The recommendations comprehensively address bio-fuel production and use, as well as the necessity of agency and private sector stakeholder cooperation for effective implementation of the recommendations . Initially, all federal agencies with authority relevant to bio-fuel production should be identified, their likely responsibilities on the invasiveness issue determined, and their ability to minimize the risk of bio-fuel escape and invasion strengthened as necessary. To reduce the risk of escape, the bio-fuel crops that are promoted should not be currently invasive or should pose a low risk of becoming invasive in the target region. In addition, bio-fuel crops should be propagated in production sites that are least likely to impact sensitive habitat or create disturbances that would facilitate invasion. Most importantly, effective mitigation protocols need to be developed to prevent dispersal of plant propagules from sites of production, transportation corridors, storage areas and processing facilities. Minimizing harvest disturbance can also reduce the potential for dispersal and off-site movement of propagules. Prior to wide scale planting, multi-year eradication protocols should be developed that are based on integrated pest management strategies. Such practices should be readily available, and appropriate information should be distributed with the purchase of bio-fuel crop seeds. These control methods are not only critical for preventing the dispersal of bio-fuel crops from abandoned production sites, they are a necessary component of an effective early detection and rapid response system for bio-fuel crop populations that do escape active management. Throughout this entire process , flood and drain table all stakeholder groups should be engaged, from bio-fuel development to conversion.The question of how species successfully invade new areas has fascinated scientists for over a century .

By studying ruderal and agricultural weeds invading empty niches, Herbert Baker began to identify characteristics associated with invasiveness, which resulted in a list of traits describing the ‘ideal weed’ . Work in subsequent decades examined a wide range of traits using comparative approaches of taxonomically-related species and regional floras . With these studies came an increasing realization that factors contributing to invasiveness are strongly influenced by the stage of invasion, characteristics of the introduced range, and which species groups are being compared. These realizations, combined with discrepancies across studies, resulted in some skepticism that traits associated with invasiveness could be generalized . However, there is support for the idea that invasive species differ from non-invasive native and non-native species in key attributes depending on the environmental context . Here, we explore how ecological and evolutionary theory has refined our understanding of the ‘ideal weed’. We do not provide an exhaustive review of all traits but rather an overview of key functional and evolutionary frameworks in which progress has been made.Baker’s ‘ideal weed’ possessed a general-purpose phenotype , life history traits that permit reproduction from a single individual , rapid growth, and high, continuous seed output . Several of these characteristics are well studied and appear to be common when evaluated across different invasive taxa such as high germination success across environments , selfing , and rapid growth rate , while others are less studied . In recent decades, researchers have broadened the search for ‘weedy’ characteristics to include traits related to resource acquisition and use that underlie rapid growth, competitive ability, and even stress tolerance. Syntheses of regional and global for as have demonstrated that, relative to non-invasive species, invasive species are generally larger, have higher specific leaf area , allocate relatively more biomass to leaves and stems at the expense of roots, and use resources more efficiently .

However, there are exceptions to every rule. Identifying traits associated with invasive species is hindered by differences in how invasiveness is defined, bias in species selection for experiments, and challenges comparing species at different stages of invasion . However, several useful frameworks have been developed to evaluate traits within relevant contexts. First, many researchers recommend controlling for a species’ commonness when selecting species for experiments as comparisons among common invasives and rare non-invasive species may lead to spurious conclusions . For example, invasive species appear to be more competitive than co-occurring natives ; however, many of these studies focus on particularly aggressive and common invaders. In a comparison of annual plants in Germany, Zhang and van Kluenen found that invasive species were stronger competitors only when comparing common invaders with rare natives. In essence, comparing species that are similarly successful should allow researchers to identify traits that promote invasion in particular, rather than commonness more generally. In another effort to standardize how invasiveness is defined, Catford et al. proposed comparing traits of invasive species within invasiveness categories based on four demographic dimensions: local abundance, geographic range, environmental range, and spread rate. One trait may promote invasiveness along one dimension but limit invasion along another . Time since introduction and propagule pressure would ideally be incorporated into invasiveness categories , but these data are not available for many species. Perhaps the most comprehensive effort to link traits to different stages of invasion is that of van Kluenen et al. who proposed a nested, multi-scale approach . Identifying a universal set of traits that explains invasiveness is challenging because traits are dependent on environmental context, including specific abiotic and biotic factors arising from, for example, climate and community composition .

