Species Invasiveness in Biological Invasions: A Modelling Approach

The study of invasiveness, the traits that enable a species to invade a habitat, and invasibility, the habitat characteristics that determine its susceptibility to the establishment and spread of an invasive species, provide a useful conceptual framework to formulate the biological invasion problem in a modelling context. Another important aspect is the complex interaction emerging among the invader species, the noninvader species already present in the habitat, and the habitat itself. Following a modelling approach to the biological invasion problem, we present a spatially explicit cellular automaton model (Interacting Multiple Cellular Automata (IMCA)). We use field parameters from the invader Gleditsia triacanthos and the native Lithraea ternifolia in montane forests of central Argentina as a case study to compare outputs and performance of different models. We use field parameters from another invader, Ligustrum lucidum, and the native Fagara coco from the same system to run the cellular automaton model. We compare model predictions with invasion values from aerial photographs. We discuss in detail the importance of factors affecting species invasiveness, and give some insights into habitat invasibility and the role of interactions between them. Finally, we discuss the relevance of mathematical modelling for studying and predicting biological invasions. The IMCA model provided a suitable context for integrating invasiveness, invasibility, and the interactions. In the invasion system studied, the presence of an invader's juvenile bank not only accelerated the rate of invasion but was essential to ensure invasion. Using the IMCA model, we were able to determine that not only adult survival but particularly longevity of the native species influenced the spread velocity of the invader, at least when a juvenile bank is present. Other factors determining velocity of invasion detected by the IMCA model were seed dispersal distance and age of reproductive maturity. We derived relationships between species' adult survival, fecundity and longevity of both theoretical and applied relevance for biological invasions. Invasion velocities calculated from the aerial photographs agreed well with predictions of the IMCA model.     

Allee Effects, Propagule Pressure and the Probability of Establishment: Risk Analysis for Biological Invasions

Colonization is of longstanding interest in theoretical ecology and biogeography, and in the management of weeds and other invasive species, including insect pests and emerging infectious diseases. Due to accelerating invasion rates and widespread economic costs and environmental damages caused by invasive species, colonization theory has lately become a matter of considerable interest. Here we review the concept of propagule pressure to inquire if colonization theory might provide quantitative tools for risk assessment of biological invasions. By formalizing the concept of propagule pressure in terms of stochastic differential equation models of population growth, we seek a synthesis of invasion biology and theoretical population biology. We focus on two components of propagule pressure that affect the chance of invasion: (1) the number of individuals initially introduced, and (2) the rate of subsequent immigration. We also examine how Allee effects, which are expected to be common in newly introduced populations, may inhibit establishment of introduced propagules. We find that the establishment curve (i.e., the chance of invasion as a function of initial population size), can take a variety of shapes depending on immigration rate, carrying capacity, and the severity of Allee effects. Additionally, Allee effects can cause the stationary distribution of population sizes to be bimodal, which we suggest is a possible explanation for time lags commonly observed between the detection of an introduced population and widespread invasion of the landscape.     

Predicting Biological Invasions

There are various approaches to explain the mechanisms of biological invasions. It is possible (1) to focus on the characteristics of invading species and (2) on those of the ecosystems invaded, (3) to investigate the relationship between these two factors (key–lock approach), or (4) to differentiate the invasion process in time. Each of these approaches may serve to improve the understanding of some aspects of biological invasions, and each of them is in some way suitable for the purpose of predicting invasions. We discuss the usefulness and the limitations of these approaches, focusing on case studies from central Europe. An example of the fourth approach, a model of steps and stages of plant invasions that describes the invasion process in greater detail, illustrates some general limitations in predicting biological invasions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Does Darwin’s Naturalization Hypothesis Explain Fish Invasions?

Darwin’s naturalization hypothesis predicts that introduced species tend not to invade areas containing congeneric native species, because they would otherwise compete with their close relatives and would likely encounter predators and pathogens that can attack them. An opposing view is that introduced species should succeed in areas where native congeners are present because they are more likely to share traits that pre-adapt them to their new environment. A test of both these hypotheses using data on fish introductions from several independent regions fails to support either viewpoints. In contrast to studies of nonindigenous plants, our results suggest that taxonomic affiliation is not an important general predictor of fish invasion success.     

