Vincent M. Eckhart

Reproductive assurance and the evolutionary ecology of self-pollination in Clarkia xantiana (Onagraceae)

The reproductive assurance hypothesis posits that selection favors self-pollination in flowering plants where mates and/or pollinators are scarce. A corollary is that self-pollinating populations are expected to be superior colonizers of mate- and pollinator-scarce environments. The California annual Clarkia xantiana includes outcrossing populations (ssp. xantiana) and autogamously self-pollinating populations (ssp. parviflora). Outcrossing is ancestral, and the subspecies have parapatric distributions with a narrow contact zone. We tested aspects of the reproductive assurance hypothesis by examining geographic and subspecies variation in the densities of mates and pollinators (native bees) and the density dependence of pollinator visitation and pollen receipt. Plant and flower densities, pollinator density, and pollinator visitation rates were lowest in the region of exclusively self-pollinating populations. Pollinator assemblages there lacked Clarkia-associated pollinator taxa that were common elsewhere. Self-pollinating populations in the contact zone generally had densities and visitation rates intermediate between allopatric self-pollinating populations and outcrossing populations. Visitation rate and pollen receipt increased significantly with plant density. These findings suggest that selection for reproductive assurance influenced the origin of self-pollination and/or that reproductive assurance influenced the geographic distribution of self-pollination. Geographic variation in pollinator assemblages may have generated variation in the value of reproductive assurance.

Spatio-Temporal Variation in Abundance and Variation in Foraging Behavior of the Pollinators of Gynodioecious Phacelia linearis (Hydrophyllaceae)

Variation in pollinator assemblages at several levels can affect the likelihood of evolutionary specialization of plants for particular pollinators and patterns of natural selection on floral characters. This paper describes variation in a pollinator assemblage (mainly solitary bees) at three levels - geographic occurrence, abundance, and foraging behavior as a function of floral characters - for the gynodioecious annual Phacelia linearis (Hydrophyllaceae). The set of Hymenoptera that visit P. linearis flowers varies geographically (...)

The Geography of Demography: Long-Term Demographic Studies and Species Distribution Models Reveal a Species Border Limited by Adaptation

Potential causes of species’ geographic distribution limits fall into two broad classes: (1) limited adaptation across spatially variable environments and (2) limited opportunities to colonize unoccupied areas. Combining demographic studies, analyses of demographic responses to environmental variation, and species distribution models, we investigated the causes of range limits in a model system, the eastern border of the California annual plant Clarkia xantiana ssp. xantiana. Vital rates of 20 populations varied with growing season temperature and precipitation: fruit number and overwinter survival of 1-year-old seeds declined steeply, while current-year seed germination increased modestly along west-to-east gradients in decreasing temperature, decreasing mean precipitation, and increasing variation in precipitation. Long-term stochastic finite rate of increase, λs, exhibited a fourfold range and varied among geologic surface materials as well as with temperature and precipitation. Growth rate declined significantly toward the eastern border, falling below 1 in three of the five easternmost populations. Distribution models employing demographically important environmental variables predicted low habitat favorability beyond the eastern border. Models that filtered or weighted population presences by λs predicted steeper eastward declines in favorability and assigned greater roles in setting the distribution to among-year variation in precipitation and to geologic surface material. These analyses reveal a species border likely set by limited adaptation to declining environmental quality.

Reduced pollinator service and elevated pollen limitation at the geographic range limit of an annual plant

Mutualisms are well known to influence individual fitness and the population dynamics of partner species, but little is known about whether they influence species distributions and the location of geographic range limits. Here, we examine the contribution of plant-pollinator interactions to the geographic range limit of the California endemic plant Clarkia xantiana ssp. xantiana. We show that pollinator availability declined from the center to the margin of the geographic range consistently across four years of study. This decline in pollinator availability was caused to a greater extent by variation in the abundance of generalist rather than specialist bee pollinators. Climate data suggest that patterns of precipitation in the current and previous year drove variation in bee abundance because of its effects on cues for bee emergence in the current year and the abundance of floral resources in the previous year. Experimental floral manipulations showed that marginal populations had greater outcross pollen limitation of reproduction, in parallel with the decline in pollinator abundance. Although plants are self-compatible, we found no evidence that autonomous selfing contributes to reproduction, and thus no evidence that it alleviates outcross pollen limitation in marginal populations. Furthermore, we found no association between the distance to the range edge and selfing rate, as estimated from sequence and microsatellite variation, indicating that the mating system has not evolved in response to the pollination environment at the range periphery. Overall, our results suggest that dependence on pollinators for reproduction may be an important constraint limiting range expansion in this system.

