Exploiting Allee effects for managing biological invasions

Biological invasions are a global and increasing threat to the function and diversity of ecosystems. Allee effects (positive density dependence) have been shown to play an important role in the establishment and spread of non-native species. Although Allee effects can be considered a bane in conservation efforts, they can be a benefit in attempts to manage non-native species. Many biological invaders are subject to some form of an Allee effect, whether due to a need to locate mates, cooperatively feed or reproduce or avoid becoming a meal, yet attempts to highlight the specific exploitation of Allee effects in biological invasions are surprisingly unprecedented. In this review, we highlight current strategies that effectively exploit an Allee effect, and propose novel means by which Allee effects can be manipulated to the detriment of biological invaders. We also illustrate how the concept of Allee effects can be integral in risk assessments and in the prioritization of resources allocated to manage non-native species, as some species beset by strong Allee effects could be less successful as invaders. We describe how tactics that strengthen an existing Allee effect or create new ones could be used to manage biological invasions more effectively.     

生物入侵对鸟类的生态影响

生物入侵是全球生物多样性面临的最主要威胁之一, 入侵种在改变入侵地环境的同时也使当地的生物受到极大影响。鸟类在生态系统中处于较高的营养级, 生态系统中任何一个环节的变化都可能对鸟类造成一定的影响。本文回顾了哺乳动物、鸟类、无脊椎动物和植物等不同生物类群的入侵对本地鸟类生态影响方面的研究进展。外来生物对鸟类的影响主要表现在以下几方面: (1)外来哺乳动物对成鸟、幼鸟或鸟卵的捕食作用; (2)外来鸟类与本地鸟类竞争栖息地和食物资源, 与当地的近缘种杂交而造成基因流失; (3)外来无脊椎动物改变本地鸟类的栖息环境和食物状况, 甚至直接捕食本地鸟类; (4)外来植物入侵改变入侵地的植物群落组成和结构, 造成本地鸟类的栖息地丧失或破碎化, 并通过改变入侵地生态系统的食物链结构而对高营养级的鸟类产生影响。最后, 作者还提出了该领域有待解决的问题和今后可能的研究方向。     

Invasion speed is affected by geographical variation in the strength of Allee effects

Allee effects can play a critical role in slowing or preventing the establishment of low density founder populations of non-indigenous species. Similarly, the spread of established invaders into new habitats can be influenced by the degree to which small founder populations ahead of the invasion front are suppressed through Allee effects. We develop an approach to use empirical data on the gypsy moth, a non-indigenous invader in North America, to quantify the Allee threshold across geographical regions, and we report that the strength of the Allee effect is subject to spatial and temporal variability. Moreover, we present what is to our knowledge the first empirical evidence that geographical regions with higher Allee thresholds are associated with slower speeds of invasion.     

An evolutionary perspective of biological invasions

The workshop on the Evolutionary Perspective of Biological Invasions in Terrestrial Ecosystems was held in Halle, Germany from 30 September to 3 October 2002.     

Mate‐finding failure as an important cause of Allee effects along the leading edge of an invading insect population

The movement of humans and goods has facilitated the arrival of non‐native insects, some of which successfully establish and cause negative consequences to the composition, services, and functioning of ecosystems. The gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), is currently invading North American forests at variable rates, spreading by local and long‐distance movement in a process known as stratified dispersal. Newly arriving colonizers often occur considerably ahead of the population front, and a key question is the degree to which they successfully establish. Prior research has highlighted mate‐finding failures in sparse populations as a cause of an Allee effect (positive density dependence). We explored this mechanism by measuring the relationship between female mating success and background male moth densities along the gypsy moth western front in Northern Wisconsin (USA) over 2 years. The mating results were then compared with analogous previous studies in southern Wisconsin, and the southern front in West Virginia and Virginia (USA). Mate‐finding failures in low‐density populations were consistently observed to be density‐dependent across all years and locations. Mate‐finding failures in low‐density populations have important ramifications to invasive species management, particularly in predicting species invasiveness, preventing successful establishment by small founder populations, and concentrating eradication efforts where they are most likely to succeed.     