Human Agency in Biological Invasions: Secondary Releases Foster Naturalisation and Population Expansion of Alien Plant Species

The human mediation of biological invasions is still an underestimated phenomenon. This paper attempts to show that introductions on varying spatial scales may strongly foster invasions throughout the whole invasion process. As shown by data from central Europe, invasions frequently result from an interplay of biological and anthropogenic mechanisms. The latter, however, cannot be explained nor predicted by ecological rules. This may be an important reason for the limited predictability of invasions. Initial introductions from a donor to a new range are here distinguished from following secondary releases within the new range. The rate of naturalisation is higher in deliberately introduced plants as compared to accidental introductions. Due to higher numbers of accidental introductions, such species contribute significantly to the pool of naturalised species. Secondary releases of alien species are frequently made over long periods subsequent to the initial introduction. They may mimic demographic and dispersal processes that lead to population growth and range expansion. They also offer a pathway to overcome spatial isolation in species whose propagules are not naturally moved long distances. This even holds for most of Germany's noxious alien plant species. Secondary releases may thus promote invasions even beyond the threshold of naturalisation. In consequence, attempts at prevention should focus on secondary releases as well as on initial introductions. In the last section of the paper, the final invasion stage subsequent to naturalisation is shown as a multi-scale phenomenon. In consequence, the classification of a species as 'invasive' depends on the perspective chosen. Using different biologically or anthropocentrically based approaches leads to sub-sets of alien species that overlap only partially. In conclusion, the term `invasive' should preferably be used in a broader sense to describe the entire invasion process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Boats, Pathways, and Aquatic Biological Invasions: Estimating Dispersal Potential with Gravity Models

Biological invaders can have dramatic effects on the environment and the economy. To most effectively manage these invaders, we should consider entire pathways, because multiple species are dispersed through the same vectors. In this paper, we use production-constrained gravity models to describe movement of recreational boaters between lakes – potentially the most important pathway of overland dispersal for many aquatic organisms. These models are advantageous because they require relatively easily acquired data, hence are relatively easy to build. We compare linear and non-linear gravity models and show that, despite their simplicity, they are able to capture important characteristics of the recreational boater pathway. To assess our model, we compared observed data based on creel surveys and mailed surveys of recreation boaters to the model output. Specifically, we evaluate four metrics of pathway characteristics: boater traffic to individual lakes, distances traveled to reach these lakes, Great Lakes usage and movement from the Great Lakes to inland waters. These factors will influence the propagule pressure (hence the probability of establishment of invasive populations) and the rate of spread across a landscape. The Great Lakes are of particular importance because they are a major entry point of non-indigenous species from other continents, hence will act as the origin for further spread across states. The non-linear model had the best fit between model output and empirical observations with r² =0.80, r² =0.35, r² =0.57, and r² =0.36 for the four metrics, respectively. For the distances traveled to individual lakes, r² improved from 0.35 to 0.76 after removal of an outlier. Our results suggest that we were able to capture distances traveled to most but not all lakes. Thus, we demonstrate that production-constrained gravity models will be generally useful for modeling invasion pathways between non-contiguous locations.     

Propagule pressure and the establishment of emergent polyploid populations

BackgroundWhereas the incidence or rate of polyploid speciation in flowering plants is modest, the production of polyploid individuals within local populations is widespread. Explanations for this disparity primarily have focused on properties or interactions of polyploids that limit their persistence.HypothesisThe emergence of local polyploid populations within diploid populations is similar to the arrival of invasive species at new, suitable sites, with the exception that polyploids suffer interference from their progenitor(s). The most consistent predictor of successful colonization by invasive plants is propagule pressure, i.e. the number of seeds introduced. Therefore, insufficient propagule pressure, i.e. the formation of polyploid seeds within diploid populations, ostensibly is a prime factor limiting the establishment of newly emergent polyploids within local populations. Increasing propagule number reduces the effects of genetic, environmental and demographic stochasticity, which thwart population survival. As with invasive species, insufficient seed production within polyploid populations limits seed export, and thus reduces the chance of polyploid expansion.ConclusionThe extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little. The numbers of auto- or allopolyploid seed in diploid populations rarely have been ascertained, as have the numbers of newly emergent polyploid plants within diploid populations. Moreover, seed production by these polyploids has yet to be assessed.     

