There are thousands of case studies documenting invasive species’ impacts and these have led to the development of over 30 hypotheses that describe how invasions occur and their impacts manifest. The ...proliferation of invasion hypotheses over the decades has spurred several efforts to identify conceptual overlap and increase clarity of impact mechanism in the field. What is still lacking, however, is a comprehensive accounting of the evidence base on invasive species impacts, especially in regard to the biological scales, temporal scales, and taxonomic groups that receive research. Identifying the ‘known unknowns’ of empirical invasion impact research serves a critical function in the effort to evaluate support for existing hypotheses and generate novel hypotheses. We built a taxonomically and geographically comprehensive database of over 1500 research articles that report measures of invasive species’ ecological impacts published over the past 18 years (1999–2016). We found that, field-wide, published measures of invasive species’ impacts are highly skewed toward those measured at the population or community-level with scarce information on impacts at other biological scales (e.g., physiology, behavior). We also show that existing impact evidence stems most often from one-off studies of single invasive species. Yet, even for species that receive consistent attention, impacts have rarely been documented across more than one biological scale, beyond very short time periods, or in several ecosystems. In order to predict and anticipate how impacts manifest in a variety of temporal and biological contexts, the evidence base informing existing invasion hypotheses must become more integrative both within and across publications.
A consistent determinant of the establishment success of alien species appears to be the number of individuals that are introduced to found a population (propagule pressure), yet variation in the ...form of this relationship has been largely unexplored. Here, we present the first quantitative systematic review of this form, using Bayesian meta-analytical methods. The relationship between propagule pressure and establishment success has been evaluated for a broad range of taxa and life histories, including invertebrates, herbaceous plants and long-lived trees, and terrestrial and aquatic vertebrates. We found a positive mean effect of propagule pressure on establishment success to be a feature of every hypothesis we tested. However, establishment success most critically depended on propagule pressures in the range of 10-100 individuals. Heterogeneity in effect size was associated primarily with different analytical approaches, with some evidence of larger effect sizes in animal rather than plant introductions. Conversely, no variation was accounted for in any analysis by the scale of study (field to global) or methodology (observational, experimental, or proxy) used. Our analyses reveal remarkable consistency in the form of the relationship between propagule pressure and alien population establishment success.
The process by which a species becomes a biological invader, at a location where it does not naturally occur, can be divided into a series of sequential stages (transport, introduction, establishment ...and spread). A species' success at passing through each of these stages depends, in a large part, on the number of individuals available to assist making each transition. Here, we review the evidence that numbers determine success at each stage of the invasion process and then discuss the likely mechanisms by which numbers affect success. We conclude that numbers of individuals affect transport and introduction by moderating the likelihood that abundant (and widespread) species are deliberately or accidentally translocated; affect establishment success by moderating the stochastic processes (demographic, environmental, genetic or Allee) to which small, introduced populations will be vulnerable; and affect invasive spread most likely because of persistent genetic effects determined by the numbers of individuals involved in the establishment phase. We finish by suggesting some further steps to advance our understanding of the influence of numbers on invasion success, particularly as they relate to the genetics of the process.
A predictive understanding of the ecological impacts of nonnative species has been slow to develop, owing largely to an apparent dearth of clearly defined hypotheses and the lack of a broad ...theoretical framework. The context dependency of impact has fueled the perception that meaningful generalizations are nonexistent. Here, we identified and reviewed 19 testable hypotheses that explain temporal and spatial variation in impact. Despite poor validation of most hypotheses to date, evidence suggests that each can explain at least some impacts in some situations. Several hypotheses are broad in scope (applying to plants and animals in virtually all contexts) and some of them, intriguingly, link processes of colonization and impact. Collectively, these hypotheses highlight the importance of the functional ecology of the nonnative species and the structure, diversity, and evolutionary experience of the recipient community as general determinants of impact; thus, they could provide the foundation for a theoretical framework for understanding and predicting impact. Further substantive progress toward this goal requires explicit consideration of within-taxon and across-taxa variation in the per capita effect of invaders, and analyses of complex interactions between invaders and their biotic and abiotic environments.
