Baker highlighted high competitive ability as a characteristic of invasive species, although his focus was on competition through “special means” such as allelopathy and choking growth . In practice, invasive plant species may coexist with and outcompete natives through a variety of mechanisms. Niche differentiation, where species possess different strategies of resource use, may allow for coexistence of native and non-native invasive species . In this case, invasive species are functionally different than the natives, either by possessing novel traits or by using resources in different ways or at different times . For example, in many Mediterranean climate systems, invasive annual species display different phenology and function compared to the largely perennial or woody native communities . Alternatively, invasive species may succeed by possessing highly competitive traits . As an example, functional similarity did not predict competitive outcomes between native species and a focal invader in a California grassland; instead, competitive natives possessed trait values consistent with high rates of below ground resource acquisition and allocation to above ground tissue . Other studies have found that both niche and fitness differences operate within a given community . For example, Fried et al. found that native species with flowering phenology similar to a focal invader were adversely impacted by the presence of the invader . At the same time, native species with larger seeds and higher rates of resource acquisition were more competitive with the invader. As the relative importance of competition mechanisms is likely to change at fine scales across resource gradients , ebb and flood table experiments that manipulate resource availability and directly measure competition outcomes are likely to elucidate the mechanisms by which non-native invasive species can coexist with or competitively exclude native species.
Baker hypothesized that species possessing more ‘ideal weed’ traits would be more invasive: “probably no existing plant has them all; if such a plant should evolve it would be a formidable weed, indeed” . Trade-ofs likely limit the capacity for any species to possess all ‘ideal weed’ traits , but particular trait combinations may act synergistically . Thus, focusing on a single trait or a small handful of traits may not accurately characterize invasiveness; rather, exploring multidimensional functional differences between invasive and non-invasive species may yield greater insight into mechanisms of invasion . Traits may act in non-additive ways, as certain combinations of traits lead to success in particular conditions. For example, species with high rates of resource uptake and poorly defended tissues have the most to gain from enemy escape . Finally, different traits can result in similar fitness highlighting the need to consider multiple traits. For example, prostrate plants with strong lateral spread may shade out native plants just as effectively as tall plants . Thus, a multi-trait approach that accurately characterizes light use would be more meaningful than comparisons of mean height among invasive and non-invasive species. Many researchers have emphasized that traits or suites of traits interact with other processes, such as habitat suitability and socioeconomic factors, to influence invasion. In an effort to identify patterns of species-ecosystem interactions leading to invasion, Kuefer et al. coined the term ‘invasion syndrome’ which Novoa et al. redefined as ‘‘a combination of pathways, alien species traits, and characteristics of the recipient ecosystem which collectively result in predictable dynamics and impacts, and that can be managed effectively using specific policy and management actions’’. This synthetic approach involves an iterative process of identifying similar invasion events and their associated syndromes .
As an example, Novoa et al. point to invasive plant species in high elevation areas, which tend to share a broad environmental tolerance and a similar pathway of introduction along transportation corridors from low and mid elevation areas. Thus, managing for invasive plant species in high elevation areas entails limiting the spread of introduced species along corridors. However, as our review highlights, traits and species interactions within communities are dynamic, so an invasion syndrome approach would have to be flexible, potentially weakening the value of this framework.Phenotypic plasticity, or the ability of a plant to adjust its phenotype in response to environmental variation, was a defining feature of Baker’s ‘ideal weed’ . First, Baker and others hypothesized that plasticity could lead to success in a wide range of novel environments . Consistent with this hypothesis, plasticity is associated with increased species range size . Second, plasticity could lead to high success in certain environments . For example, invaders may be particularly adept at capitalizing on high resource conditions , opening ‘invasion windows’ when resources become abundant that allow for explosive population growth . Third, as we discuss in “Evolutionary considerations” section, plasticity can facilitate rapid evolution. Empirical evidence for the role of plasticity in invasions is mixed, however. While several large multi-species studies or meta-analyses find that invaders are more plastic than natives or non-invasive non-natives , others find that on average invasive and non-invasive species do not differ in plasticity . Interestingly, heightened plasticity is only adaptive and helps maintain fitness in a subset of species and only in response to resource increases; non-invasive plant taxa were better able to maintain fitness homeostasis in low resource conditions .