By accounting for spatial scale, the framework proposed by van Kluenen et al. avoids inappropriate comparisons of traits across different stages of invasion and resolves inconsistencies associated with context dependency. For example, studies have found that invasive species can have smaller, similar, or larger seeds compared to native or non-invasive species . However, this inconsistency likely reflects different ecological filters or processes across stages: smaller seeds are likely to be dispersed to new sites, but larger seeds have more resources for establishment and growth . Conversely, some traits may enhance invasiveness at multiple stages of invasion. For example, fast growth rates can assist with colonization of new or disturbed habitats , lead to priority effects , and ultimately affect competition outcomes in established communities . Finally, a trait-based community assembly framework may also elucidate mechanisms of invasion . Community assembly theory allows for both stochastic and niche-based processes at various scales. Species composition within a community is determined by a series of ecological filters that sort species based on their traits . As an example, seed predation is a strong biotic filter on recruitment in some systems and this may favor species with smaller seeds that are more likely to evade predation from rodents . Investigating how trait-performance relationships change when a filter is manipulated can indicate if non-native invaders are succeeding by acting like the natives or by doing something different . Trait analyses can also determine if invasive species occupy empty niches. Work in desert annual communities in the southwest U.S. show that invasive annuals have unique trait combinations that allow them to grow fast and use water efficiently . Below, we expand on how traits and trait plasticity interact with abiotic and biotic filters to regulate invasion.Many invasive species thrive in resource-rich environments . Environments with ample light, water, or nutrient availability could favor fast-growing species that quickly take up available resources. Species associated with a resource acquisitive strategy have trait values aligned with the ‘fast-return’ end of leaf, plant, and root economic spectra . This includes cheaply constructed, short-lived tissues designed for high rates of carbon and nutrient assimilation and biomass allocation patterns that favor light interception and growth . These species may alter the system in a way that prevents slower-growing species from establishing and dominating. For example, hydroponic flood table the proliferation of invasive grasses in many systems suppresses woody seedling establishment via competition for limiting resources or increased fire firequency leading to a type conversion or invasion by other species . Many species can also invade low resource environments and they succeed by employing a wide range of strategies . Community assembly theory predicts that strong abiotic filters in stressful environments will result in co-occurring species with similar traits and there is some evidence for this in invaded systems. For example, species invading low resource systems are similarly or more efficient at using limiting resources relative to native species adapted to those systems .

There is also evidence that invasive species can succeed in low resource environments by possessing resource acquisitive traits. While native and invasive non-native annuals in semi-arid Mediterranean-climate ecosystems are similar with respect to most traits, invasive annuals were taller and had larger seeds and thinner roots—which likely enhances establishment and resource acquisition . Phenological differences, such as early germination, may allow invasive species to avoid competition from co-occurring species in low resource environments . Early phenology coupled with high resource-use efficiency or rapid growth may be particularly effective in low resource environments, such as deserts and coastal sage scrub in the southwestern U.S. . In sum, the fast growth rates and competitive strategies hypothesized by Baker appear to promote invasion in a range of habitats, but the specific physiological traits underlying these strategies differ across environments. Resource acquisition traits may be particularly useful in high resource environments, while efficient resource use or competitive strategies like early phenology may lead to invasion success in low resource environments. Finally, a central tenet of Baker’s ideology is that some invaders display broad environmental tolerance and are able to move past environmental filters by possessing traits that promote high fitness under low and high resource conditions. Some invasive species exhibit broad environmental tolerance by not conforming to growth-stress tolerance tradeoffs. For example, Norway maple is a common invader in North American forests and has high survival under low light conditions and high growth rates in full sun . Tree of heaven is one of the most invasive woody species in Europe and North America and its broad geographic distribution is driven by a combination of traits aligned with high resource acquisition as well as the ability to alter morphological traits and biomass allocation patterns across environments . During the invasion process plants may escape specialist enemies that limit their population growth in the native range . Such escape is typically transient, however, as invaders accumulate new enemies over time . The initial escape from enemies could allow for rapid establishment but, over longer timescales, three traits of invaders may make them particularly adept at overcoming the biotic filter created by enemies and promoting invasion. First, ruderal invaders can escape their enemies by virtue of their high dispersal, short lifespan, and low allocation to defense, fireeing up resources for rapid growth or competitive ability . Second and relatedly, many invaders appear to have high growth rates, which tend to reduce the cost of damage . This high growth rate means that invaders can withstand high amounts of enemy damage with limited effects on fitness . Consistent with this idea, in a multi-species study, invasive vines received just as much herbivory as natives or naturalized species, but were also more tolerant of damage , although other multispecies studies and metaanalyses find that invasives are similarly or even less tolerant to herbivory than natives . Third, native generalist enemies may have reduced preferences for non-native species with which they have no evolutionary history , although this appears not to be a general phenomenon across invasive species . Thus, both innate traits of the invader that Baker hypothesized would facilitate invasion and the match between invader traits and the invaded community may reduce the capacity for enemies to limit invader population growth. Like enemies, mutualists may also be left behind during the invasion process. As a result, successful invaders might be less dependent on mutualists , more generalist and able to interact with a wide variety of partners as predicted by Baker , or rely on co-invasion of mutualist partners . For example, selfng was one of Baker’s ‘ideal weed’ characteristics because it would allow reproduction in the absence of suitable pollinators and at low population densities. Selfers do appear to be over represented in invasive taxa although it is not clear whether this is because of the advantages of selfng when suitable pollinators aren’t available or because of Allee effects. For other species that fail to meet Baker’s criteria of generalized dispersal or pollination mechanisms , like that of highly specialized fgs which require a specific species of wasp pollinator or pines limited by appropriate mycorrhizae, invasion can still occur but only once the mutualist also invades.