Nickolas M. Waser

Effects of local density on pollination and reproduction in Delphinium nuttallianum and Aconitum columbianum (Ranunculaceae).

Plant populations vary in density both naturally and as a consequence of anthropogenic impacts. Density in turn can influence pollination by animals. For example, plants in dense populations might enjoy more frequent visitation if pollinators forage most efficiently in such populations. We explored effects of plant density on pollination and seed set in the larkspur Delphinium nuttallianum and monkshood Aconitum columbianum. At our site in the Colorado Rocky Mountains, flowers of D. nuttallianum are pollinated primarily by queen bumble bees, solitary bees, and hummingbirds, whereas those of A. columbianum are pollinated primarily by queen and worker bumble bees. We found that the quantity of pollination service to both species (pollinator visitation rate and pollen deposition) was at best weakly related to density. In contrast, seed set declined by approximately one-third in sparse populations relative to nearby dense populations. This decline may stem from the receipt of low-quality pollen, for example, inbred pollen. Alternatively, sparsity may indicate poor environmental conditions that lower seed set for reasons unrelated to pollination. Our results demonstrate the value of simultaneously exploring pollinator behavior, pollen receipt, and seed set in attempting to understand how the population context influences plant reproductive success.

Experimental studies of pollen carryover: Hummingbirds and Ipomopsis aggregata

SummaryWe present results of experiments designed to identify floral characteristics that influence patterns of pollen carryover by hummingbirds visiting Ipomopsis aggregata flowers. We used fluorescent dye powders as pollen analogues. For all four experimental treatments considered, amounts of dye deposited on recipient stigmas declined linearly as a function of flower position in a visitation sequence. The decline was significantly steeper when recipient flowers had pollen-carrying anthers than when they did not; whereas degree of stigma clogging and presence or absence of empty anthers did not influence carryover. From this we conclude that presence of pollen on recipient flowers significantly reduces the average number of subsequent flowers reached by donor pollen. We discuss mechanisms for this effect and its significance for the evolution of floral structure.

PATTERNS OF SEED DISPERSAL AND POPULATION DIFFERENTIATION IN MIMULUS GUTTATUS

Traditionally, species have been viewed as homogeneous evolutionary units whose cohesion is maintained by gene flow within and between populations (e.g., Mayr, 1963, 1970; Dobzhansky, 1970; Grant, 1971). Recently this view has been questioned, especially for plants. Evidence now available suggests that gene flow in plant species usually is localized (Ehrlich and Raven, 1969; Levin and Kerster, 1974; Endler, 1977; Harper, 1977; Levin, 1979), and that microdifferentiation within and among populations is extensive (e.g., Epling et al., 1960; Solbrig and Simpson, 1974; Allard, 1975; Schaal, 1975; Levin, 1977). Indeed, Ehrlich and Raven (1969) and Levin (1979) have stressed that populations within a species may be kept from diverging extensively not because of gene flow but because they occupy habitats with similar sets of selective forces (see also Wright, 1932). It may be possible to determine the relative importance of gene flow and selection in maintaining the integrity of a species by examining local patterns of differentiation among populations. If gene flow is of primary importance, we expect similarity of two populations to be influenced by their physical proximity, whereas if selection is of importance, we expect similarity to be influenced by similarity of environments occupied. In this paper we correlate patterns of microdifferentiation among populations of the monkey flower Mimulus guttatus Fisch ex. D.C. (Scrophulariaceae) in the Wasatch Mountains of Utah to expected pat-

Spatial genetic heterogeneity in a population of the montane perennial plant Delphinium nelsonii

The montane perennial herb Delphinium nelsonii experiences restricted pollen and seed movement and spatial environmental heterogeneity, features likely to promote genetic differentiation within populations. To explore this possibility, allele frequencies at 5 polymorphic loci were characterised along transects through a meadow. F-statistics indicated substantial heterozygote excess within transect samples (overall FIS = −0·096). Possible contributors are sex-biased gene flow, and events occurring between pollination and ovule fertilisation that favor pollen from an “optimal outcrossing distance” of 3–10 m. Higher mating success over this optimal distance should cause longer realised gene dispersal than expected from pollen and seed movement, which are <1 m on average. It also may bring about a “leapfrog” dispersal pattern instead of a monotonie decline with distance from a source plant. F-statistics also indicated moderate genetic differentiation among samples (overall FST =0·069). However, this did not correspond to organised spatial pattern. Spatial autocorrelations showed that alleles were randomly distributed along transects. Although mean genetic distance increased with physical distance, the pattern was not statistically significant. Apparent random spatial heterogeneity is expected if gene flow is extensive enough to prevent a rapid decline in genetic correlation with distance; it also might be promoted by a leapfrog pattern of gene flow.

