Jason P. Sexton

PLASTICITY AND GENETIC DIVERSITY MAY ALLOW SALTCEDAR TO INVADE COLD CLIMATES IN NORTH AMERICA

Two major mechanisms have been proposed to explain the ability of intro- duced populations to colonize over large habitat gradients, despite significant population bottlenecks during introduction: (1) Broad environmental tolerance—successful invaders possess life history traits that confer superior colonizing ability and/or phenotypic plasticity, allowing acclimation to a wide range of habitats. (2) Local adaptation—successful invaders rapidly adapt to local selective pressures. However, even with bottlenecks, many introduced species exhibit surprisingly high levels of genetic variation and thus the potential for evolutionary increases in invasive traits and plasticity. Here we assess the invasive potential of Tamarix ramosissima, by examining the degree of genetic differentiation within and among populations from the latitudinal extremes of its introduced range. Using growth chamber experiments we examined ecologically important variation in seedlings, both in trait means and their reaction norms across temperature environments. Although we found no genetic variation for gas exchange traits, within or among populations, we did find significant genetic variation for growth traits, both in the trait means and in the degree of plasticity in these traits. Northern ecotypes invested more in roots relative to southern ecotypes but only under low temperatures. Both ecotypes increased shoot investment in warm temperatures. Increased root investment in cold temperatures by northern ecotypes may increase their first winter survival. Genetic differences in seedling root investment may contribute to the ability of this species to successfully tolerate and invade a broader latitudinal range. Our data support a model in which both plasticity and adaptive evolution can contribute to the invasive potential of introduced species.

GENETIC ISOLATION BY ENVIRONMENT OR DISTANCE: WHICH PATTERN OF GENE FLOW IS MOST COMMON?

Gene flow among populations can enhance local adaptation if it introduces new genetic variants available for selection, but strong gene flow can also stall adaptation by swamping locally beneficial genes. These outcomes can depend on population size, genetic variation, and the environmental context. Gene flow patterns may align with geographic distance (IBD—isolation by distance), whereby immigration rates are inversely proportional to the distance between populations. Alternatively gene flow may follow patterns of isolation by environment (IBE), whereby gene flow rates are higher among similar environments. Finally, gene flow may be highest among dissimilar environments (counter‐gradient gene flow), the classic “gene‐swamping” scenario. Here we survey relevant studies to determine the prevalence of each pattern across environmental gradients. Of 70 studies, we found evidence of IBD in 20.0%, IBE in 37.1%, and both patterns in 37.1%. In addition, 10.0% of studies exhibited counter‐gradient gene flow. In total, 74.3% showed significant IBE patterns. This predominant IBE pattern of gene flow may have arisen directly through natural selection or reflect other adaptive and nonadaptive processes leading to nonrandom gene flow. It also precludes gene swamping as a widespread phenomenon. Implications for evolutionary processes and management under rapidly changing environments (e.g., climate change) are discussed.

Controlling invasive species in complex social landscapes

Control of biological invasions depends on the collective decisions of resource managers across invasion zones. Regions with high land-use diversity, which we refer to as “management mosaics”, may be subject to severe invasions, for two main reasons. First, as land becomes increasingly subdivided, each manager assumes responsibility for a smaller portion of the total damages imposed by invasive species; the incentive to control invasives is therefore diminished. Secondly, managers opting not to control the invasion increase control costs for neighboring land managers by allowing their lands to act as an invader propagule source. Coordination among managers can help mitigate these effects, but greater numbers ‐ and a wider variety ‐ of land managers occupying a region hinder collective action. Here, we discuss the challenges posed by management mosaics, using a case study of the yellow starthistle (Centaurea solstitialis) invasion in the Sierra Nevada foothills of California. We suggest that the incorporation of management mosaic dynamics into invasive species research and management is essential for successful control of invasions, and provide recommendations to address this need.

