The effect of ecological factors on the mating system of a South American shrub species (Helicteres brevispira)

Abstract

Mating systems are in¯uenced by several ecological factors, including plant density, number of owers per plant, and pollinator movements. In this study, we investigated the simultaneous eects of these three factors on the mating system of a self-compatible Brazilian shrub species: Helicteres brevispira St. Hil. Outcrossing rate is directly correlated with plant density. Changes in the number of owers per plant aect outcrossing rate through their eect on the density of¯owers. Variation in foraging behaviour of hummingbird pollinators is a consequence of the interaction between plant density and number of¯owers per plant. Territorialist pollinators are common in high density areas but visit few¯owers on each plant, thereby promoting outcrossing. In areas of low plant density, trapliners and rare territorialists visit several¯owers per plant, thus increasing sel®ng. Our results indicate that outcrossing rate is a dynamic parameter, with the extent of variation depending on a number of ecological factors. In successional species such as those in the genus Helicteres, demographic changes may be accompanied by alterations in mating system parameters, with concomitant eects on the genetic structure of populations.Keywords:¯ower density, Helicteres, mating system, outcrossing rate, plant density, pollinator behaviour. IntroductionMating systems in plants can be in¯uenced by several ecological factors. Wright (1946) demonstrated theoretically that inbreeding is inversely correlated with plant density and gene dispersal, if mating within neighbourhoods is random. As density increases, the eective number of reproducing individuals inside the neighbourhood areas also increases, as does the outcrossing rate. This is particularly true in plant species pollinated by animals, because pollinators tend to¯y to the closest neighbour and most of the pollen is exchanged among neighbouring plants (Levin, 1986; Parra et al., 1993; Boshier et al., 1995; Stacy et al., 1996).Several authors have suggested that high concentration of rewards in space may enhance pollinator activity between plants and thus pollen¯ow (e.g. Ausgspurger, 1980; Wyatt, 1980; Gerber, 1985). Others have suggested that this concentration may restrict pollinator movement and gene¯ow between plants and within populations (e.g. Linhart, 1973;Feinsinger, 1978;Schmitt, 1983; Stacy et al., 1996). Species with dierent spatial patterns of¯oral rewards can also attract guilds of pollinators with distinct foraging strategies. For example, Linhart (1973) andFeinsinger (1978) found that species that produce few¯owers per plant attract trapliner hummingbirds (which visit sequentially the same plants over a wide geographical area), whereas species that produce many¯owers per plant are pollinated by territorial hummingbirds (which visit and defend owering plants that are within their feeding territories). These authors have suggested that traplining and foraging over long distances should promote outcrossing, but that territorial foraging should result in inbreeding (see also B...

The post-fire effects on the outcrossing rate of a Brazilian savannah shrub, Helicteres sacarolha A.St.-Hil

“…These pollinators visited almost every open flower during each visit to a plant, which may increase correlated mating and decrease pollen flow and crossing between distant plants. In the studied population of H. brevispira, traplining behaviour is common only in low plant density areas, where outcrossing rates also showed low values (Franceschinelli & Bawa 2000).…”

“…Comparison with a cogeneric species -In 1994, a sympatric population of a cogeneric species, Helicteres brevispira, had higher outcrossing rates (tm = 0.49 to 0.68, Franceschinelli & Bawa 2000) than H. sacarolha (tm = 0.37 to 0.58). This may be explained by the lower plant density of H. sacarolha population and by the traplining behaviour of the H. sacarolha pollinators.…”

“…The mating system of a species may be measured through its outcrossing rate. This rate is influenced by several ecological factors, such as plant density, number of flowers per plant, and pollinator movements (Franceschinelli & Bawa 2000). Outcrossing rate increases with plant density (Farris & Mitton 1984, Burdon et al 1988, Wolff et al 1988, Murawski et al 1990, Murawski & Hamrick 1991, 1992a, b, Van Treuren et al 1993.…”

“…Large plants that produce many flowers can induce the pollinator to visit high number of flowers per plant, thereby increasing the probability of selfing (Burdon et al 1988, Wolff et al 1988. However, pollinators may be induced to visit higher number of flowers per plant in populations that are less dense regardless of the plant size (Franceschinelli & Bawa 2000).…”

“…In 1994, we studied the effect of plant and flower densities on the outcrossing rate of this species. Also, we compared the outcrossing rates of H. sacarolha with its cogeneric H. brevispira A.St.-Hil which has populations with higher plant and flower densities (Franceschinelli & Bawa 2000). The studied population of H. sacarolha occurs in an area that has been protected against fire for more than 30 years until the end of 1994, when a deliberate fire lit by unknown people occurred.…”