Michael Zimmerman

Reproduction in Polemonium: assessing the factors limiting seed set

Evidence of pollen and/or resource limitation of seed production in the herbaceous perennial plant Polemonium foliosissimum was investigated. Large numbers of open flowers on selected individuals were hand-pollinated at regular intervals, and seed set was compared with that of control blossoms on those same plants as well as with that of control flowers on control individuals. Experimental and control individuals were monitored and reproductive output measured during the following flowering season as well. Although hand-pollinated flowers set significantly more seeds than did either set of control blossoms, the results suggest that individual reproduction (i.e., the total number of seeds produced by a plant) was limited by resources other than pollen. Pollen availability may also have limited seed production, but to a lesser extent. A proper protocol for examining the limitation of seed set is developed, and the biases inherent in most of the simplified procedures currently in use are discussed.

Reproduction in Polemonium: Competition for Pollinators

The flowering season of Polemonium foliosissimum Gray (Polemoniaceae) can be divided into two distinct periods. In the first, the bee:flower ratio is relatively high and large numbers of seeds per ovary are set. In addition, no correlation exists between the ollinator visitation rate to flowers and seed—set. In the second period the bee:flower ratio and seed—set are lower and a very strong correlation exists between pollinator service and seed—set. Pollinators are limiting resources during this portion of the season and those individual plants which can best attract pollinators have a strong selective advantage at this time. The evidence suggests that intraspecific exploitation competition for pollinator service might be occurring during this portion of the flowering season. See full-text article at JSTOR

REPRODUCTION IN POLEMONIUM: PRE-DISPERSAL SEED PREDATION'

Evidence is presented indicating that the anthomyiid seed predator, Hylemya sp., of Polemonium foliosissimum Gray (Polemoniaceae) preferentially attacks those plants which receive high numbers of pollinator visits. Seed mortality is high in ovaries which set many seeds (i.e., mortality is density-dependent) during the portion of the flowering season when pollinators are limiting re- sources and plants compete for pollinator service. The actions of both the seed predator and the pollinators of P. foliosissimum are responsible, in part, for the observed flowering phenology of this species.

Experimental manipulations of Polemonium foliosissimum: effects on subsequent nectar production, seed production and growth

(1) The carbon budgets of Polemonium foliosissimum individuals were experimentally manipulated. The amount of available carbon was decreased by defoliating 50% of each plant, while available carbon was enhanced by daily watering and by removing either 50% or 100% of the flower buds. (2) Rates of nectar production per flower were constant across all treatments except for a significant increase observed in the watered individuals. (3) There were no consistent changes in average seed set per flower or per plant across treatment groups. (4) Defoliated and control individuals did not differ significantly from one another with respect to average seed weight, but end-of-season underground biomass was significantly less for defoliated plants relative to controls. (5) In the year following treatment, no differences were observed in nectar production per flower, seed production per flower or seed production per plant, although a significant difference in rate of growth was observed. Defoliated plants grew least during the 2-year period, followed by control individuals. The debudded plants increased in size by the greatest margin. (6) The trade-offs between resources allocated to nectar, flowers, seeds and vegetative growth can apparently be expressed in terms of biomass.

Patchiness in the dispersion of nectar resources: Probable causes

SummaryPopulations of Delphinium nelsonii have been shown to be patchy with regard to standing crop of nectar available to pollinators (Pleasants and Zimmerman 1979). Plants with relatively large amounts of nectar (“hot plants”) are associated with other hot plants while plants with relatively small amounts of nectar (“cold plants”) are found near other cold plants. Two possible explantations for this pattern exist: 1. Plants in close proximity to one another may have similar nectar production rates or; 2. The patterns of foraging bumblebees might create the observed nectar distribution pattern. By sampling standing crop of nectar in a D. nelsonii population during periods of both high and low bumblebee abundance evidence was gathered indicating that hot and cold spots are caused by bumblebees.

Optimal foraging: Random movement by pollen collecting bumblebees

SummaryTwo bumblebee species, Bombus bifarius and B. flavifrons, forage randomly with respect to direction when gathering pollen on Potentilla gracilis. Bees avoid revisiting flowers by being able to differentiate recently visited from unvisited flowers. This recognition occurs while bees are flying over open flowers and appears to be a response to the amount of available pollen within flowers. Random foraging with respect to direction is the optimal strategy when the probability of flower revisitation is low. Bumblebees appear to be moving preferentially between nearest neighbors, again as predicted by foraging theory. This behavior causes the establishment of pollen patches in the P. gracilis population. Unlike other pollinators studied in similar situations, bumblebees on P. gracilis do not forage utilizing an area-restricted searching behavior. Because floral reward quality can be assessed at low cost by bees foraging on P. gracilis, their tendency to move to nearby flowers even after encountering a poor quality blossom apparently yields a higher rate of net energy intake than does area-restricted searching. The data indicate that bumblebees exhibit great plasticity in foraging behavior and that they are able to forage efficiently under a wide range of environmental conditions.

