Biological invasions mediated by human activities are a huge source of change in ecosystems, with escalating impacts, both ecological and economic, worldwide (Brooks et al., 2004; Rejmánek et al., 2005; Mason, French & Russell, 2007; Yelenik, Stock & Richardson, 2007; Batten, Scow & Espeland, 2008; Marchante et al., 2008b; Gaertner et al., 2009; Jäger, Kowarik & Tye, 2009; Pejchar & Mooney, 2010; Pyšek & Richardson, 2010; Vilá et al., 2010), often resulting in an “invasional meltdown” 1 (sensu Simberloff & Von Holle, 1999). The origin of biological invasions is lost in time with some authors pointing to Late Devonian (Stigall, 2010). Nevertheless, it has increased consistently under human-mediation, reaching a peak in the second half of the 20th century (Pyšek & Richardson, 2010) when it gained “formal” recognition in the late 1950s with the pioneering work of Charles Elton (Elton, 1958), often considered as the “father of Invasion Biology” (Richardson & Pyšek, 2008). Scientists, politicians and Global Organizations (Ministério do Ambiente, 1999; Millennium Ecosystem Assessment, 2005; Commission of the European Communities, 2008; ISSG/IUCN, 2008; Pyšek & Richardson, 2010; TEEB, 2010) all recognize that invasive species damage ecosystem services, disrupt human well-being and threaten biodiversity. Ecosystem services affected by invasive species include supporting (e.g. alteration of succession patterns and soil and nutrient cycling), provisioning (e.g. threats to native species, alteration of genetic resources), regulating (e.g. changing pollination services and fire regimes) and cultural services (e.g. effects on ecotourism, changes in perception of landscape) (Millennium Ecosystem Assessment, 2005; Vilá et al., 2010). The alterations that occur at one trophic level may have repercussions into several other trophic levels (e.g. alterations in plant communities affect herbivores and parasitoids which are interlinked in food webs) amplifying the more frequently measured impacts of invasive alien species (IAS) (Carvalheiro, Buckley & Memmott, 2010).
Invasive species are reported to rank as the second agent of species endangerment and extinction (Pejchar & Mooney, 2010). Nevertheless, and despite the unquestionably extensive and profound impacts of IAS, there is no quantitative data that supports this universal ranking. Such level of threat has been justified for particular systems and taxonomic groups, where invasive species may even be
Invasional meltdown refers to the interactions among invaders that accelerate invasions and amplify their effects on native communities
5 the predominant threat (Miller, 1989; Wilcove et al., 1998). On a global scale, it is more correct to include biological invasions amongst the five mains causes of biodiversity decline (Millennium Ecosystem Assessment, 2005).
Invasive alien species (see Table 1.1 for definitions) are found amongst all living organisms, including many plants, animals and microrganisms (Elton, 1958; Vitousek, 2001; Perrings, Mooney & Williamson, 2010). Some of those species are “born” invaders (i.e. without major limitations on their adaptation to new environments) others are “made” invaders (i.e. evolve after being introduced in the new environment in order to adapt and become more fit) (Pyšek & Richardson, 2010). This work focuses on invasive plants, in particular the Leguminosae tree Acacia longifolia.
Table 1.1 Main concepts and definitions used in plant invasion biology (source Richardson et al., 2000).
Concept Definition
Exotic, alien, aloctonous, non-native, non-
indigenous
Plant taxa whose presence in a given area is due to introduction, intentional or accidental, as a result of human activity.
Casual, occasional escape, transient
Subset of alien plants that may flourish, and even reproduce occasionally in an area, but which do not form self-replacing populations, and which rely on repeated introductions to persist.
Naturalized, sub- spontaneous
Subset of alien plant that reproduce consistently and sustain populations over many life cycles without direct intervention by humans (or in spite of human intervention); often recruit offspring freely, usually close to adult plants, and do not necessarily spread into natural, semi-natural or human-made ecosystems. Invasive, environmental
weed
Subset of naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from parent plants (approximate scales: > 100 m; < 50 years for taxa spreading by seeds and other propagules; > 6 m/3 years for taxa spreading by roots, rhizomes, stolons, or creeping stems), and thus have the potential to spread over a considerable area; frequently adversely affecting native biodiversity and/or ecosystem functioning. Transformer Subset of invasive plants which change the character, condition, form or nature
of ecosystems over a substantial area relative to the extent of that ecosystem. Weeds, plant pests,
harmful species
Plants (not necessarily exotic) that grow in sites where they are not wanted by humans and which usually have detectable economic or environmental effects. Invasive alien species are a small fraction of exotic species that are increasingly moved around the world, intentionally or accidentally, crossing geographic barriers (and then a series of environmental and other barriers, see Figure 1.5 and Richardson et al. (2000) for more details) that had once limited their distribution to their natural ranges. Often, such species are moved without carrying the
6
natural enemies that in their native environment are one of the factors contributing to keeping them in check - enemy release hypothesis ERH (Keane & Crawley, 2002). ERH when associated with some species traits, e.g. prolific seed production, efficient mechanisms of dispersal, high growth rates, plasticity, altogether facilitate a plant species becoming invasive. Some invasive species are able to significantly modify the ecosystems over a substantial area; these are named transformers (Richardson et al., 2000).