We argue that 'propagule pressure', a key term in invasion biology, has been attributed at least three distinct definitions (with usage of a related term causing additional confusion). All of the ...definitions refer to fundamental concepts within the invasion process, with the result that the distinct importance of these different concepts has been at best diluted, and at worst lost. Global. We reviewed pertinent literature on propagule pressure to resolve confusion about different uses of the term 'propagule pressure' and we introduced a new term for one variant, colonization pressure. We conducted a computer simulation whereby the introduction of species is represented as a simple sampling process to elucidate the relationship between propagule and colonization pressure. We defined colonization pressure as the number of species introduced or released to a single location, some of which will go on to establish a self-sustaining population and some of which will not. We subsequently argued that colonization pressure should serve as a null hypothesis for understanding temporal or spatial differences in exotic species richness, as the more species that are introduced, the more we should expect to establish. Finally, using a simple simulation, we showed that propagule pressure is related to colonization pressure, but in a non-linear manner. We suggest that the nature of the relationship between propagule pressure and colonization pressure, as well as the efficacy of various proxy measures of each, require more detailed exploration if invasion ecology is to continue to develop into a more predictive science.
1. The number of alien reptiles and amphibians introduced and established worldwide has increased over the last decades. The legal pet trade is now the dominant pathway by which individuals of these ...species arrive in their non-native locale. Despite its importance, specific factors of pet trade pathway that influence the release (introduction) of exotic reptiles and amphibians have not yet been examined. 2. We set out to identify broadscale and easily measured biological and economic factors that influence the release of these exotic pets by their owners. We hypothesize that biological factors reflect the cost of care, and economic factors reflect the value that owners place on their pet, both of which can influence the probability when a pet is released. We collected life history and economic data on the 1,722 species of reptiles and amphibians sold within the US as pets over the last 18 years. We also compiled a list of pet trade-attributed releases in the US (i.e., all free-living species regardless of whether they successfully established). We used boosted regression trees to correlate species release status with their life-history traits and economic attributes (r² = 0.51, AUC = 0.89). 3. We found that species with a high probability of being released were imported at higher quantities over our period of record, have a relatively large adult mass and commanded cheaper retail prices. The number imported and price interacted with longevity and adult mass to produce nonlinear increases in release probability. The most important interaction revealed that large-bodied species imported in high quantities have a three times higher release probability compared to largebodied species imported in lower quantities. 4. Policy implications. Our results provide guidance towards targeting exotic pet reptile and amphibian species that are at a high risk of being released. Species that are both prevalent in the pet trade and large-bodied or long-lived have the highest probability of being released. This will aid in developing education and policy solutions aimed at decreasing the rate at which these pets are released, thus curtailing the invasion process before these species can establish and impacts can occur.
When pets become pests Lockwood, Julie L; Welbourne, Dustin J; Romagosa, Christina M ...
Frontiers in ecology and the environment,
August 2019, Letnik:
17, Številka:
6
Journal Article
Recenzirano
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The annual trade in exotic vertebrates as pets is a multi-billion-dollar global business. Thousands of species, and tens of millions of individual animals, are shipped both internationally and within ...countries to satisfy this demand. Most research on the exotic pet trade has focused on its contribution to native biodiversity loss and disease spread. Here, we synthesize information across taxa and research disciplines to document the exotic pet trade’s contribution to vertebrate biological invasions. We show recent and substantial worldwide growth in the number of non-native animal populations introduced via this invasion pathway, which demonstrates a strong potential to increase the number of invasive animals in the future. Key to addressing the invasion threat of exotic pets is learning more about the socioeconomic forces that drive the massive growth in the exotic pet market and the socioecological factors that underlie pet release by owners. These factors likely vary according to cultural pet-keeping traditions across regions and whether purchases were legal or illegal. These gaps in our understanding of the exotic pet trade must be addressed in order to implement effective policy solutions.