A neutral terminology to define 'invasive' species

The use of simple terms to articulate ecological concepts can confuse ideological debates and undermine management efforts. This problem is particularly acute in studies of nonindigenous species, which alternatively have been called ‘exotic’, ‘introduced’, ‘invasive’ and ‘naturalised’, among others. Attempts to redefine commonly used terminology have proven difficult because authors are often partial to particular definitions. In an attempt to form a consensus on invasion terminology, we synthesize an invasional framework based on current models that break the invasion process into a series of consecutive, obligatory stages. Unlike previous efforts, we propose a neutral terminology based on this framework. This ‘stage‐based’ terminology can be used to supplement terms with ambiguous meanings (e.g. invasive, introduced, naturalized, weedy, etc.), and thereby improve clarity of future studies. This approach is based on the concept of ‘propagule pressure’ and has the additional benefit of identifying factors affecting the success of species at each stage. Under this framework, invasions can be more objectively understood as biogeographical, rather than taxonomic, phenomena; and author preferences in the use of existing terminology can be addressed. An example of this recommended protocol might be: ‘We examined distribution data to contrast the characteristics of invasive species (stages IVa and V) and noninvasive species (stages III and IVb)’.     

Null model of biotic homogenization: a test with the European freshwater fish fauna

In recent years, there has been growing concern about how species invasions and extinctions could change the distinctiveness of formerly disparate fauna and flora, a process called biotic homogenization. In the present study, a null model of biotic of homogenization was developed and applied to the European freshwater fish fauna. We found that non-native fish species led to the greatest homogenization in south-western Europe and greatest differentiation in north-eastern Europe. Comparing these observed patterns to those expected by our null model empirically demonstrated that biotic homogenization is a non-random ecological pattern, providing evidence for previous assumptions. The place of origin of non-native species was also considered by distinguishing between exotic (originating from outside Europe) and translocated species (originating from within Europe). We showed that exotic and translocated species generated distinct geographical patterns of biotic homogenization across Europe because of their contrasting effects on the changes in community similarity among river basins. Translocated species promoted homogenization among basins, whereas exotic species tended to decrease their compositional similarity. Quantifying the individual effect of exotic and translocated species is therefore an absolute prerequisite to accurately assess the spatial dynamics of biotic homogenization.     

Biological Invasions in Austria: Patterns and Case Studies

This paper provides a review of the first national inventory of non-indigenous species in Austria. In summary, 1110 vascular plant species (27 of the entire flora), 83 mycetes and at least 500 animal species (approximately 1 of the entire fauna) were documented for Austria, which are introduced intentionally or unintentionally by humans after 1492 and reported from the wild. About 25 of non-indigenous vascular plant species have become naturalized. Most non-indigenous vascular plants are native to the Palaearctic region (55%; with 33% originating from the Mediterranean subregion) and North America (20%). More than 90% of non-indigenous plant species are confined to naturally and anthropogenically disturbed (ruderal, urban, arable land, and riverine) habitats. Aquatic ecosystems are more affected and vulnerable to changes in their animal species composition. The current data demonstrate that non-indigenous species continue to invade and disperse and it also emphasize the necessity and responsibility to develop scientific strategies to minimize the impact of biological invasions and to raise public awareness of the problem.     

外来种入侵的过程、机理和预测

生物入侵是指某种生物从原来的分布区域扩展到一个新的（通常也是遥远的）地区,在新的区域里,其后代可以繁殖、扩散并持续维持下去,生物入侵成功的原因,即与入侵者本身的生物学,生态学特征有关,也与群落的脆弱性有关,入侵者可能较本地种的竞争能力强,更适应当地的环境,有的入侵者还可以改变环境,使之对已有利,而不利于本地种。缺乏天敌制约。群落的稳定性低和异常的环境扰动往往导致生物入侵,生物入侵的预测包括哪一种外来种会变成入侵种？哪些生态系统区域会被入侵？影响程度如何？入侵种的扩散态势如何等内容,对有关的理论和模型作了评介。     

A NEW RECORD AND ERADICATION OF THE NORTHERN ATLANTIC ALGA ASCOPHYLLUM NODOSUM (PHAEOPHYCEAE) FROM SAN FRANCISCO BAY,CALIFORNIA, USA1

