Rapid adaptation post-introduction could be necessary for successful establishment and persistence or might catalyze increased spread or impacts on native ecosystems . Indeed, many of the traits posited to characterize invasive species, by Baker and others more recently, show evolutionary change post-invasion . Several examples of rapid adaptation come from studies of invasive species . In part, this may be due to their suitability as evolutionary models ; by definition invaders are colonizing new environments and likely encountering different selection agents than they experienced in the past. However, it could also reflect unique characteristics of invasive species that make them particularly adept at rapid evolution. First, successful invaders are likely to escape constraints, at least in the short-term. When invasive species colonize new areas devoid of their enemies, they escape many of the strong selective agents that could constrain their evolutionary responses to other selective agents . Such constraints can limit adaptation and appear to do so for invasions . For example, resistance to generalist herbivores may be negatively genetically correlated with resistance to specialist herbivores . In the native range, grow table this strong trade-of may constrain evolutionary responses if the plant population is faced with both generalist and specialist herbivores.
In the invaded range, because specialist herbivores are likely absent, the direction of selection is no longer perpendicular to the direction of the genetic correlation, so stronger and more rapid evolutionary responses are possible . Second, admixture or the mixing of genetically differentiated populations following repeated invasion can enhance genetic variation, increase heterozygosity, and can sometimes yield extreme phenotypes that may be more successful at invading novel habitats than parental populations . Finally, some invaders are notoriously plastic, and plasticity plays two important roles in rapid adaptation: it can promote evolutionary rescue by ‘buying time’ and promoting population persistence until evolutionary changes occur , and it can potentially allow for genetic accommodation because plastic genotypes are likely to have the machinery underlying key adaptive traits that can then become canalized . For example, plastic increases in clonality in wetter environments allowed for increased likelihood of persistence in introduced sunfowers colonizing riparian habitats. Selection favoring increased clonality in these habitats then led to the evolution of increased invasiveness . Together, these factors make rapid evolution of invasive populations a common phenomenon and suggest that invaders may be particularly good at adapting to new conditions.
In fact, despite their shorter evolutionary history, invaders can be similarly or more locally adapted to local environmental conditions than natives , which is counter to Baker’s prediction that natives would be more likely to show finescale ecotypic differentiation while invasives may be more likely to rely on plasticity . Interestingly and unsurprisingly, given that particular traits are likely to be advantageous during some but not all invasion stages , the traits favored by natural selection also are likely to differ across invasion stages. For example, the North American forest under story invader Alliaria petiolota is likely successful because of its chemical warfare on the mycorrhizae that benefit competing natives. Over the course of the invasion, as native diversity declines and the competitive environment for Alliaria shifts from interspecific to intraspecific competition, the benefits of this chemical production are reduced and Alliaria evolves to produce less of the chemical, reducing its impacts on native communities . In this case, a novel weapon was useful during early colonization, but was selected against during later invasion stages.It is often assumed that invasive species, given their proven ability to successfully colonize and persist in novel environments, will be less affected by climate change than non-invasive species. ‘Ideal weed’ characteristics like broad environmental tolerance, flexible phenology and germination cues, and high propagule pressure may enable invasive species to weather changes in abiotic factors or take advantage of extreme climatic events . For example, early phenology coupled with rapid growth may increase the average size of reproductive individuals and the proportion of individuals that survive to reproduce, yielding higher seed production and population growth rates .
A meta-analyses of simulated climate change experiments suggests that invasive plant species respond positively to elevated temperature, precipitation, N deposition, and CO2 . However, another meta-analysis suggests that native and non-native terrestrial invaders responded similarly to many of these same global changes . In some regions, climate change may increase stress, and this may put resource-acquisitive invasive species at a disadvantage. For example, studies in California grassland and shrubland systems found that drought decreases individual and community level performance of annual invaders . Species responses to climate change are not just determined by their immediate short-term response to climate variables. Species may also succeed under future climates by migrating to more suitable habitats or by adapting. The traits favored at various stages of the invasion process directly affect these two longer-term responses. First, successful invaders are often good dispersers. This heightened capacity for dispersal should increase their ability to migrate and keep pace with climate change. Second, a number of characteristics of invaders may speed up the pace of adaptation, both to the novel environments faced during invasion but also to the novel environments faced post-establishment in response to climate change . While many studies have considered the direct, immediate effects of simulated climate change on invasive vs. native species performance, the propensity for traits to promote migration and adaptation in invasive species and what this means for longer term responses to climate change is less well-studied.Despite widespread recognition that traits are not static and can evolve, researchers often focus on species mean trait values while ignoring the substantial variation both within and between populations of a given species . This intraspecific trait variation can sometimes rival the effects of interspecific variation on ecological processes and is likely important to invasion success. First, intraspecific trait variation may contribute to the strong associations between propagule number and invasion success and may also help explain why multiple introductions often increase invasion success. Both higher propagule densities and multiple introductions are likely to increase the number of genotypes introduced and, therefore, the likelihood of including a genotype well-matched to the introduced environment. For example, increased genetic diversity of Arabidopsis thaliana accessions increased colonization success both through sampling effects and complementarity effects . Second, intraspecific trait variation combined with multiple introductions could lead to rapid increases in range size in the invaded region. Observed clines in introduced populations can result from the repeated introduction of different populations rather than post-introduction evolution . As a result, range expansions post-invasion may benefit from additional introductions rather than the slower process of evolution, leading to local adaptation. One might also expect intraspecific trait variation and the introduction of multiple populations to play a similar role in the colonization of disparate environmental conditions. For example, invaders originating from high nutrient sites in the native range may be the colonizers of high nutrient environments in the invaded range, while invaders originating from low nutrient stressful conditions may promote the colonization of low nutrient habitats.Where trait differences between invasive and non-invasive species exist, it is critical to demonstrate that these differences lead to enhanced fitness for the invader .