Plant-soil water relations and species border of clarkia xantiana SSP. xantiana (onagraceae)

Low precipitation can limit plant distributions. Soil variation might interact with precipitation gradients to define species borders. Analyzing the eastern species border of the California annual Clarkia xantiana ssp. xantiana, we assessed the following: (1) the geography of plant water status, precipitation, and soil; (2) soil control of plant water status; and (3) water status control of plant performance. Plant water potential declined toward the border in consecutive years. Precipitation declined in parallel and was lower in the year of lower water potential. Many border soils are derived from metasedimentary rock, while igneous rock dominates the species range to the west and unoccupied areas to the east. Compared with igneous soils, metasedimentary soils were found to have greater penetration resistance and finer texture, and they exhibited greater water storage capacity but also stronger restrictions on water release. Within populations, plant water status was inferior on shallow slopes with hard, fine soils. Among populations, plant size and fruit production increased with water potential, though seedling‐to‐adult mortality was independent. Low precipitation and an abrupt change in bedrock likely contribute to the species border of C. xantiana ssp. xantiana. Understanding adaptation and distribution shifts under climate change requires a consideration of both climate and soils.

Experimental studies of adaptation in Clarkia xantiana. III. Phenotypic selection across a subspecies border

Sister taxa with distinct phenotypes often occupy contrasting environments in parapatric ranges, yet we generally do not know whether trait divergence reflects spatially varying selection. We conducted a reciprocal transplant experiment to test whether selection favors “native phenotypes” in two subspecies of Clarkia xantiana (Onagraceae), an annual plant in California. For four quantitative traits that differ between subspecies, we estimated phenotypic selection in subspecies’ exclusive ranges and their contact zone in two consecutive years. We predicted that in the arid, pollinator‐scarce eastern region, selection favors phenotypes of the native subspecies parviflora: small leaves, slow leaf growth, early flowering, and diminutive flowers. In the wetter, pollinator‐rich, western range of subspecies xantiana, we expected selection for opposite phenotypes. We investigated pollinator contributions to selection by comparing naturally pollinated and pollen‐supplemented individuals. For reproductive traits and for subspecies xantiana, selection generally matched expectations. The contact zone sometimes showed distinctive selection, and in ssp. parviflora selection sometimes favored nonnative phenotypes. Pollinators influenced selection on flowering time but not on flower size. Little temporal variation in selection occurred, possibly because of plastic trait responses across years. Though there were exceptions and some causes of selection remain obscure, phenotypic differentiation between subspecies appears to reflect spatially variable selection.

The ‘Hutchinsonian niche’ as an assemblage of demographic niches: implications for species geographic ranges

Hutchinson (1957) defined the ecological niche as a hypervolume shaped by the environmental conditions under which a species can “exist indefinitely”. Although several authors further discussed the need to adopt a demographic perspective of the ecological niche theory, very few have investigated the environmental requirements of different components of species’ life cycles (i.e. vital rates) in order to examine their internal niche structures. It therefore remains unclear how species’ demography, niches and distributions are interrelated. Using comprehensive demographic data for two well-studied, short-lived plants (Plantago coronopus, Clarkia xantiana), we show that the arrangement of species’ demographic niches reveals key features of their environmental niches and geographic distributions. In Plantago coronopus, opposing geographic trends in some individual vital rates, through different responses to environmental gradients (demographic compensation), stabilize population growth across the range. In Clarkia xantiana, a lack of demographic compensation underlies a gradient in population growth, which could translate in a directional geographic range shift. Overall, our results highlight that occurrence and performance niches cannot be assumed to be the same, and that studying their relationship is essential for a better understanding of species’ ecological niches. Finally, we argue for the value of considering the assemblage of species’ demographic niches when studying ecological systems, and predicting the dynamics of species geographical ranges. This article is protected by copyright. All rights reserved.

Biotic interactions limit the geographic range of an annual plant: herbivory and phenology mediate fitness beyond a range margin

Species’ range limits offer powerful opportunities to study environmental factors regulating distributions and probe the limits of adaptation. However, we rarely know what aspects of the environment are actually constraining range expansion, much less which traits are mediating the organisms’ response to these environmental gradients. Though most studies focus on climatic limits to species’ distributions, biotic interactions may be just as important. We used field experiments and simulations to estimate contributions of mammal herbivory to a range boundary in the annual plant Clarkia xantiana ssp. xantiana. A steep gradient of increasing probability of herbivory occurs across the boundary, and herbivory drives several-fold declines in lifetime fitness at and beyond the boundary. By including in our analyses data from a sister taxon with more rapid phenology, we show that delayed phenology drives C. xantiana ssp. xantiana’s susceptibility to herbivory and low fitness beyond its border.