Direct tracking of pollen with quantum dots reveals surprising uniformity in dispersal distance across 11 populations of an annual plant.

PREMISE: Pollen movement is a crucial component of dispersal in seed plants. Although pollen dispersal is well studied, methodological constraints have made it challenging to directly track pollen flow within multiple populations across landscapes. We labeled pollen with quantum dots, a new technique that overcomes past limitations, to evaluate the spatial scale of pollen dispersal and its relationship with conspecific density within 11 populations of Clarkia xantiana subsp. xantiana, a bee-pollinated annual plant. METHODS: We used experimental arrays in two years to track pollen movement across distances of 5-35 m within nine populations and across distances of 10-70 m within two additional populations. We tested for distance decay of pollen dispersal, whether conspecific density modulated dispersal distance, and whether dispersal kernels varied among populations across an environmentally complex landscape. RESULTS: Labeled pollen receipt did not decline with distance over 35 m within eight of nine populations or over 70 m within either of two populations. Pollen receipt increased with conspecific density. Overall, dispersal kernels were consistent across populations. CONCLUSIONS: The surprising uniformity in dispersal distance within different populations was likely influenced by low precipitation and plant density in our study years. This suggests that spatiotemporal variation in the abiotic environment substantially influences the extent of gene flow within and among populations.

Are genetic variation and demographic performance linked?

Quantifying relationships between genetic variation and population viability is important from both basic biological and applied conservation perspectives, yet few populations have been monitored with both long-term demographic and population genetics approaches. To empirically test whether and how genetic variation and population dynamics are related, we present one such paired approach. First, we use eight years of historical demographic data from five populations of Boechera fecunda (Brassicaceae), a rare, self-compatible perennial plant endemic to Montana, USA, and use integral projection models to estimate the stochastic population growth rate (λ S) and extinction risk of each population. We then combine these demographic estimates with previously published metrics of genetic variation in the same populations to test whether genetic diversity within populations is linked to demographic performance. Our results show that in this predominantly inbred species, standing genetic variation and demography are weakly positively correlated. However, the inbreeding coefficient was not strongly correlated with demographic performance, suggesting that more inbred populations are not necessarily less viable or at higher extinction risk than less inbred populations. A contemporary re-census of these populations revealed that neither genetic nor demographic parameters were consistently strong predictors of current population density, although populations showing lower probabilities of extinction in demographic models had larger population sizes at present. In the absence of evidence for inbreeding depression decreasing population viability in this species, we recommend conservation of distinct, potentially locally adapted populations of B. fecunda rather than alternatives such as translocations or reintroductions.

Moving beyond the "Diversity Paradox": The Limitations of Competition-Based Frameworks in Understanding Species Diversity.

AbstractOver the past century, ecologists have attempted to understand patterns of species diversity by showing stable coexistence arising from a baseline expectation of competitive exclusion. This expectation stems from an explicit assumption of resource scarcity and implicit assumptions of Malthusian struggle and winner-takes-all dynamics. Fidelity to the competitive exclusion principle (CEP) presents species diversity as a paradox: if species compete for limited resources, how can they coexist? In this article, we investigate the contradiction between the theoretical expectation of competitive exclusion and the empirical prevalence of multispecies communities. We trace the persistence of the CEP in ecological research despite numerous challenges and explore publishing trends suggesting that this framework has resulted in a disproportionate focus on competition and exclusion in contemporary research. From a critical science studies perspective, we analyze the sociopolitical factors that have contributed to these patterns. We argue that we must excavate the ideological foundation on which competition-based coexistence research has been built to move beyond the current perceived "diversity paradox." To that end, we propose shifting the baseline expectation of coexistence research, introducing the notion of a coexistence principle, which positions the persistence of multispecies communities as the rule rather than the exception in nature.

Ecological factors influence balancing selection on leaf chemical profiles of a wildflower.

Balancing selection is frequently invoked as a mechanism that maintains variation within and across populations. However, there are few examples of balancing selection operating on loci underpinning complex traits, which frequently display high levels of variation. We investigated mechanisms that may maintain variation in a focal polymorphism-leaf chemical profiles of a perennial wildflower (Boechera stricta, Brassicaceae)-explicitly interrogating multiple ecological and genetic processes including spatial variation in selection, antagonistic pleiotropy and frequency-dependent selection. A suite of common garden and greenhouse experiments showed that the alleles underlying variation in chemical profile have contrasting fitness effects across environments, implicating two ecological drivers of selection on chemical profile: herbivory and drought. Phenotype-environment associations and molecular genetic analyses revealed additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, probably caused by pleiotropic effects of secondary chemical biosynthesis genes on herbivore defence and drought response.

Relaxation of putative plant defenses in a tropical agroecosystem.

