Ecological explanations for successful invasion of exotic plants

The intentional introduction of exotic species can increase the level of local biodiversity, enrich people's material lives, and bring significant social and economic benefits that are also the symbols of human progress. However, along with the frequent intercourse among countries and regions, the frequency of uncontrolled crossregional migration of species is increased and there is a lack of scientific management strategy for the intentional introduction of exotic species. Exotic species invasion, which is behind habitat fragmentation, has become the second largest threatening factor to the maintenance of the global-scale level of biological diversity. Exotic species invasion can destroy the structure of an ecosystem, disturb the economic life of a society, and do harm to human health. In this paper, the authors review some of the ecological explanations for issues such as "what causes or mechanisms have led to the successful invasion of exotic species", including the "ideal weeds characteristics", "biodiversity resistance hypothesis", "enemies release hypothesis", "evolution of increased competitive ability hypothesis", "niche opportunity hypothesis", and "novel weapon hypothesis". The authors also analyze and evaluate the background and theoretical basis of the hypotheses, providing explanations for some phenomena, as well as the deficiencies of these explanations.     

The advantages and disadvantages of being introduced

Introduced species, those dispersed outside their natural ranges by humans, now cause almost all biological invasions, i.e., entry of organisms into habitats with negative effects on organisms already there. Knowing whether introduction tends to give organisms specific ecological advantages or disadvantages in their new habitats could help understand and control invasions. Even if no specific species traits are associated with introduction, introduced species might out-compete native ones just because the pool of introduced species is very large (“global competition hypothesis”). Especially in the case of intentional introduction, high initial propagule pressure might further increase the chance of establishment, and repeated introductions from different source populations might increase the fitness of introduced species through hybridization. Intentional introduction screens species for usefulness to humans and so might select for rapid growth and reproduction or carry species to suitable habitats, all which could promote invasiveness. However, trade offs between growth and tolerance might make introduced species vulnerable to extreme climatic events and cause some invasions to be transient (“reckless invader hypothesis”). Unintentional introduction may screen for species associated with human-disturbed habitats, and human disturbance of their new habitats may make these species more invasive. Introduction and natural long-distance dispersal both imply that species have neither undergone adaptation in their new habitats nor been adapted to by other species there. These two characteristics are the basis for many well-known hypotheses about invasion, including the “biotic resistance”, “enemy release”, “evolution of increased competitive ability” and “novel weapon” hypotheses, each of which has been shown to help explain some invasions. To the extent that biotic resistance depends upon local adaption by native species, altering selection pressures could reduce resistance and promote invasion (“local adaptation hypothesis”), and restoring natural regimes could reverse this effect.     

Saving camels from straws: how propagule pressure-based prevention policies can reduce the risk of biological invasion

Nonnative species that harm or have the potential to cause harm to the environment, economy, or human health are known as invasive species. Propagule pressure may be the most important factor in establishment success of nonnative species of various taxa in a variety of ecosystems worldwide, and strong evidence is emerging that propagule pressure determines both the scale of invasion extent and impact. In a limited way, the US government is applying a “propagule pressure approach” in a variety of prevention policy contexts aimed at minimizing the impact of harmful organisms. However, there are also readily apparent opportunities for enacting propagule pressure-based measures to fill current gaps in invasive species prevention and control at national, state, and local levels. An explicit focus on propagule pressure-based policies could substantially increase the effectiveness of US efforts to prevent the introduction of invasive species through by intentional and unintentional introductions. The views expressed in this paper are solely those of the authors and do not necessarily reflect those of the US government. “As the last straw breaks the laden camel’s back...” -Charles Dickens, Dombey and Son     

Plantations of exotic tree species in Britain: irrelevant for biodiversity or novel habitat for native species?