Some of the characteristics of the species, e.g. prolific production of seeds which accumulate in long-lived, viable seed banks, are particularly important, because besides being responsible for initial invasion they ensure persistence into the future by allowing resurgence after control efforts or other forms of disturbance. Whether a species is invasive or not depends not only on the characteristics of the species but also on the characteristics of the new environment itself including factors such as resources availability, levels of antagonist species (e.g. predators, herbivores, pathogens) and intactness of the system. Propagule pressure (i.e. introduction effort), other human activities (reflected in variables such as Human wealth and demography), and intensity of disturbance (Essl et al., 2010; Perrings, Mooney & Williamson, 2010; Pyšek et al., 2010) can also play an important role in the process of whether or not a species will become invasive.
Although some ecosystems are intrinsically more susceptible to invasion than others, disturbance, fragmentation and changes in land-use are known to greatly increase ecosystems vulnerability to invasion, even with the same activity affecting different ecosystems types in distinct ways (Pyšek, Chytrý & Vojtech, 2010). Other authors (Blossey & Notzold, 1995) noted another factor, expressed as the EICA (Evolution of Increased Competitive Ability) hypothesis, according to which, species, after introduced into a new location, evolve and adapt to the new environment being able to reallocate resources in order to gain competitive advantage. This hypothesis makes it more difficult to predict which species will became invasive because unpredicted evolution, following introduction, can make a difference.
The time lag between the introduction of a species and the revelation of its invasive behaviour can be extremely variable. Even so, a recent study highlights that many of the most problematic invasive species (for all major taxonomic groups) are species that were introduced several decades ago, calling it an “invasion debt” and alerting to the fact that current human activities (including globalization and economic growth) will have their consequences on the extent of biological invasions several decades into the future (Essl et al., 2010).
7 The economic and ecological dimensions of biological invasions are interconnected and are far reaching. The changes in ecosystems that frequently increase their susceptibility to invasion impacts (e.g. disturbance, habitat fragmentation) are frequently economically mediated; and the resulting ecological impacts (normally considered as externalities to the economic activities and as such not taken into account by its promoters) have direct implications on the “natural capital” with repercussions which are manifest as economical problems. An increasing number of regional trade agreements (more than 420 reported to the World Trade Organization) aim to reduce barriers to movement of commodities and people, associated with increased trade, transport and travel all over the world (as a result of globalization). Although these agreements have many benefits for human well-being, they are increasingly facilitating the introduction of more species (some new, others already problematic), and consequently increasing probabilities of establishment and dispersal of such species (Perrings, Mooney & Williamson, 2010). Additional problems arise from the ever- increasing online trade, which facilitates unregulated movement of species, including invasives, to virtually every part of the globe (Derraik & Phillips, 2010).
Economic impacts of IAS, measured so far, include mainly management costs and also losses to provisioning ecosystem services (Pimentel, Zuniga & Morrison, 2005; Pejchar & Mooney, 2010; Vilá et al., 2010). Some of these costs could be partially balanced /attenuated if probability of success of costly management options was first evaluated, in a systematic manner, in order to weigh the possibility of “doing nothing” against the management actions and thus avoiding some of the huge funding efforts in systems without clear chances of success.
Besides environmental and economic effects of IAS, cultural impacts, centred in human uses and traditions, are quite diverse being possible to find invasive species culturally impoverishing (i.e. imply loss or replacement of culturally important native species), culturally enriching (i.e. augment cultural traditions), or culturally facilitating (i.e. continuity and reformulation of traditional ethnobiological practices) (Pfeiffer & Voeks, 2008). When considering management of invasive species, all these dimensions of impacts have to be balanced in an integrative way.