One possibility for these conflicting empirical observations is that plasticity, like other traits, may only be advantageous during certain invasion stages . A large, phylogenetically-controlled study investigating phenological plasticity in response to warming found that on average invasive species show strong phenological shifts in response to warming, while native species do not . These phenological shifts were strongest for species characterized as invasive and much weaker for non-invasive non-native species, and phenological plasticity was stronger for species that had invaded long ago, suggesting that phenological plasticity may be most important during the spread and impact stages and may increase over time through evolution .Baker and G. Ledyard Stebbins brought together evolutionary biologists and ecologists to consider the problem of invasive species and, in doing so, inserted an evolutionary perspective into the field of invasion biology . Evolutionary studies of invasive species were relatively slow to take of compared to the rapid increase in ecological works following Elton’s seminal work and the SCOPE series that followed several decades later . However, we now recognize that prior adaptation and rapid evolution during or post invasion can allow for establishment and promote the spread of invasive species. Evolutionary history reflects challenges a population has experienced in the past, and overcoming particular challenges may make it more likely for a species to be transported to, establish in, and successfully invade new areas. Post-introduction, rapid evolutionary responses to novel aspects of the invaded environment may be necessary for the invasive species to establish and spread. Because a population’s evolutionary history determines its traits, incorporating evolution into invasion biology may help explain why certain bio-geographic regions produce so many invasive species . Using quantitative genetics approaches that link traits to fitness may help inform which traits promote success in particular environments. Such studies could help explain the context dependency so firequently observed in ecological studies linking traits to invasions. Interestingly, only a few of Baker’s traits have been well investigated from an evolutionary perspective . One study explicitly focusing on Baker’s ‘ideal weed’ traits found evidence for genetic variation in traits related to competitive ability and seed production, indicating that such traits have the potential to evolve pre- or post-introduction, but growth rate exhibited little genetic variation . Furthermore, grow racks these traits were often genetically correlated, although not always in the same direction across the two populations studied, suggesting that genetic constraints may sometimes limit and other times accelerate the evolution of ‘ideal weed’ traits.The idea that evolutionary history determines invasion success has a long, but relatively sparse, history going back to at least Darwin’s seminal works.
Much of this work has investigated Darwin’s Naturalization Hypothesis which proposes that species lacking close relatives in the community are more likely to invade . This hypothesis assumes that because close relatives are likely to be functionally similar, competition may strongly limit closely related invaders compared to more distantly related invaders . The counter argument is that closely related species may have similar environmental tolerances and species interactions leading to increased likelihood of invasion by close relatives in the introduced range . Support for these competing hypotheses is decidedly mixed, but Ma et al. suggest that this may result from different processes acting across scales and invasion stages . For example, Darwin’s Naturalization Hypothesis specifically invoked competition, which occurs at very local scales. In contrast, the Pre-Adaptation Hypothesis more likely applies to the climatic factors more prevalent at regional scales. Across invasion stages, Darwin’s Naturalization Hypothesis most likely applies to the species interactions that come into play at later invasion stages post-establishment , while the Pre-Adaptation Hypothesis is more likely to pertain to the filtering processes that occur earlier in invasion . Darwin’s Naturalization Hypothesis and the Pre-Adaptation Hypothesis are both less focused on a general role for specific traits and more on the match between traits and the invaded environment. More recently, Fridley and Sax proposed the Evolutionary Imbalance Hypothesis, predicting that species from richer biotas with more stable environments and larger habitat sizes are more likely to be ecologically optimized with better solutions to ecological challenges. Essentially, these biogeographic regions have had a larger number of ‘evolutionary experiments.’ Because ecological conditions repeat across the world, better solutions in the native range are likely to lead to better solutions elsewhere too. In support of this hypothesis, phylogenetic diversity in the native range predicts invasiveness . While this hypothesis does not focus on particular traits underlying this success, it does point to a strong role for traits promoting competitive ability, like allelopathy and other mechanisms highlighted by Baker, and suggests that the traits that have evolved in the native range determine success in the invaded range. Evolutionary responses to human-modifed environments also have the potential to promote invasion. The Anthropogenically Induced Adaptation to Invade hypothesis posits that prior adaptation to human-disturbed environments in the native range facilitates invasion into similarly disturbed environments across the globe because human-disturbed environments share many similarities regardless of location . Adaptation to disturbed environments will also lead to increased abundance in areas firequented by humans, potentially contributing to increased dispersal. In this way, adaptation to disturbed environments increases the likelihood of transport and the probability of establishment once transported . While this hypothesis does not strongly focus on specific traits, instead generally focusing on adaptation to a particular environment, many traits highlighted by Baker are also thought to be adaptive in disturbed environments, including rapid growth rates, a propensity for selfng or vegetative reproduction, and high and continuous seed production. While challenging to definitively test, three types of evidence support the hypothesis. First, European taxa associated with human altered environments are much more likely to invade other continents than taxa found only in natural habitats, although it is less clear whether this advantage results from adaptation to those disturbed environments, from species sorting , or from increased likelihood of transport given their abundance in human-visited habitats . Second, in animal systems, association with human-altered habitats appears to allow for expansion of the climatic niche in the invaded range, suggesting that adaptation to human-disturbance may facilitate invasion and range expansion . Finally, laboratory studies suggest that pre-adaptation to novel environments rivals the effects of propagule pressure on introduction success . While the Anthropogenically Induced Adaptation to Invade hypothesis focuses more on adaptation to cultivated habitats, invasive species are also adapting to urban environments. This urban adaptation could lead to further trait-matching and colonization of geographically distant but environmentally similar habitats, particularly given the high abundance of invasives in cities and the high likelihood of human transport . Interestingly, some traits favored by urban environmental conditions may further facilitate invasiveness in other areas ; for example, the reduced pollinator abundance in urban ecosystem is predicted to select for increased selfing and clonality , two traits characterizing Baker’s ‘ideal weed’. However, urban conditions also have the potential to select for traits that inhibit invasion. For example, increased fragmentation in city landscapes can select for reduced dispersal that is likely to reduce the spread of invasive species at larger spatial scales .Over the past three decades, increasing evidence suggests that many invaders rapidly adapt to the novel environments they encounter post-introduction .