Novel or emergent ecosystems arising from human action present both threats and opportunities for biodiversity. It has been suggested that exotic species can “facilitate” or “inhibit” native biodiversity through habitat modification. In Britain, there is a discussion over the contribution to biodiversity of plantations of exotic conifer species as these are commonly thought to have little relevancy as a habitat for native biodiversity. To address this we compared the species richness of a range of different taxonomic groups (lichens, bryophytes, fungi, vascular plants, invertebrates and songbirds) in exotic and native forest stands of differing structural stages in northern and southern Britain. In terms of overall native species-richness there was no significant difference between the exotic and the native stands. In the north, six species groups showed higher values in the exotic Sitka spruce (Picea sitchensis) stands with the remaining six showing higher values in the native Scots pine (Pinus sylvestris) stands. Most notably, lichen species richness was much lower in the exotic stands compared to the native stands, whereas bryophyte and fungal species richness was proportionately higher in the exotic stands. In the south, five species groups (all invertebrate taxa) showed higher species richness in exotic Norway spruce (Picea abies) stands compared to native oak (Quercus robur) stands. Five species groups had higher species-richness in the oak stands, in particular lichens and fungi. It is concluded that emergent ecosystems of exotic conifer species are not irrelevant to biodiversity. Where already well-established they can provide habitat for native species particularly if native woodland is scarce and biodiversity restoration is an immediate priority.     

Indicators for plant species richness in pine (<Emphasis Type="Italic">Pinus sylvestris</Emphasis> L.) forests of Germany

Forestry is obliged to record as well as maintain and/or enhance biological diversity in forests due to national and international agreements. Accordingly, it is necessary to work out methodological approaches for the assessment of biodiversity in forests. In the study presented here, we focus on the total plant species pool (563 vascular plant and bryophyte species) of pine (Pinus sylvestris L.) forests in NE Germany to identify indicators for plant species richness. We distinguished several groups like “herb”, “grass-like”, “woody”, “endangered”, and “exotic species”, for which we detected indicators for low (class #1), intermediate (class #2), and high (class #3) species numbers. From a total of 84 species, which were identified by a three-step procedure, most indicators were found for class #3. Only few indicators have been revealed for intermediate species numbers, i.e. class #2. With help of Ellenberg’s ecological indicator values and information on the main occurrence in Central European vegetation types and plant communities, respectively, we characterized the indicator species ecologically. The ecological site preferences of the indicator species in general reflect the fact that species richness is highest in base-rich, light, and anthropogenically disturbed pine forests. On the contrary, species-poor forests were revealed by indicators, which mainly occur on acidic sites. It is concluded that a considerable set of indicators for species richness can help facilitate biodiversity assessments in forestry and ecosystem restoration practice. Electronic Supplementary Material  The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

A reassessment of historical records of avian introductions to Australia: no case for propagule pressure

Introduced species are widely believed to represent a significant threat to conservation of biological diversity. A better understanding of the ecological factors associated with successful species establishment should lead to improved management and mitigation of these introductions. The “propagule pressure hypothesis”, implying a greater chance of successful introduction with greater numbers introduced, has been widely accepted as a principal ecological factor in explaining establishment of exotic species. The historical record of bird introductions in a few locations, including the state of Victoria in Australia, has been advanced as the principal quantitative support for the hypothesis. We compiled lists of bird species introductions into Australia from several sources, and discovered inconsistencies in the records of introductions. In a series of comparisons, we found that the historical record of passerine introductions to Australia does not support the propagule pressure hypothesis unless superfluous introductions of already successful species are included. An additional problem with previous analyses is the inclusion of unsuccessful haphazard cage escapes.     

Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass

Exotic plants are generally considered a serious problem in wildlands around the globe. However, some argue that the impacts of exotic plants have been exaggerated and that biodiversity and other important plant community characteristics are commonly improved with invasion. Thus, disagreement exists among ecologists as to the relationship of exotic plants with biodiversity and native plant communities. A better understanding of the relationships between exotic plants and native plant communities is needed to improve funding allocation and legislation regarding exotic plants, and justify and prioritize invasion management. To evaluate these relationships, 65 shrub–bunchgrass plant communities with varying densities of an exotic annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead), were sampled across 160,000 ha in southeastern Oregon, United States. Environmental factors were generally not correlated with plant community characteristics when exotic annual grass density was included in models. Plant diversity and species richness were negatively correlated with exotic annual grass density. Exotic annual grass density explained 62% of the variation in plant diversity. All native plant functional groups, except annual forbs, exhibited a negative relationship with T. caput-medusae. The results of this study suggest that T. caput-medusae invasions probably have substantial negative impacts on biodiversity and native plant communities. The strength of the relationships between plant community characteristics and T. caput-medusae density suggests that some exotic plants are a major force of change in plant communities and subsequently threaten ecosystem functions and processes. However, experimental studies are needed to fully substantiate that annual grass invasion is the cause of these observed correlations.     