Gene flow increases fitness at the warm edge of a species’ range

According to theory, gene flow to marginal populations may stall or aid adaptation at range limits by swamping peripheral populations with maladaptive gene flow or by enhancing genetic variability and reducing inbreeding depression, respectively. We tested these contrasting predictions by manipulating patterns of gene flow of the annual plant, Mimulus laciniatus, at its warm range limit. Gene flow was experimentally applied by using crosses within warm-limit populations (selfed and outcrossed), between warm-limit populations, and between warm-limit and central range populations across two elevational transects. We measured the fitness of offspring in a common garden at the warm-edge species range limit. All sources of gene flow increased seedling emergence at the range limit, suggesting local inbreeding depression at both range limit populations; however, lifetime reproductive success only increased significantly when pollen originated from another warm-limit population. Center–to–warm-edge gene flow was maladaptive by delaying time to development at this warm, fast-drying range limit, whereas edge-to-edge gene flow hastened emergence time and time to reproduction. By empirically testing theory on the effects of gene flow on the formation of geographic range limits, we find benefits of gene flow among populations to be greatest when gene flow is between populations occupying the same range limit. Our results emphasize the overlooked importance of gene flow among populations occurring near the same range limit and highlight the potential for prescriptive gene flow as a conservation option for populations at risk from climate change.

Niche and range size patterns suggest that speciation begins in small, ecologically diverged populations in North American monkeyflowers (Mimulus spp.).

Closely related species (e.g., sister taxa) often occupy very different ecological niches and can exhibit large differences in geographic distributions despite their shared evolutionary history. Budding speciation is one process that may partially explain how differences in niche and distribution characteristics may rapidly evolve. Budding speciation is the process through which new species form as initially small colonizing populations that acquire reproductive isolation. This mode of species formation predicts that, at the time of speciation, sister species should have highly asymmetrical distributions. We tested this hypothesis in North American monkeyflowers, a diverse clade with a robust phylogeny, using data on geographical ranges, climate, and plant community attributes. We found that recently diverged sister pairs have highly asymmetrical ranges and niche breadths, relative to older sister pairs. Additionally, we found that sister species occupy distinct environmental niche positions, and that 80% of sister species have completely or partially overlapping distributions (i.e., are broadly sympatric). Together, these results suggest that budding speciation has occurred frequently in Mimulus, that it has likely taken place both inside the range and on the range periphery, and that observed divergences in habitat and resource use could be associated with speciation in small populations.

Niche partitioning between close relatives suggests trade-offs between adaptation to local environments and competition

Niche partitioning among close relatives may reflect trade-offs underlying species divergence and coexistence (e.g., between stress tolerance and competitive ability). We quantified the effects of habitat and congeneric species interactions on fitness for two closely related herbaceous plant species, Mimulus guttatus and Mimulus laciniatus, in three common habitat types within their sympatric range. Drought stress strongly reduced survival of M. guttatus in fast-drying seeps occupied by M. laciniatus, suggesting that divergent habitat adaptation maintains this niche boundary. However, neither seedling performance nor congeneric competition explained the absence of M. laciniatus from shady streams where M. guttatus thrives. M. laciniatus may be excluded from this habitat by competition with other species in the community or mature M. guttatus. Species performance and competitive ability were similar in sympatric meadows where plant community stature and the growing season length are intermediate between seeps and streams. Stochastic effects (e.g., dispersal among habitats or temporal variation) may contribute to coexistence in this habitat. Habitat adaptation, species interactions, and stochastic mechanisms influence sympatric distributions for these recently diverged species.

OCCURRENCE, PERSISTENCE, AND EXPANSION OF SALTCEDAR (TAMARIX SPP.) POPULATIONS IN THE GREAT PLAINS OF MONTANA

Abstract Saltcedar (Tamarix spp.), a shrub native to Eurasia, is associated with major alterations to wetland and riparian systems in the southwestern United States. Since the 1960s saltcedar has been naturalized in northern states of the U.S. where its growth potential and impacts are not well known. Here, we describe the occurrence, age, size, and relative cover of saltcedar populations in several river basins in central eastern Montana, USA, to identify potential patterns of spread across the region and changes in individual populations as they age. Stands were aged according to the oldest saltcedar individuals and were sampled for dominant plant cover and soil properties. Multiple introductions appear to have occurred in Montana, with the oldest stands occurring on the Bighorn River in southern Montana. Saltcedar absolute and relative cover and stand area increased significantly with stand age, while native tree and shrub relative cover remained low across all stand ages. These results suggest that saltcedar stands establish where woody natives are not abundant and that they persist and expand over time. Although soil salinity remained constant, soil pH decreased with saltcedar stand age, indicating a possible effect of organic matter inputs. An analysis of annual wood increment of saltcedar and sandbar willow (a native with analogous growth form) stems along a latitudinal gradient showed that stem growth of both species did not differ significantly among regions. Stem growth decreased inversely with elevation for both species while growth responses to elevation did not differ between species. Our results show an increase in number of populations and continued viability of these populations. Mechanisms of saltcedar increases in this region are yet to be determined. Anthropogenic influences, such as saltcedar plantings, watershed alterations (e.g., river flow control), and habitat disturbances (e.g., cattle grazing or habitat clearing) may facilitate its spread in similar climates of the Great Plains.

