Philip W. Lambdon

Disentangling the role of environmental and human pressures on biological invasions across Europe

The accelerating rates of international trade, travel, and transport in the latter half of the twentieth century have led to the progressive mixing of biota from across the world and the number of species introduced to new regions continues to increase. The importance of biogeographic, climatic, economic, and demographic factors as drivers of this trend is increasingly being realized but as yet there is no consensus regarding their relative importance. Whereas little may be done to mitigate the effects of geography and climate on invasions, a wider range of options may exist to moderate the impacts of economic and demographic drivers. Here we use the most recent data available from Europe to partition between macroecological, economic, and demographic variables the variation in alien species richness of bryophytes, fungi, vascular plants, terrestrial insects, aquatic invertebrates, fish, amphibians, reptiles, birds, and mammals. Only national wealth and human population density were statistically significant predictors in the majority of models when analyzed jointly with climate, geography, and land cover. The economic and demographic variables reflect the intensity of human activities and integrate the effect of factors that directly determine the outcome of invasion such as propagule pressure, pathways of introduction, eutrophication, and the intensity of anthropogenic disturbance. The strong influence of economic and demographic variables on the levels of invasion by alien species demonstrates that future solutions to the problem of biological invasions at a national scale lie in mitigating the negative environmental consequences of human activities that generate wealth and by promoting more sustainable population growth.

Plant extinctions and introductions lead to phylogenetic and taxonomic homogenization of the European flora

Human activities have altered the composition of biotas through two fundamental processes: native extinctions and alien introductions. Both processes affect the taxonomic (i.e., species identity) and phylogenetic (i.e., species evolutionary history) structure of species assemblages. However, it is not known what the relative magnitude of these effects is at large spatial scales. Here we analyze the large-scale effects of plant extinctions and introductions on taxonomic and phylogenetic diversity of floras across Europe, using data from 23 regions. Considering both native losses and alien additions in concert reveals that plant invasions since AD 1500 exceeded extinctions, resulting in (i) increased taxonomic diversity (i.e., species richness) but decreased phylogenetic diversity within European regions, and (ii) increased taxonomic and phylogenetic similarity among European regions. Those extinct species were phylogenetically and taxonomically unique and typical of individual regions, and extinctions usually were not continent-wide and therefore led to differentiation. By contrast, because introduced alien species tended to be closely related to native species, the floristic differentiation due to species extinction was lessened by taxonomic and phylogenetic homogenization effects. This was especially due to species that are alien to a region but native to other parts of Europe. As a result, floras of many European regions have partly lost and will continue to lose their uniqueness. The results suggest that biodiversity needs to be assessed in terms of both species taxonomic and phylogenetic identity, but the latter is rarely used as a metric of the biodiversity dynamics.

Alien Vascular Plants of Europe

In terms of invasion biology, vascular plants are the most intensively researched taxonomic group; at least 395 plant invaders have been addressed in detailed case studies globally, accounting for 44% of all invasive taxa studied; after North America, Europe is the continent enjoying the most intensive study with at least 80 invasive plant species having been addressed (Pysek et al. 2008). However, although there is a considerable body of information on major plant invaders in Europe (see also Weber 2003), the situation is much less satisfactory as far as complete national inventories of alien plants are concerned. Prior to the DAISIE project (www.europe-aliens.org), only few countries had a sound information on the composition of their alien floras, available in specialised checklists, notably Austria (Essl and Rabitsch 2002), the Czech Republic (Pysek et al. 2002), Germany (Klotz et al. 2002; Kuhn and Klotz 2003), Ireland (Reynolds 2002) and the UK (Clement and Foster 1994; Preston et al. 2002, 2004). This situation directly translated into poor knowledge across the European continent. The only available continental analysis of plant invasion patterns in Europe (Weber 1997) was based on data from Flora Europaea (Tutin et al. 1964–1980), the only syn-thetic source of information on floras of particular countries, including alien spe-cies. This source is, however, nowadays outdated and contains numerous inaccuracies in data for individual countries (Pysek 2003). In general, information on the presence and distribution of alien plant species for most European countries was scattered in a variety of published and unpublished accounts and databases; this is the case in other continents too (Meyerson and Mooney 2007). On the plant side, DAISIE was thus a major challenge of collating and assessing existing data on the most numerous group of European aliens and concentrating this informa-tion in an authoritative continental inventory.The European area covered (Fig. 4.1) by the plant team of DAISIE was partly determined by the geographical coverage of source floras, but it was broadly attempted to use the limits set by Flora Europaea (Tutin et al. 1964–1980) for the north and central continental boundaries (i.e., as far east as the Urals, to the bor-der of the Black Sea but excluding the Caucasus). In the south-east, Cyprus was

Do alien plants on Mediterranean islands tend to invade different niches from native species