Overcoming Allee effects through evolutionary,genetic, and demographic rescue

Despite the amplified threats of extinction facing small founder populations, successful colonization sometimes occurs, bringing devastating ecological and economic consequences. One explanation may be rapid evolution, which can increase mean fitness in populations declining towards extinction, permitting persistence and subsequent expansion. Such evolutionary rescue may be particularly important, given Allee effects. When a population is introduced at low density, individuals often experience a reduction in one or more components of fitness due to novel selection pressures that arise from diminished intraspecific interactions and positive density dependence (i.e. component Allee effects). A population can avoid extinction if it can adapt and recover on its own (i.e. evolutionary rescue), or if additional immigration sustains the population (i.e. demographic rescue) or boosts its genetic variation that facilitates adaptation (i.e. genetic rescue). These various forms of rescue have often been invoked as possible mechanisms for specific invasions, but their relative importance to invasion is not generally understood. Within a spatially explicit modelling framework, we consider the relative impact of each type of rescue on the probability of successful colonization, when there is evolution of a multi-locus quantitative trait that influences the strength of component Allee effects. We demonstrate that when Allee effects are important, the effect of demographic rescue via recurrent immigration overall provides the greatest opportunity for success. While highlighting the role of evolution in the invasion process, we underscore the importance of the ecological context influencing the persistence of small founder populations.     

集合种群的Allee效应研究进展

随着全球环境破坏的加剧,物种丧失的速度加快,人们日益关注生物多样性的保护。种群生物学和自然保护生物学的一些研究表明,如果一个局域种群受到Allee效应的影响,最终可能走向灭绝。从物种保护的角度考虑,分别介绍了集合种群水平上的Allee效应的和似Allee效应,比较了集合种群的Allee效应和似Allee效应产生的原因,以及集合种群的Allee效应和局域种群的Allee效应之间的关系、集合种群的似Allee效应和局域种群的Allee效应之间的关系,并提出集合种群的Allee效应还需要进一步的研究。     

Overlooking the smallest matter: viruses impact biological invasions

Parasites and pathogens have recently received considerable attention for their ability to affect biological invasions, however, researchers have largely overlooked the distinct role of viruses afforded by their unique ability to rapidly mutate and adapt to new hosts. With high mutation and genomic substitution rates, RNA and single‐stranded DNA (ssDNA) viruses may be important constituents of invaded ecosystems, and could potentially behave quite differently from other pathogens. We review evidence suggesting that rapidly evolving viruses impact invasion dynamics in three key ways: (1) Rapidly evolving viruses may prevent exotic species from establishing self‐sustaining populations. (2) Viruses can cause population collapses of exotic species in the introduced range. (3) Viruses can alter the consequences of biological invasions by causing population collapses and extinctions of native species. The ubiquity and frequent host shifting of viruses make their ability to influence invasion events likely. Eludicating the viral ecology of biological invasions will lead to an improved understanding of the causes and consequences of invasions, particularly as regards establishment success and changes to community structure that cannot be explained by direct interspecific interactions among native and exotic species.     

Biotic indirect effects: a neglected concept in invasion biology

Indirect effects involve more than two species and are defined as how one species alters the effect that another species has on a third. These complex interactions are often overlooked in studies of interactions between alien and native species, and their role in influencing biological invasions has been rarely considered. Based on a comprehensive review of the invasion biology literature, we examine the evidence for the occurrence of four of the most commonly documented indirect effects (apparent competition, indirect mutualism/commensalism, exploitative competition, and trophic cascades) in the invasion process. Studies investigating indirect effects in the context of invasion biology are relatively rare, but have been increasing in recent years, and there are sufficient examples to indicate that this kind of interaction is likely to be more common than is currently recognized. Whether indirect interactions are mediated by an alien or a native species, and whether they occur between ecologically similar or dissimilar alien and native species, depends in part on the type of interaction considered and no predictable patterns were detected in the literature. Further empirical studies will help to elucidate such patterns. At this stage, the inherent unpredictability of indirect interactions means that their impacts in relation to invasions are particularly challenging for land managers to deal with, and their role in invasions is a complex, but is a valuable area of investigation for researchers.     

