Maureen L. Stanton

Seed Variation in Wild Radish: Effect of Seed Size on Components of Seedling and Adult Fitness

Seed mass in wild radish (Raphanus raphanistrum) varies up to sixfold within single fruits. The impact of such variation on subsequent growth and fecundity was studied. Seeds from single fruits were used to minimize genetic differences among individuals. Weighed seeds were planted close together in a disturbed area typical of Raphanus habitats. Large seeds (> 6 mg) were more likely to emerge as seedlings than were small seeds (<4 mg). Seed size had no effect on emergence time. Seedlings from large seeds grew more rapidly and produced more flowers than did those from related smaller seeds. Results from the field experiment contrasted with those obtained in greenhouse growth studies, where seed size had no significant effect on final plant size. This study points out two factors that have led to inconsistent results in previous studies of seed size and seedling success: (1) differences in the timing of growth measurements, and (2) the presence or absence of competitive inequities among neighbors within the experimental design.

Floral Evolution: Attractiveness to Pollinators Increases Male Fitness

Because availability of resources often limits seed or fruit set, increased visits by pollinators may not always lead to increases in maternal reproduction. This observation has led evolutionary biologists to hypothesize that a plants ability to attract pollinators may have its primary impact on male fitness achieved through the fertlization of ovules. This interpretation of angiosperm reproductive ecology is supported by field experiments. Pollinating insects strongly discriminated between two Mendelian petal-color morphs in Raphanus raphanistrum, a widespread, self-incompatible crucifer. In experimental populations composed of petal-color homozygotes. color discrimination by naturally occurring pollinators had no statistically significant effect on relative maternal function (fruit and seed production) in the two morphs. In contrast, yellow-flowered individuals were far more successful as fathers (pollen donors) than were the less visited whites. These results suggest that the evolution of floral signals such as petal color may be driven primarily by selection on male function.

INFLUENCES OF FLORAL VARIATION ON POLLEN REMOVAL AND SEED PRODUCTION IN WILD RADISH

The effects of corolla size, pollen production, and pollinator visitation fre- quency on pollen removal (an estimate of male reproductive success) and pollen deposition and seed production (female success) were studied in wild radish (Raphanus sativus, Bras- sicaceae) in 1987 and 1988. Wild radish exhibits variation in corolla size, petal coloration, and anther size. A previous study (Stanton and Preston 1988) found no correlation between corolla size and estimates of female success, suggesting that flower size may be selected upon through variation in male reproductive success. For flowers receiving a single honey bee visit, corolla size was positively correlated with pollen removal, but not with pollen deposition on stigmas or seed production. Flowers receiving an average of 3.7 honey bee visits (in the 1987 study) also showed a correlation between corolla size and pollen removal (seed production was not monitored in this study). In contrast, when flowers were visited an average of 5.7 times by honey bees (the 1988 study), there was no significant relationship between corolla size and either pollen removal or estimates of female reproductive success (pollen deposition on stigmas and seed pro- duction). Honey bees demonstrated a preference for large flowers, either by significantly overvisiting large flowers or by visiting the large flowers before visiting smaller flowers. Under natural conditions, most radish flowers receive numerous visits, but even with frequent visitation, at least 20% of the pollen a flower produced was not removed. At the same time, flowers with greater pollen production had a larger proportion of pollen re- moved. Pollen removal was a positive decelerating function of honey bee visitation fre- quency. The number of pollen grains removed per visit declined with increasing honey bee visits. The male fitness function (pollen removal plotted against pollen production) was positive and decelerating in two experiments; positive and linear in one study. Large flowers may have greater male success than small flowers by virtue of their greater pollen pro- duction, their greater pollinator visitation frequency, and their earlier pollinator visitation. If selection is acting on floral traits in wild radish through variation in male success, increases in both corolla size and pollen production are likely to be selected for.

Invasibility of experimental habitat Islands in a California winter annual grassland

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica). at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m 2 to 32 m 2 , had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species-rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species-rich plots were more invasible.

Interacting guilds: moving beyond the pairwise perspective on mutualisms.