Exotic species richness and native species endemism increase the impact of exotic species on islands

Aim Exotic species pose one of the most significant threats to biodiversity, especially on islands. The impacts of exotic species vary in severity among islands, yet little is known about what makes some islands more susceptible than others. Here we determine which characteristics of an island influence how severely exotic species affect its native biota. Location We studied 65 islands and archipelagos from around the world, ranging from latitude 65° N to 54° S. Methods We compiled a global database of 10 island characteristics for 65 islands and determined the relative importance of each characteristic in predicting the impact of exotic species using multivariate modelling and hierarchical partitioning. We defined the impact of exotic species as the number of bird, amphibian and mammal (BAM) species listed by the International Union for Conservation of Nature (IUCN) as threatened by exotics, relative to the total number of BAM species on that island. Results We found that the impact of exotic species is more severe on islands with more exotic species and a greater proportion of native species that are endemic. Unexpectedly, the level of anthropogenic disturbance did not influence an island's susceptibility to the impacts of exotic species. Main conclusions By coupling our results with studies on the introduction and establishment of exotic species, we conclude that colonization pressure, or invasion opportunities, influences all stages of the invasion process. However, species endemism, the other important factor determining the impact of exotic species, is not known to contribute to introduction and establishment success on islands. This demonstrates that different factors correlate with the initial stages of the invasion process and the subsequent impacts of those invaders, highlighting the importance of studying the impacts of exotic species directly. Our study helps identify islands that are at risk of impact by exotics and where investment should focus on preventing further invasions.     

Wikipedia as an encyclopaedia of life

In a 2003 essay E. O. Wilson outlined his vision for an “encyclopaedia of life” comprising “an electronic page for each species of organism on Earth”, each page containing “the scientific name of the species, a pictorial or genomic presentation of the primary type specimen on which its name is based, and a summary of its diagnostic traits.” Although biodiversity informatics has generated numerous online resources, including some directly inspired by Wilson’s essay (e.g., iSpecies and EOL), we are still some way from the goal of having available online all relevant information about a species, such as its taxonomy, evolutionary history, genomics, morphology, ecology, and behaviour. While the biodiversity community has been developing a plethora of databases, some with overlapping goals and duplicated content, Wikipedia has been slowly growing to the point where it now has over 100,000 pages on biological taxa. My goal in this essay is to explore the idea that, largely independent of the aims of biodiversity informatics and well-funded international efforts, Wikipedia has emerged as potentially the best platform for fulfilling E. O. Wilson’s vision.     

Alien arthropod predators and parasitoids: an ecological approach

Invasive alien species (IAS) coupled with climate change have been referred to as a “deadly duo”. Until recently research on invasion biology has centred mainly on alien plants and vertebrates, despite the numerical dominance of alien arthropods. Arthropods are the largest group of IAS worldwide and many can play a beneficial role, particularly in controlling insect and mite pests. Indeed, 1590 terrestrial arthropod species have been identified as alien to Europe but only a fraction has been shown to cause either an ecological or economical impact, yet knowledge is severely limited by a paucity of data. The IOBC/WPRS Working Group “Benefits and Risks of Exotic Biological Control Agents” developed the theme of this special issue to begin to address the limitations in understanding of this important research area. It represents a timely synthesis of current ecological knowledge and research on alien arthropod predators and parasitoids.     

The role of trade-offs in biodiversity conservation planning: Linking local management, regional planning and global conservation efforts

Biodiversity conservation planning requires trade-offs, given the realities of limited resources and the competing demands of society. If net benefits for society are important, biodiversity assessment cannot occur without other sectoral factors “on the table”. In trade-offs approaches, the biodiversity value of a given area is expressed in terms of the species or other components of biodiversity that it has that are additional to the components protected elsewhere. That “marginal gain” is called thecomplementarity value of the area. A recent whole-country planning study for Papua New Guinea illustrates the importance of complementarity-based tradeoffs in determining priority areas for biodiversity conservation, and for designing economic instruments such as biodiversity levies and offsets. Two international biodiversity programs provide important new opportunities for biodiversity trade-offs taking complementarity into account. Both the Millennium Ecosystem Assessment and the Critical Ecosystems or “hotspots” programs can benefit from an explicit framework that incorporates tradeoffs, in which a balance is achieved not only by land-use allocation among areas, but also by the crediting of partial protection of biodiversity provided by sympathetic management within areas. For both international programs, our trade-offs framework can provide a natural linkage between local, regional and global planning levels.     