A principal impact of invasive species is that they reduce local species richness. However, it is unknown whether the magnitude of the richness decrease has been consistent over the past two decades ...of published research. We used cumulative meta-analysis to synthesize evidence from 240 articles evaluating whether this cumulative evidence base generally supports, or refutes, the association between invasive species presence and richness declines. First, we determined whether evidence accumulation lowered the mean effect size of invasive species on local native richness through time; termed the “decline effect.” Then, as mean effect sizes changed over time, we identified when accumulated evidence reached sufficiency, indicating that the mean effect direction (positive or negative) was unlikely to be reversed by unpublished research. We also assessed whether the mean effect size reached a threshold of stability over publication years. To date, no research has tested mechanisms of the decline effect, and here we determine whether publication bias, sample size, time since invasion, or invader trophic position are driving a decline effect in the published evidence base. We found a clear decline in the cumulative mean effect of invasive species on local native species richness as published evidence accumulated between 1999 and 2016. Despite this decline, an average negative association was stable and sufficiently robust to unpublished studies by 2007, showing a 21% mean richness decrease by 2016. Contrary to our expectation, the decline effect manifested consistently regardless of invasive species trophic position, time since invasion, or journal rank. Within taxonomic subgroups, trees, insects, and herbaceous plants exhibit a decline effect, yet still show sufficient and stable negative impacts on richness. However, many other taxonomic subgroups (e.g., crustaceans, fish, mammals) lack evidence for average negative impacts on richness, or have not met sufficiency or stability thresholds.
Human-mediated species invasions are a significant component of current global environmental change. There is every indication that the rate at which locations are accumulating non-native species is ...accelerating as free trade and globalization advance. Thus, the need to incorporate predictive models in the assessment of invasion risk has become acute. However, finding elements of the invasion process that provide consistent explanatory power has proved elusive. Here, we propose propagule pressure as a key element to understanding why some introduced populations fail to establish whereas others succeed. In the process, we illustrate how the study of propagule pressure can provide an opportunity to tie together disparate research agendas within invasion ecology.
There is considerable evidence that keeping propagule pressure low can drastically reduce establishment probability of potential invasive species. Yet, most management plans and research efforts fail ...to explicitly acknowledge all three of the components of propagule pressure: size, number, and the risk–release relationship. It is unclear how failing to specify one or more of these components can influence the efficacy of management plans in preventing invasive species establishment. Furthermore, even if all components are acknowledged and quantified, there currently is no mathematical tool available to calculate the levels of propagule pressure that ensure attainment of a predetermined, and system-specific, target establishment probability. Here, we quantify the resulting uncertainty in establishment probability when one or more components of propagule pressure is unknown by using parameter uncertainty analysis on realistic values of propagule pressure. In addition, to aid in the development of management plans that explicitly set propagule pressure limits, we develop a propagule-pressure sensitivity analysis that we use to determine the required reduction in levels for propagule size and number (representative of management actions) to maintain a target establishment probability. We show that the precision of establishment estimates is highly dependent on knowledge of all three propagule pressure components, where the possible range of values for establishment probability can vary by over 50% without full specification. In addition, our sensitivity analysis showed that propagule size and number can be altered independently or in conjunction to lower establishment probability below a target level. Importantly, our sensitivity analysis was able to specifically quantify how much reduction in a propagule pressure component(s) is needed to reach a given target establishment probability. Our findings suggest that quantifying the three components of propagule pressure should be a priority for invasive species prevention moving forward. Furthermore, our sensitivity analysis tool can serve to guide the development of new invasive species management plans in a transparent and quantitative manner. Together with information on the costs associated with approaches to reducing propagule pressure, our tool can be used to identify the most cost-effective approach to prevent invasive species establishments.
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