Evolutionary dynamics and strong Allee effects

We describe the dynamics of an evolutionary model for a population subject to a strong Allee effect. The model assumes that the carrying capacity k(u), inherent growth rate r(u), and Allee threshold a(u) are functions of a mean phenotypic trait u subject to evolution. The model is a plane autonomous system that describes the coupled population and mean trait dynamics. We show bounded orbits equilibrate and that the Allee basin shrinks (and can even disappear) as a result of evolution. We also show that stable non-extinction equilibria occur at the local maxima of k(u) and that stable extinction equilibria occur at local minima of r(u). We give examples that illustrate these results and demonstrate other consequences of an Allee threshold in an evolutionary setting. These include the existence of multiple evolutionarily stable, non-extinction equilibria, and the possibility of evolving to a non-evolutionary stable strategy (ESS) trait from an initial trait near an ESS.     

Residence time and potential range: crucial considerations in modelling plant invasions

A prime aim of invasion biology is to predict which species will become invasive, but retrospective analyses have so far failed to develop robust generalizations. This is because many biological, environmental, and anthropogenic factors interact to determine the distribution of invasive species. However, in this paper we also argue that many analyses of invasiveness have been flawed by not considering several fundamental issues: (1) the range size of an invasive species depends on how much time it has had to spread (its residence time); (2) the range size and spread rate are mediated by the total extent of suitable (i.e. potentially invasible) habitat; and (3) the range size and spread rate depend on the frequency and intensity of introductions (propagule pressure), the position of founder populations in relation to the potential range, and the spatial distribution of the potential range. We explored these considerations using a large set of invasive alien plant species in South Africa for which accurate distribution data and other relevant information were available. Species introduced earlier and those with larger potential ranges had larger current range sizes, but we found no significant effect of the spatial distribution of potential ranges on current range sizes, and data on propagule pressure were largely unavailable. However, crucially, we showed that: (1) including residence time and potential range always significantly increases the explanatory power of the models; and (2) residence time and potential range can affect which factors emerge as significant determinants of invasiveness. Therefore, analyses not including potential range and residence time can come to misleading conclusions. When these factors were taken into account, we found that nitrogen‐fixing plants and plants invading arid regions have spread faster than other species, but these results were phylogenetically constrained. We also show that, when analysed in the context of residence time and potential range, variation in range size among invasive species is implicitly due to variation in spread rates, and, that by explicitly assuming a particular model of spread, it is possible to estimate changes in the rates of plant invasions through time. We believe that invasion biology can develop generalizations that are useful for management, but only in the context of a suitable null model.     

External morphology explains the success of biological invasions

Biological invasions have become major players in the current biodiversity crisis, but realistic tools to predict which species will establish successful populations are still unavailable. Here we present a novel approach that requires only a morphometric characterisation of the species. Using fish invasions of the Mediterranean, we show that the abundance of non‐indigenous fishes correlates with the location and relative size of occupied morphological space within the receiving pool of species. Those invaders that established abundant populations tended to be added outside or at the margins of the receiving morphospace, whereas non‐indigenous species morphologically similar to resident ones failed to develop large populations or even to establish themselves, probably because the available ecological niches were already occupied. Accepting that morphology is a proxy for a species' ecological position in a community, our findings are consistent with ideas advanced since Darwin's naturalisation hypothesis and provide a new warning signal to identify invaders and to recognise vulnerable communities.     