An alien approach to invasive species: objectivity and society in invasion biology

Several authors have recently argued that invasion biologists should adopt a more objective and dispassionate stance towards invasive species. Brown and Sax (Austral Ecol 29:530–536, 2004; Austral Ecol 30:481–483, 2005) assert that invasion biologists risk their objectivity, “commit the naturalist fallacy” or “embark on a slippery slope” with engaged concern about invasive species. Elsewhere, Colautti and MacIsaac (Divers Distrib 10:135–141, 2004) propose a neutral language for invasion biology, one that insulates scientific from popular discussion about invasive species. While there is certainly hyperbole about the effects of some invasive species, the type of objectivity promoted in these papers may often be inappropriate for invasion biology. It implies a policy of non-action that is inconsistent with the conservation values of many invasion biologists. To engage these values, invasion biologists can adopt deliberative methods for environmental problem-solving that involve stakeholders in their research design and which still promote high standards of scientific rigor.     

Multi-scale analysis of butterfly diversity in a Mediterranean mountain landscape: mapping and evaluation of community vulnerability

This paper is an attempt to outline a protocol for animal diversity census and evaluation aimed for areas in view of landscape planning of territories of hundred square kilometres and more, that may work utilising different faunal groups and be anyway useful at various scales. Many papers are addressed to elaborate tools for landscape planning starting from biodiversity evaluation and butterflies are often utilised because of their sensitivity to landscape modifications. In this work, the biodiversity evaluation has been performed using three hierarchically linked landscape units at micro-, meso- and macroscale. Being species diversity values often inadequate to define the conservation interest of a landscape portion, more importance has been given to which species compose the species assemblages. A community vulnerability Index was coded and used for evaluating potential consequences of human disturbance on butterfly assemblages. Forty-four year samples were gained by visual census in the Sila Greca, Southern Italy, on an area of approximately 520 square kilometres. During 5 years work, 2,535 specimens and 94 species were recorded, equal to 75.8% of the whole Calabrian fauna. Four vulnerability levels have been established and used for mapping butterfly assemblage vulnerability in the area, starting from a vegetation map. Species richness was found somewhat contradictory at micro-scale, where the community vulnerability Index gives a sounder approach. S diversity gives a more reliable picture of naturalness at meso-scale, a level we identified with the “ecotope”. At this more “geomorphic” scale level, biological functions reflected by butterfly assemblages revealed to be clearly linked to seral processes. Similarity analysis results show that the ecotope species richness, here called “eta-diversity”, could be an useful measure of zoological landscape (faunation) potentialities.     

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.     

Lengths and correlates of lag phases in upper-Midwest plant invasions

There are often lag phases in plant invasions, seemingly dormant periods between arrival in a new range and rapid population growth. Lags impede prioritization of invasive-species control efforts: when eradication is most feasible, it is often unclear whether a species is benign or a potentially harmful “sleeper weed.” I used herbarium records to estimate lag phases for invasive or potentially invasive plant species in three regions of the upper Midwest. I tested whether factors related to species’ invasion epidemiology, traits, or the habitats they invade were correlated with lag lengths. From an initial pool of 151 species, there were sufficient records to test for lags in 76 for northern Wisconsin, 90 for southern Wisconsin, and 91 for the southern Lake Michigan region. Lags were identified in 77% (197) of these 257 datasets and ranged from 3–140 years with a mean of 47.3 ± 34.6 (SD). Lags differed by native range, introduction pathway, growth form and habit, dispersal mode, flowering phenology, pollination mode for a subset of species, and breadth, light availability, and water availability of invaded habitats. However, estimated lags were highly variable and tested factors did not have strong explanatory power. Exotic species comprised an increasing proportion of total herbarium records. Of the species with known introduction pathways, 85% were intentionally introduced, mainly as ornamentals. The long durations, high variability, and low predictability of lags, along with human culpability for an increasingly non-native flora, support a cautious approach to species introductions.     