Studies that examine how traits influence vital rates can be challenging to implement for a large number of species, grow rack but may be particularly insightful . For example, sexual reproduction enhanced population growth rates of some invasive plant species relative to their noninvasive relatives, although this did not apply to all invaders examined . The effect of trait-environment interactions on invader performance and demographic processes is even less explored . Traits may align with some ecological filters but not others; for example, resource acquisitive traits such as high SLA and rapid growth may be advantageous in grazed systems but disadvantageous if mean annual precipitation declines . Understanding trait-environment interactions has important implications for invasive species management and may complement existing tools like habitat suitability models, which currently do not include traits or account for trait evolution .Interest in using organic cover crops and soil amendments is rapidly increasing in California as organic acreage expands. In the northeast corner of the state, several Klamath Basin producers are experimenting with transferring substantial acreage to organic production. Crops commonly grown in rotation in the area include small grains, firesh-market potatoes and alfalfa. In 2016, over 4,200 acres of potatoes and 13,100 acres of wheat were produced organically in California . Prices for wholesale organic crops are regularly higher than prices for conventional crops . In the case of firesh-market potatoes, organic prices can exceed 185% of conventional prices . On the other hand, organic management of nutrient deficiencies and pest problems is challenging. Nitrogen is a limiting nutrient in many California soils, especially when potatoes and grass crops are grown in multi-year rotations . Most potato varieties require at least 200 pounds of nitrogen per acre, from all sources, to maximize yield and quality . Potatoes also require a steady source of nitrogen throughout the growing season to prevent yield reductions and physiological disorders . Common organic farming practices for increasing soil nitrogen include using certified amendments, such as manures, or growing cover crops . Manure, compost and organic fertilizers derived from animal and plant matter contain several plant nutrients, including nitrogen . Manures are especially beneficial to soils deficient in phosphorus or potassium because the percentages of phosphorus and potassium found in most manure types are similar to or greater than the percentage of nitrogen found in the same manure type . Cover crops have long been identified as beneficial to soil health because of their ability to increase soil carbon, decrease soil erosion and increase water infiltration . Cover crops also influence soil nutrient recycling and nutrient availability. This is especially true of legumes — which, through a symbiotic relationship with bacteria, fix atmospheric nitrogen . When legume leaves and stems decompose, plant-available nitrogen is added to the soil . A challenging aspect of using amendments and cover crops to fertilize potatoes is accurately predicting when the nitrogen in these products will become available to the crop . Adequate nitrogen must be available at potato planting to support vegetative vine growth and tuber set, while nitrogen availability in mid-summer is critical for tuber bulking . Nutrient mineralization is driven by the decomposition of organic compounds into soluble inorganic forms that are available to plants . Since cover crops and manures are composed of organic material, farmers rely on the mineralization process to draw from these products plant-available nitrogen that can feed their crops. Many factors influence a material’s mineralization, including the carbonto-nitrogen ratio of the material, soil temperature, soil moisture and soil type. Cover crops can have a positive or negative influence on potato pests such as weeds, nematodes, diseases and insects . Several plant species in the Brassica genus have been shown to produce high levels of glucosinolates, which can facilitate bio-fumigation when incorporated into the soil . Oilseed radish has been shown to serve as a trap crop for cyst nematode . Some cover crops can promote potato diseases and nematodes by serving as a host and green bridge . Barriers to widespread use of cover crops and other organic amendments in potatoes include costs related to materials, labor, transportation and application. Cover crops require time and resources to manage and do not provide the benefit of crop revenue. In urban areas, strong odors from manures are a disincentive . Cover crops and amendments with a high carbon-to-nitrogen ratio, such as grasses and brown composts, can often lead to a temporary immobilization of plant-available nitrogen , which is the opposite of the effect that potato growers are pursuing. In northeast California, nitrogen immobilization and the opportunity cost of cover cropping can be particularly problematic because growers have a small window of frost-firee days in which to grow crops.For this research project, multiple studies were conducted from 2014 to 2017 at the UC ANR Intermountain Research and Extension Center in Tulelake, Siskiyou County, to evaluate the influence of cover crops, amendments and combinations of the two in a potato crop grown without synthetic fertilizers and pesticides. All cover crop and amendment trials were conducted alongside control treatments that included an unamended control as well as urea applications of 75 and 150 pounds of nitrogen per acre. All treatments were replicated four times.