The effect of nectar production on neighborhood size

SummaryBumblebee foraging behavior was observed on two plant species with similar floral and inflorescence structures. One species produces nectar while the other does not. Bees, upon visiting nectar producing flowers tend to empty them of nectar and by frequently moving between close neighbors, create a patchily distributed resource base. Bees maximize their foraging efficiency in such an environment by using an area-restricted searching behavior and flying distances inversely correlated with the quality of reward received. Pollen collecting bumblebees do not create a patchy environment and maximize their foraging efficiency by more consistently moving shorter distances. Pollen collecting bumblebees are significantly more likely to revisit flowers and to visit more flowers per inflorescence than are nectar gathering bumblebees. These differences in foraging behavior increase the neighborhood size for nectar producing species and make it increasingly unlikely that random drift will be a dominant mode of evolution in populations of these species.

Pollen competition and sporophyte fitness in Brassica campestris: does intense pollen competition result in individuals with better pollen?

In flowering plants, competition among pollen grains can result in positively correlated responses in sporophytic vigor. Whether this indirect selection on the sporophyte can drive the evolution of sporophytic traits depends in part upon whether the,strength of pollen competition in any given (F0) generation is positively related to both the vigor of the F1 sporophytes and the ability of pollen from those sporophytes to sire seeds. We performed two experiments to address this question. In the first experiment we simulated three levels of pollen competition by manipulating pollen load size in the F0 generation and examined correlated responses in the F1 sporophytes (...)

Pollination biology of montane plants : Relationship between rate of nectar production and standing crop

Rates of floral nectar production and volumes of standing crops of floral nectar were measured for eight species of Colorado montane plants. Although significant positive correlations exist between the two variables, the magnitudes of the coefficients are not very great. This surprising result may be due to the large variability in rate of nectar production within each species. The patterns suggest that pollinators, by responding to standing crop volumes, may not exert very strong selective pressure on rate of floral nectar production. All species examined had similarly variable patterns of standing crop indicating that pollinators encounter a wide range of reward variability while collecting nectar, regardless of which plant species is being visited. INTRODUCTION It has become increasingly obvious in recent years that successful plant reproduction occurs not only as a function of the frequency of pollinator visitation but of subtler aspects of pollinator behavior as well (e.g, Pyke, 1981; Waser, 1983; Zimmerman, 1988). Not all combinations of matings, for example, yield equal results. Price and Waser (1979) and Waser and Price (1983) have found that optimal outcrossing distances exist for both Delphinium nelsonii and Ipomopsis aggregata; pollen moving this distance before landing on a stigma has a higher probability of producing more, high-quality seeds than does pollen arriving from either a shorter or longer distance. Also, a growing body of literature suggests that pollinator flight distances, and thus, ultimately, the distances pollen grains are carried, are significantly influenced by the amount of nectar reward received from flowers by pollinators (e.g., Pyke, 1978a; Heinrich, 1979; Zimmerman, 1982; Galen and Plowright, 1985; Zimmerman and Cook, 1985). Therefore the possibility exists that pollinators might provide selective pressure on plants to produce optimal quantities of floral nectar (Zimmerman, 1988). The concept that rate of floral nectar production by plants might be under selective pressure from pollinators is certainly not novel (Pyke, 1981; Pyke and Waser, 1981; Pleasants, 1983; Pleasants and Chaplin, 1983). Nectar production in a number of species has been found to have high heritability (Pedersen, 1953; Hawkins, 1971; Teuber and Barnes, 1978, 1979; Teuber et al., 1980, 1983) and to respond quickly to artificial selection (Hawkins, 1971). Although both of these characteristics are necessary if pollinators are to provide the selective force necessary to influence rate of nectar production, a third condition must also be met. Selection will operate only if a significant, positive relationship exists between rate of nectar production and standing crop of nectar. From the perspective of pollinators, standing crop is the more important of the two values because it, more directly, reflects the nature of the reward received. Standing crops, however, are determined both by the rate of nectar production and by the foraging behavior of pollinators. Surprisingly, not a single study has examined the relationship between rate of floral nectar production and the volume of standing crop of nectar found in flowers. It is certainly reasonable to expect that a significant positive relationship between nectar production and standing crop will exist. What is important from an evolutionary standpoint, however, is the strength of that relationship. Selection pressure on rate of nectar production cannot be very great when the two variables are only weakly linked. A weak correlation can arise in at least two manners. First, if high per flower visitation

Competition among Pollinators: Quantification of Available Resources

In a study of the bee community of short-grass prairie Tepedino and Stanton (1981) attempted to verify several predictions of competition theory by comparing abundances of bees and flowers. Their results were inconclusive suggesting, they said, that bees do not continuously compete for floral resources. We questioned their use of the number of open flowers as a measure of resource availability to bees because it is the nectar contained within those flowers that is the resource actually utilized. In two Rocky Mountain meadows we attempted to correlate bumblebee abundance with both the number of available flowers and the number of flowers multiplied by their 24 hour nectar production rates. Our results demonstrated that merely using floral abundance as an estimate of resource availability can be very misleading. The more time consuming method of quantifying the resources actually used by bees should be performed if meaningful conclusions concerning competition are to be drawn.