Rapid shifts in the chemical composition of aspen forests: an introduced herbivore as an agent of natural selection

The global ecological impacts of introduced and exotic species can be dramatic, leading to losses in biodiversity and ecosystem “meltdown”, however, the evolutionary impacts of introduced species are much less understood. Further, very few studies have examined whether mammalian herbivores can act as agents of natural selection for plant traits. We examined the hypothesis that variation in aspen phytochemistry resulted in selective herbivory by Cervus elaphus (elk), an introduced mammalian herbivore. With the experimental removal of a large elk exclosure, elk selectively eliminated 60% of an aspen population previously protected from herbivory resulting in a dramatic shift in the phytochemical composition of the aspen forest. Selection gradients (β) varied from 0.52 to 0.66, well above average relative to other studies of selection. These results indicate that introduced herbivores can have rapid evolutionary consequences even on long lived native species. Because there are fundamental links between phytochemistry, biodiversity and ecosystem processes, the effects of an introduced herbivore are likely to have cascading impacts on the services ecosystems provide.     

Predicting habitat use and trophic interactions of Eurasian ruffe,round gobies,and zebra mussels in nearshore areas of the Great Lakes

The Laurentian Great Lakes have been subject to numerous introductions of nonindigenous species, including two recent benthic fish invaders, Eurasian ruffe (Gymnocephalus cernuus) and round gobies (Neogobius melanostomus), as well as the benthic bivalve, zebra mussel (Dreissena polymorpha). These three exotic species, or “exotic triad,” may impact nearshore benthic communities due to their locally high abundances and expanding distributions. Laboratory experiments were conducted to determine (1) whether ruffe and gobies may compete for habitat and invertebrate food in benthic environments, and (2) if zebra mussels can alter those competitive relationships by serving as an alternate food source for gobies. In laboratory mesocosms, both gobies and ruffe preferred cobble and macrophyte areas to open sand either when alone or in sympatry. In a 9-week goby–ruffe competition experiment simulating an invasion scenario with a limited food base, gobies grew faster than did ruffe, suggesting that gobies may be competitively superior at low resource levels. When zebra mussels were added in a short-term experiment, the presence or absence of mussels did not affect goby or ruffe growth, as few zebra mussels were consumed. This finding, along with other laboratory evidence, suggests that gobies may prefer soft-bodied invertebrate prey over zebra mussels. Studies of interactions among the “exotic triad”, combined with continued surveillance, may help Great Lakes fisheries managers to predict future population sizes and distributions of these invasive fish, evaluate their impacts on native food webs, and direct possible control measures to appropriate species.     

Plant species diversity in remnant grasslands on arable soils in the cropping Pampa

In order to provide a baseline for management decisions, we evaluated vegetation heterogeneity among 38 remnant grasslands located on high agronomic potential soils along a five degree-east–west transect in the cropping Pampa. This extensive region has been cultivated for well over a century, small-scale traditional forms of agriculture were progressively replaced with large-scale industrial forms and its cropped area has more than doubled in the last 25 years, threatening the maintenance of biodiversity associated to productive soils. We characterized the diversity of this grassland patches, identified its main environmental drivers and searched for factors that favor their invasion by exotic plant species. Vegetation samples were analysed through Canonical Correspondence Analysis, proportional distribution of functional groups was compared and regression models were estimated to identify variables associated with exotic species importance. Water availability, determined by climate and soil characteristics, was the main driving factor of floristic heterogeneity in these grasslands. Exotic richness, mostly of Eurasian species, increases toward the east, related to proximity to the entrance port. The functional group of C₄ native grasses seems to be responsible for biotic resistance to invasion. More than 80% of the species recorded during the first vegetation registers in 1930 are still present, while the number of exotics was seven-fold increased. Despite covering small areas as isolated stands, these are hot spots of native biodiversity and must be the object of increasing attention for conservation and restoration in order to protect the biodiversity of an extensive agricultural landscape.