A new record of the Northern Atlantic fucoid Ascophyllum nodosum (L.) Le Jolis (Knotted wrack) was discovered on a shoreline in San Francisco Bay, California during a survey of intertidal habitats in 2001–2002. The alga showed no signs of deterioration 2.5 months after its initial detection. The healthy condition, presence of receptacles with developing oogonia, potential for asexual reproduction, and ability to withstand environmental conditions, both inside the Bay and on the outer Pacific coast, prompted a multiagency eradication effort. Given the relatively small area of shoreline inhabited by the alga, in combination with its absence in 125 other surveyed locations, we decided that manual removal of the seaweed would be the most environmentally sensitive yet effective eradication approach. No A. nodosum has been detected at the site since December 2002, and the species is thought to have been locally eradicated. The site continues to be monitored to assess the success of the eradication efforts.     

A null model of temporal trends in biological invasion records

Biological invasions are a growing aspect of global biodiversity change. In many regions, introduced species richness increases supralinearly over time. This does not, however, necessarily indicate increasing introduction rates or invasion success. We develop a simple null model to identify the expected trend in invasion records over time. For constant introduction rates and success, the expected trend is exponentially increasing. Model extensions with varying introduction rate and success can also generate exponential distributions. We then analyse temporal trends in aquatic, marine and terrestrial invasion records. Most data sets support an exponential distribution (15/16) and the null invasion model (12/16). Thus, our model shows that no change in introduction rate or success need be invoked to explain the majority of observed trends. Further, an exponential trend does not necessarily indicate increasing invasion success or 'invasional meltdown', and a saturating trend does not necessarily indicate decreasing success or biotic resistance.     

Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plant species diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicum Desf. ex Fisch.‐invaded and noninvaded plots from 12 locations in northern Norway. There was lower native cover, evenness, taxonomic diversity, native biomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two native species fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the native cover, evenness, and native biomass, and a positive association with the height of the native plants. Plant communities containing only native species appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant community by H. persicum exerts consistent negative pressure on vascular plant diversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant community to further invasion, it severely reduces the abundance of native species and makes them more vulnerable to competitive exclusion.     

Invasion Gateways and Corridors in the Carpathian Basin: Biological Invasions in Hungary

Biological invasions in Hungary are causing severe problems as a result of recent introductions and rapid land use changes. Poorly managed agricultural and rural, disturbed areas, and aquatic ecosystems are the most prone to plant invasions. Dry grasslands and semi-natural forests are less prone to invasions. A few plant species have led to human health (allergenic) problems. Some insect species have caused economic problems to crop production. A number of monitoring networks and control measures are in place for selected plants and insects. This revised version was published online in July 2006 with corrections to the Cover Date.     

Patterns of Plant Invasions: A Case Example in Native Species Hotspots and Rare Habitats

Land managers require landscape-scale information on where exotic plant species have successfully established, to better guide research, control, and restoration efforts. We evaluated the vulnerability of various habitats to invasion by exotic plant species in a 100,000 ha area in the southeast corner of Grand Staircase-Escalante National Monument, Utah. For the 97 0.1-ha plots in 11 vegetation types, exotic species richness (log₁₀) was strongly negatively correlated to the cover of cryptobiotic soil crusts (r = −0.47, P < 0.001), and positively correlated to native species richness (r = 0.22, P < 0.03), native species cover (r = 0.23, P < 0.05), and total nitrogen in the soil (r = 0.40, P < 0.001). Exotic species cover was strongly positively correlated to exotic species richness (r = 0.68, P < 0.001). Only 6 of 97 plots did not contain at least one exotic species. Exotic species richness was particularly high in locally rare, mesic vegetation types and nitrogen rich soils. Dry, upland plots (n = 51) had less than half of the exotic species richness and cover compared to plots (n = 45) in washes and lowland depressions that collect water intermittently. Plots dominated by trees had significantly greater native and exotic species richness compared to plots dominated by shrubs. For the 97 plots combined, 33% of the variance in exotic species richness could be explained by a positive relationship with total plant cover, and negative relationships with the cover of cryptobiotic crusts and bare ground. There are several reasons for concern: (1) Exotic plant species are invading hot spots of native plant diversity and rare/unique habitats. (2) The foliar cover of exotic species was greatest in habitats that had been invaded by several exotic species.(3) Continued disturbance of fragile cryptobiotic crusts by livestock, people, and vehicles may facilitate the further invasion of exotic plant species. This revised version was published online in July 2006 with corrections to the Cover Date.