In order to understand invasions, it is important to know how alien species exploit opportunities in unfamiliar ecosystems. For example, are aliens concentrated in niches under-exploited by native communities, or widely distributed across the ecological spectrum? To explore this question, we compared the niches occupied by 394 naturalized alien plants with a representative sample from the native flora of Mediterranean islands. When niche structure was described by a functional group categorization, the distribution of native and alien species was remarkably similar, although “succulent shrubs” and “trees with specialized animal pollination mechanisms” were under-represented in the native species pool. When niche structure was described by Grime’s CSR strategy, the positioning of aliens and natives differed more strongly. Stress-tolerance was much rarer amongst the aliens, and a competitive strategy was more prevalent at the habitat level. This pattern is similar to previous findings in temperate Europe, although in those regions it closely reflects patterns of native diversity. Stressed environments are much more dominant in the Mediterranean. We discuss a number of factors which may contribute to this difference, e.g., competitive and ruderal niches are often associated with anthropogenic habitats, and their high invasibility may be due partly to introduction patterns rather than to a greater efficiency of aliens at exploiting them. Thus far, the reasons for invasion success amongst introduced species have proved difficult to unravel. Despite some differences, our evidence suggests that alien species naturalize across a wide range of niches. Given that their ecologies therefore vary greatly, one may ask why such species should be expected to share predictable traits at all?

Consistent performance of invasive plant species within and among islands of the Mediterranean basin

Since the success of an invasive species depends not only upon its intrinsic traits but also on particular characteristics of the recipient habitat, assessing the performance of an invader across habitats provides a more realistic analysis of risk. Such an analysis will not only provide insights into the traits related to invasiveness, but also the habitat characteristics that underpin vulnerability to invasion that, taken together, will facilitate the selection of management strategies to mitigate the invader’s effect. In the present study, we considered the Mediterranean basin islands as an excellent study region to test how the same invasive species perform in different habitats within a single island, and to scale up differences among islands with similar climate. We tested how the performance of three widespread plant invaders with clonal growth but contrasting life-history traits, a deciduous tree Ailanthus altissima, a succulent subshrub Carpobrotus spp., and an annual geophyte Oxalis pes-caprae, varied depending upon the species identity, habitat, and invaded island. The environmental parameters considered were habitat type, elevation, species diversity in the invaded plot, and several soil traits (% C, % N, C/N, pH, and relative humidity). The study documents that the performance of these three important and widespread plant invaders is dependent mainly on species identity, and less upon the invaded island’s general features. Likewise, differences in performance among habitats were only significant in the case of Ailanthus, whereas Carpobrotus and Oxalis appear to perform equally well in different environments. Ailanthus thus appears to have a broader spectrum of invasiveness, being able to invade a larger number of habitat types. On the contrary, Carpobrotus spp. have not yet invaded habitats different from those where the species have been originally introduced and where they are still commonly spread by humans. Oxalis distribution is mainly related to agricultural activities and disturbed sites, and the total area infested by this geophyte may be more reflection of the extent of suitable habitats than of invasiveness or ecological impact. Our results confirm the potential for these species to significantly alter the functioning of ecosystems in the Mediterranean islands and highlight the risk to other islands not yet invaded.

Consistency in the habitat degree of invasion for three invasive plant species across Mediterranean islands

Habitats are known to vary in their vulnerability to invasion by alien plants and different species often colonise distinct habitats. To assess the consistency in the degree of invasion of particular habitats, this study examined the frequency of occurrence and local abundance of three invasive plant taxa: Ailanthus altissima, Carpobrotus spp., and Oxalis pes-caprae across different habitat types on four representative Mediterranean islands. We conducted systematic field surveys recording the presence-absence and cover of these taxa on the islands of Mallorca, Corsica, Sardinia and Crete. Drawing on the results of 5,285 sample points, the frequency of occurrence of the three invaders tends to be higher than expected in urban, ruderal and roadside habitats. In contrast, scrub habitats rarely contain any of the three invaders, indicating that they may be more resistant to invasion. The degree of invasion, determined by the local abundance of an invasive plant in any one habitat, varies according to the identity of the invader and the island. However, based on average abundance, Oxalis pes-caprae exhibits the highest degree of invasion, and Carpobrotus spp. the least. There is no indication that any one of the four islands is more prone to either higher frequencies or abundances of the three invaders. These patterns suggest that anthropogenic changes in Mediterranean islands will increase the vulnerability of certain habitats to invasion and increase the distribution of these three invasive taxa at any of the four islands.

Alien Bryophytes and Lichens of Europe

Bryophytes (Bryophyta) include mosses (Bryopsida), liverworts (Hepaticopsida) and species-poor hornworts (Anthoceratopsida) (Soderstrom et al. 2002; Hill et al. 2006). Lichens are composite organisms, arising from a mutualistic association between a saprophytic fungus and a photosynthetic alga or bacterium (Ahmadjian 1993). The photosynthetic partner is usually also found as a common free-living species, and only the highly specific fungal partner is likely to be alien within Europe. Lichens are taxonomically disparate, united by common trophic strategy which has been adopted across a diverse range of fungal lineages. Lichens are dis-tantly related to bryophytes, and biologically very different.Why therefore do we consider the two groups together in this chapter? In the context of invasions they share a number of important features which present strong practical parallels in the issues they create: (1) they are poorly recorded, so we have little information to assess their invasion history; (2) they are dispersed efficiently by spores, and have much greater natural colonizing ability than other major taxa; (3) since they have few cultivated uses there is a near-absence of deliberate intro-ductions; (4) being small organisms and rarely parasitic, their impacts tend to be measurable only on a micro-scale (5) the possibility of subtle but long-term effects of such invasions has yet to be considered by the scientific community.