Niche-based modelling as a tool for predicting the risk of alien plant invasions at a global scale

Predicting the probability of successful establishment of plant species by matching climatic variables has considerable potential for incorporation in early warning systems for the management of biological invasions. We select South Africa as a model source area of invasions worldwide because it is an important exporter of plant species to other parts of the world because of the huge international demand for indigenous flora from this biodiversity hotspot. We first mapped the five ecoregions that occur both in South Africa and other parts of the world, but the very coarse definition of the ecoregions led to unreliable results in terms of predicting invasible areas. We then determined the bioclimatic features of South Africa's major terrestrial biomes and projected the potential distribution of analogous areas throughout the world. This approach is much more powerful, but depends strongly on how particular biomes are defined in donor countries. Finally, we developed bioclimatic niche models for 96 plant taxa (species and subspecies) endemic to South Africa and invasive elsewhere, and projected these globally after successfully evaluating model projections specifically for three well‐known invasive species (Carpobrotus edulis, Senecio glastifolius, Vellereophyton dealbatum) in different target areas. Cumulative probabilities of climatic suitability show that high‐risk regions are spatially limited globally but that these closely match hotspots of plant biodiversity. These probabilities are significantly correlated with the number of recorded invasive species from South Africa in natural areas, emphasizing the pivotal role of climate in defining invasion potential. Accounting for potential transfer vectors (trade and tourism) significantly adds to the explanatory power of climate suitability as an index of invasibility. The close match that we found between the climatic component of the ecological habitat suitability and the current pattern of occurrence of South Africa alien species in other parts of the world is encouraging. If species' distribution data in the donor country are available, climatic niche modelling offers a powerful tool for efficient and unbiased first‐step screening. Given that eradication of an established invasive species is extremely difficult and expensive, areas identified as potential new sites should be monitored and quarantine measures should be adopted.     

Dynamics of an age-structured population with Allee effects and harvesting

We propose a discrete-time, age-structured population model to study the impact of Allee effects and harvesting. It is assumed that survival probabilities from one age class to the next are constants and fertility rate is a function of weighted total population size. Global extinction is certain if the maximal growth rate of the population is less than one. The model can have multiple attractors and the asymptotic dynamics of the population depend on its initial distribution if the maximal growth rate is larger than one. An Allee threshold depending on the components of the unstable interior equilibrium is derived when only the last age class can reproduce. The population becomes extinct if its initial population distribution is below the threshold. Harvesting on any particular age class can decrease the magnitude of the possible stable interior equilibrium and increase the magnitude of the unstable interior equilibrium simultaneously.     

Boom‐bust dynamics in biological invasions: towards an improved application of the concept

Boom‐bust dynamics – the rise of a population to outbreak levels, followed by a dramatic decline – have been associated with biological invasions and offered as a reason not to manage troublesome invaders. However, boom‐bust dynamics rarely have been critically defined, analyzed, or interpreted. Here, we define boom‐bust dynamics and provide specific suggestions for improving the application of the boom‐bust concept. Boom‐bust dynamics can arise from many causes, some closely associated with invasions, but others occurring across a wide range of ecological settings, especially when environmental conditions are changing rapidly. As a result, it is difficult to infer cause or predict future trajectories merely by observing the dynamic. We use tests with simulated data to show that a common metric for detecting and describing boom‐bust dynamics, decline from an observed peak to a subsequent trough, tends to severely overestimate the frequency and severity of busts, and should be used cautiously if at all. We review and test other metrics that are better suited to describe boom‐bust dynamics. Understanding the frequency and importance of boom‐bust dynamics requires empirical studies of large, representative, long‐term data sets that use clear definitions of boom‐bust, appropriate analytical methods, and careful interpretations.     

General population growth models with Allee effects in a random environment

Allee effects on population growth are quite common in nature, usually studied through deterministic models with a specific growth rate function.In order to seek the qualitative behaviour of populations induced by such effects, one should avoid model-specific behaviours. So, we use as a basis a general deterministic model, i.e. a model with a general growth rate function, to which we add the effect on the growth rate of the random fluctuations in environmental conditions. The resulting model is the general stochastic differential equation (SDE) model that we propose here.We consider two possible cases, weak Allee effects and strong Allee effects, which lead to different qualitative behaviours of the model.We will study the model properties for both cases in terms of existence and uniqueness of the solution, extinction and stationary behaviour of the population. The two cases will be compared with each other and with the general density-dependent SDE model without Allee effects.We then consider as an example the particular case of the classic logistic model and an Allee effect version of it.