Evidence of the effects of agriculture on natural systems is widespread, but potential evolutionary responses in nontarget species are largely uncharacterized. To explore whether exposure to agrochemicals may influence selective pressures and phenotypic expression in nonagricultural plant populations, we characterized the expression of putative antiherbivore defense phenotypes in three nonagricultural species found upstream and downstream of irrigated rice fields in Guanacaste Province, Costa Rica. We found that plants downstream of chemically intensive agriculture showed shifts toward reduced expression of putative antiherbivore defenses relative to upstream counterparts. In two of three tested species, leaf extracts from downstream plants were more palatable to a generalist consumer, suggesting a possible reduction of chemical defenses. In one species with multiple modes of putative defenses, we observed parallel reductions of three metrics of putative biotic and physical defenses. These reductions were concurrent with reduced herbivore damage on downstream plants. Together, these results suggest that agriculture has the potential to alter intraspecific phenotypic expression, ecological interactions, and natural selection in nontarget plant populations.

Long-Term Effects of Copper Nanopesticides on Soil and Sediment Community Diversity in Two Outdoor Mesocosm Experiments.

The use of novel pesticides containing nanomaterials (nanopesticides) is growing and is considered a promising approach to reduce the impacts of agriculture on the environment and human health. However, the environmental effects of these novel agrochemicals are not fully characterized, and more research is needed to determine the benefits and risks they confer. Here, we assessed the impacts of repeated exposures to a Cu(OH)2 nanopesticide on the soil and sediment biodiversity of target (terrestrial) and nontarget (wetland) ecosystems by performing long-term outdoor mesocosm experiments. As pesticides are often used concomitantly with other agrochemicals, we also tested for interactive effects between nanopesticide exposure and fertilization treatments in both ecosystems. We used high-throughput sequencing on three marker genes to characterize effects on bacterial, fungal, and total eukaryotic community structure and diversity. Interestingly, we found limited effects of nanopesticide exposure on the terrestrial soil communities. Conversely, we found significant shifts in the sediment communities of the wetland mesocosms, especially for eukaryotes (protists, fungi, and algae). In the absence of fertilization, fungal and total eukaryotic community compositions exposed to nanopesticides for long periods of time were distinct from unexposed communities. We identified 60 taxa that were significantly affected by nanopesticide exposure, most of which were microeukaryotes affiliated to cercozoans, Gastrotricha, or unicellular algal taxa. Our study suggests that this nanopesticide has limited effects on the soil biodiversity of a target terrestrial agroecosystem, while nontarget aquatic communities are more sensitive, particularly among protists which are not targeted by this bactericide/fungicide.

Effects of maternal source and progeny microhabitat on natural selection and population dynamics in Alliaria petiolata.

PREMISE: The success or failure of propagules in contrasting microhabitats may play a role in biological invasion. We tested for variation in demographic performance and phenotypic trait expression during invasion by Alliaria petiolata in different microhabitats. METHODS: We performed a reciprocal transplant experiment with Alliaria petiolata from edge, intermediate, and forest understory microhabitats to determine the roles of the environment and maternal source on traits, fecundity, population growth rates (λ), and selection. RESULTS: Observations of in situ populations show that edge populations had the highest density and reproductive output, and forest populations had the lowest. In experimental populations, population growth rates and reproductive output were highest in the edge, and the intermediate habitat had the lowest germination and juvenile survival. Traits exhibited phenotypic plasticity in response to microhabitat, but that plasticity was not adaptive. There were few effects of maternal source location on fitness components or traits. CONCLUSIONS: Alliaria petiolata appears to be viable, or nearly so, in all three microhabitat types, with edge populations likely providing seed to the other microhabitats. The intermediate microhabitat may filter propagules at the seed stage, but discrepancies between in situ observations and experimental transplants preclude clear conclusions about the role of each microhabitat in niche expansion. However, edge microhabitats show the highest seed output in both analyses, suggesting that managing edge habitats might reduce spread to the forest understory.

Ecological causes and consequences of flower color polymorphism in a self-pollinating plant (Boechera stricta).

Intraspecific variation in flower color is often attributed to pollinator-mediated selection, yet this mechanism cannot explain flower color polymorphisms in self-pollinating species. Indirect selection mediated via biotic and abiotic stresses could maintain flower color variation in these systems. The selfing forb, Boechera stricta, typically displays white flowers, but some individuals produce purple flowers. We quantified environmental correlates of flower color in natural populations. To disentangle plasticity from genotypic variation, we performed a multiyear field experiment in five gardens. In controlled conditions, we evaluated herbivore preferences and the effects of drought stress and soil pH on flower color expression. In natural populations, purple-flowered individuals experienced lower foliar herbivory than did their white-flowered counterparts. This pattern also held in the common gardens. Additionally, low-elevation environments induced pigmented flowers (plasticity), and the likelihood of floral pigmentation decreased with source elevation of maternal families (genetic cline). Viability selection favored families with pigmented flowers. In the laboratory, herbivores exerted greater damage on tissue derived from white- vs purple-flowered individuals. Furthermore, drought induced pigmentation in white-flowered lineages, and white-flowered plants had a fecundity advantage in the well-watered control. Flower color variation in selfing species is probably maintained by herbivory, drought stress, and other abiotic factors that vary spatially.