Most textbook treatments imply, and almost all theoretical analyses assume, that mutualistic interactions take place between a single pair of interacting partner species. A major goal of this symposium is to broaden and shift this pairwise perspective and make it concordant with the emerging view that locally exclusive mutualisms between just two species are the exception and that many communities include guilds of mutualistic species on one or both sides of the interaction. Many pollination and seed‐dispersal mutualisms have long been recognized as diffuse, but recent molecular analyses are revealing unrecognized partner diversity in mutualistic interactions previously thought to be locally species specific. Co‐occurring species within a mutualist guild are unlikely to be ecologically equivalent in the way they locate, compete for, and/or reward partners, and so intraguild interactions have the potential to influence population dynamics and patterns of selection in species on both sides of the mutualistic interaction. To illustrate some of these potential complexities for population dynamics, I use simple path analytic models to show that positive pairwise interactions between mutualists do not necessarily translate into positive net interactions within a mutualism involving more than two species. For example, when there is intraguild competition for partners, or even for resources external to the mutualism, the presence of a lower‐quality mutualist can negatively affect the partner population by reducing associations it can form with better mutualists. Variation in quality among potential partners is likely to place a premium on traits or behaviors that enhance association with better mutualists. More investigations are needed to determine how variation among interacting mutualists, with respect to characteristics such as longevity, dispersal capability, and competitive ability, influence population dynamics and selection in multispecies mutualisms.

Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna

Mutualisms are key components of biodiversity and ecosystem function, yet the forces maintaining them are poorly understood. We investigated the effects of removing large mammals on an ant-Acacia mutualism in an African savanna. Ten years of large-herbivore exclusion reduced the nectar and housing provided by plants to ants, increasing antagonistic behavior by a mutualistic ant associate and shifting competitive dominance within the plant-ant community from this nectar-dependent mutualist to an antagonistic species that does not depend on plant rewards. Trees occupied by this antagonist suffered increased attack by stem-boring beetles, grew more slowly, and experienced doubled mortality relative to trees occupied by the mutualistic ant. These results show that large mammals maintain cooperation within a widespread symbiosis and suggest complex cascading effects of megafaunal extinction.

Evolution in stressful environments. I. Phenotypic variability, phenotypic selection, and response to selection in five distinct environmental stresses.

Abstract.— Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life‐history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life‐history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near‐optimal conditions (control). Full‐sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size‐ and growth‐related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grimes triangle model of life‐history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near‐optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress‐ avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress‐avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life‐history strategies.

Beyond Floral Batemania: Gender Biases in Selection for Pollination Success

For over a decade, Batemans principle has been used to argue that the showy petals and sweet nectar of flowers are evolutionarily more male than female-that they are adaptations principally for promoting the export of pollen rather than the setting of seed. Here we present alternative views. (1) We question whether the assumptions of Batemans principle have been generally upheld for angiosperms. (2) We present a path model that contradicts Batemans principle by asserting that floral attractiveness characters might well affect fitness more deterministically through female than through male function. (3) We envision an episodic selection scenario that has the same outcome as Batemans principle but is based specifically on the ecology and mechanics of pollination. In the end, we recognize that selection on the displays and rewards of flowers is probably often gender biased (one way or the other), but we warn against reflexive invocation of Batemans principle, which is neither the only nor the best way to think about the problem.

Influence of environmental quality on pollen competitive ability in wild radish.

Pollen of Raphanus raphanistrum produced under low nutrient conditions sired fewer seeds than pollen produced under better conditions when the two types were applied on a stigma together. No difference was seen in single-donor crosses. Male mating success can be strongly influenced by the environmental conditions of pollen-bearing plants, a factor overlooked in studies of plant reproductive biology and in standard quantitative genetic crossing designs, where effects of male parent are equated with heritable genetic variation.

Seasonal Variation in Pollination Dynamics of Sexually Dimorphic Sidalcea Oregana SSP. Spicata (Malvaceae)

We explored the combined effects of seasonal variation in both pollinator assemblage and availability of pollen donors on pollination in a gynodioecious species, Sidalcea oregana ssp. spicata (Malvaceae). Hermaphrodites produced flowers with signif- icantly larger petals but maintained fewer open flowers per inflorescence than females. Flowers of hermaphrodites produced 50% more nectar sugar in the 24 h after anthesis than the flowers of females. Nectar sugar production was also significantly and positively cor- related with petal length. Pollinator visitation rates were influenced more by differences in petal length than by differences in flower number per inflorescence. Consequently, her- maphrodites experienced higher visitation rates on a per-flower basis. Female flowers tended to receive pollen at a lower rate than hermaphrodites, but remained in the receptive female-phase longer than hermaphrodites. On average, the length of the period of flower receptivity declined as pollen deposition rate increased. These opposing processes resulted in the sex morphs receiving equivalent levels of pollination. Seasonal variation in the rate of pollen reception was more strongly influenced by the efficiency of the available pollinator pool than by rates of visitation. Dramatic seasonal shifts in the composition of the pollinator assemblage and pollen availability were correlated with increased pollination intensity as the season progressed. Not only were more pollen grains received, but they arrived in a shorter period of time and the number of potential pollen donors (hermaphrodites) in- creased. These findings suggest that pollen competition in both sex morphs may be more intense late in the season.