have reported higher seed or fruit set at higher conspecific densities (e.g., Sih and Baltus 1987;Feinsinger et al 1991;Kunin 1993;Roll et al 1997;Bosch and Waser 2001;Feldman 2006Feldman , 2008ZornArnold and Howe 2007), and others have detected no effect of density (Caruso 1999;Moeller 2004), we found that seed set of E. nanum was higher at lower conspecific density. Working with a biennial gentian, Spigler and Chang (2008) similarly reported a decline in fruit set with increasing conspecific density at spacings of \1 m. Intriguingly, they saw the opposite result with spacings of 1-4 m. These researchers suggested that negative density effects among near neighbors might arise from resource competition. Conceivably, resource competition, rather than intraspecific competition for pollination services, might also explain our result, since our plants grew in situ and could have interacted via shared resources other than pollinators.…”