Adaptive Speciation: Ecological Speciation in Flowering Plants

Introduction Speciation refers to divergence of phenotype, and the evolution of reproductive barriers between populations that exhibit the different phenotypes. Either of these components of speciation may involve adaptation by natural selection. In this context several terms have been coined (see Chapter 1, in particular Box 1.1). Any speciation in which reproductive isolation follows directly or indirectly from adaptation to ecological conditions has been termed ecological speciation (e.g., Hatfield and Schluter 1999). In contrast, this volume defines the term adaptive speciation as lineage splitting in sympatry (or parapatry) through a process of frequency-dependent selection. This latter definition thus includes some, but not all, forms of ecological speciation and vice versa. In this chapter we adopt the broader term ecological speciation, as we review the forms it may take in flowering plants, or angiosperms, and explore whether there is evidence for adaptive speciation more specifically. Most of the about 300000 angiosperm species rely on animals for pollination, and the dramatic evolutionary radiation of the angiosperms over the past 100 million years is roughly contemporaneous with radiation in several animal taxa from which modern pollinators are drawn. As a result, pollination is thought by many investigators to be a critical ecological factor in speciation (Baker 1963; Crepet 1984; Eriksson and Bremer 1992; Grimaldi 1999; but see Ricklefs and Renner 2000). We outline several scenarios for pollinator-mediated speciation, paying special attention to the unusual role of pollinators as external agents of gene transfer.

Nectar Standing Crops in Delphinium Nelsonii Flowers: Spatial Autocorrelation among Plants?

Several aspects of nectarivore foraging behavior have been interpreted as responses to spatial reward patchiness of the kind documented for Delphinium nelsonii floral nectar by Pleasants and Zimmerman (1979). Working with this same species over 3 yr, however, we were unable to detect substantial pattern in nectar standing volumes, either through contingency analyses or spatial autocorrelation. Although spatial autocorrelations between rewards of neighboring plants were positive in 5 of 6 samples examined, only one value was statistically significant. Spatial autocorrelations over longer distances were erratic. We used computer simulations of nectarivores foraging in a large plant population to explore factors that promote reward patchiness. Simulations suggest that moderate patch- iness will develop at all but extremely low or high flower visitation rates. Rates were intermediate at our sites and those of Pleasants and Zimmerman, however, so visitation intensity does not seem to explain the discrepancy between our results and theirs. On the other hand, reward patchiness in simulations declined substantially as nectarivores exhib- ited less area-restricted foraging. Hummingbirds were important visitors at our sites, and fly farther between plants than the bumble bees that predominated at Pleasants and Zim- mermans sites. Finally, simulations suggest that spatial patchiness is lower when interplant coefficient of variation in nectar production rate is large, as in our populations.

Interaction rewiring and the rapid turnover of plant–pollinator networks

Whether species interactions are static or change over time has wide-reaching ecological and evolutionary consequences. However, species interaction networks are typically constructed from temporally aggregated interaction data, thereby implicitly assuming that interactions are fixed. This approach has advanced our understanding of communities, but it obscures the timescale at which interactions form (or dissolve) and the drivers and consequences of such dynamics. We address this knowledge gap by quantifying the within-season turnover of plant-pollinator interactions from weekly censuses across 3xa0years in a subalpine ecosystem. Week-to-week turnover of interactions (1) was high, (2) followed a consistent seasonal progression in all years of study and (3) was dominated by interaction rewiring (the reassembly of interactions among species). Simulation models revealed that species phenologies and relative abundances constrained both total interaction turnover and rewiring. Our findings reveal the diversity of species interactions that may be missed when the temporal dynamics of networks are ignored.