Speciation on a local geographic scale: the evolution of a rare rock outcrop specialist in Mimulus

Speciation can occur on both large and small geographical scales. In plants, local speciation, where small populations split off from a large-ranged progenitor species, is thought to be the dominant mode, yet there are still few examples to verify speciation has occurred in this manner. A recently described morphological species in the yellow monkey flowers, Mimulus filicifolius, is an excellent candidate for local speciation because of its highly restricted geographical range. Mimulus filicifolius was formerly identified as a population of M. laciniatus due to similar lobed leaf morphology and rocky outcrop habitat. To investigate whether M. filicifolius is genetically divergent and reproductively isolated from M. laciniatus, we examined patterns of genetic diversity in ten nuclear and eight microsatellite loci, and hybrid fertility in M. filicifolius and its purported close relatives: M. laciniatus, M. guttatus and M. nasutus. We found that M. filicifolius is genetically divergent from the other species and strongly reproductively isolated from M. laciniatus. We conclude that M. filicifolius is an independent rock outcrop specialist despite being morphologically and ecologically similar to M. laciniatus, and that its small geographical range nested within other wide-ranging members of the M. guttatus species complex is consistent with local speciation.

Climate structures genetic variation across a species' elevation range: a test of range limits hypotheses

Gene flow may influence the formation of species range limits, and yet little is known about the patterns of gene flow with respect to environmental gradients or proximity to range limits. With rapid environmental change, it is especially important to understand patterns of gene flow to inform conservation efforts. Here we investigate the species range of the selfing, annual plant, Mimulus laciniatus, in the California Sierra Nevada. We assessed genetic variation, gene flow, and population abundance across the entire elevation‐based climate range. Contrary to expectations, within‐population plant density increased towards both climate limits. Mean genetic diversity of edge populations was equivalent to central populations; however, all edge populations exhibited less genetic diversity than neighbouring interior populations. Genetic differentiation was fairly consistent and moderate among all populations, and no directional signals of contemporary gene flow were detected between central and peripheral elevations. Elevation‐driven gene flow (isolation by environment), but not isolation by distance, was found across the species range. These findings were the same towards high‐ and low‐elevation range limits and were inconsistent with two common centre‐edge hypotheses invoked for the formation of species range limits: (i) decreasing habitat quality and population size; (ii) swamping gene flow from large, central populations. This pattern demonstrates that climate, but not centre‐edge dynamics, is an important range‐wide factor structuring M. laciniatus populations. To our knowledge, this is the first empirical study to relate environmental patterns of gene flow to range limits hypotheses. Similar investigations across a wide variety of taxa and life histories are needed.

Extensive variation, but not local adaptation in an Australian alpine daisy

Abstract Alpine plants often occupy diverse habitats within a similar elevation range, but most research on local adaptation in these plants has focused on elevation gradients. In testing for habitat‐related local adaptation, local effects on seed quality and initial plant growth should be considered in designs that encompass multiple populations and habitats. We tested for local adaptation across alpine habitats in a morphologically variable daisy species, Brachyscome decipiens, in the Bogong High Plains in Victoria, Australia. We collected seed from different habitats, controlled for maternal effects through initial seed size estimates, and characterized seedling survival and growth in a field transplant experiment. We found little evidence for local adaptation for survival or plant size, based on three adaptation measures: Home versus Away, Local versus Foreign, and Sympatric versus Allopatric (SA). The SA measure controlled for planting site and population (site‐of‐origin) effects. There were significant differences due to site‐of‐origin and planting site effects. An important confounding factor was the size of plants directly after transplantation of seedlings, which had a large impact on subsequent seedling survival and growth. Initial differences in plant width and height influenced subsequent survival across the growing season but in opposing directions: wide plants had higher survival, but tall plants had lower survival. In an additional controlled garden experiment at Cranbourne Royal Botanic Gardens, site‐of‐origin effects detected in the field experiments disappeared under more benign homogeneous conditions. Although B. decipiens from different source areas varied significantly when grown across a range of alpine habitats, these differences did not translate into a local or habitat‐related fitness advantage. This lack of local advantage may signal weak past selection, and/or weak adaptive transgeneration (plasticity) effects.