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What sorts of species become 'invasive aliens' in a world of climatic change?

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Introduction

What sorts of species become 'invasive aliens' in a world of climatic change? Illustrate your answer with examples from both the plant and animal kingdoms. "Invasive alien species have caused untold damage to natural ecosystems and human economies alike over the past few centuries. Globalization is accelerating the destruction, as expanding tourism and trade offer more and more opportunities for unwanted species to hitchhike to new homes." - Mr. H. Zedan, Executive Secretary of the CBD, 6th meeting of COP of CBD, 2002 "The concept of 'alien' can be seen as irrational if we look back at the history of vegetation dispersal, survival and evolution and a time reference point is needed if we are to justify what is native and what is alien." - Trudgill, 2001: 681 There is a long history of concern about the impact of non-native species upon 'natural' environments, as illustrated by the 1905 publication date of Dunn's Alien Flora of Britain. The study of plant and animal invasions remains popular and has recently been described as one of the "hottest current topics in ecology" (Sol, 2001 cited in Henderson, forthcoming). A number of different disciplines have contributed to the study of 'invasive aliens'. As such the terminology is frequently misused and confused, and will be therefore be defined at the outset (Richardson et al., 2000). The term 'invasive alien' has two components; the first relates to the relative degree of success experienced by a species, whilst the second infers a description of the origin of the species (Henderson, forthcoming). The two components are not necessarily mutually causative, a species may be classified as an 'alien' without being 'invasive', or vice versa. Aliens (also known as exotics, non-natives or non-indigenous) may be defined as species 'that have been transported into a region by humans across a barrier that has apparently prevented natural dispersal' (Alpert et al., 2000). Invasives are 'naturalized plants [or animals] that produce reproductive offspring, often in very large numbers, at considerable distances from the parent plants [or site of introduction] and thus have the potential to spread over a considerable area' (Richardson et al., 2000). ...read more.

Middle

(1996) found that alien species tended to be taller, have larger seeds and were more likely to show protracted seed dormancy. Crawley et al. (1996) also distinguished several predictable patterns for the establishment of invasive alien plants; weeds in one country are likely to be weeds when introduced to another climactically matched country, the rate of establishment of alien plants will be proportional to the frequency and intensity of disturbance of the habitat, and that alien plants will grow bigger and have greater ecosystem impact than equivalent native plant species as a result of their release from their specialist pathogens. However, competitive ability is not only determined by intrinsic factors but also by extrinsic biotic and abiotic variables, including the identity of competing species and environmental conditions (ibid., 1996). The proportion of alien species differs considerably according to habitat, for example, alien species comprise 0.8% of UK waste ground communities compared with less than 0.1% in grass and heath land communities (ibid., 1996). Therefore, in many situations the traits which determine species invasiveness potential are highly contingent upon the local context and its associated processes. Whilst invasive species have been the subject of increasing popular and academic attention, few studies have related this problem to scenarios of global climate change (although see Dukes & Mooney, 1999; Mooney & Hobbs, 2000 for notable exceptions). Predictions of climatic change associated with anthropogenically-induced global warming are complex and contested, largely due to uncertainty about the magnitude and direction of numerous feedback mechanisms that may be induced. As well as forecasting both positive and negative changes in temperature and precipitation according to locality, models have also suggested that elevated levels of atmospheric CO2 and increased nitrogen deposition will be experienced (Dukes & Mooney, 1999). There are a number of possible interactions between alien species and climate change, for example, the severity of the impact of existing invasive species may be altered or formerly benign alien species may become invasive through a newly advantageous competitive trait (ibid., 1999). ...read more.

Conclusion

be counted as native. In the context of global climate change, a conceptualization of invasive aliens which is reliant upon relict ecological and arbitrary political boundaries will become unsuitable and unsustainable. Migration is likely to be the key survival strategy for those species unable to tolerate environmental changes in their present habitat as a result of climate change (Huntley, 1995). In a world of climatic change, it is necessary to re-assess the values behind our perception of what species will become 'invasive aliens'. Henderson (forthcoming) has taken the first step towards this goal, suggesting a more fluid and adaptable definition of 'alien' that is based upon the recent dispersal history of a species. Indeed, in the context of rapid climate change, many potentially invasive alien (or native) species with slow migration rates may have to be actively introduced (or translocated) if we are to ensure their long term global survival. In conclusion, if we accept that the present conceptualization of an 'invasive alien' as valid, the sorts of species that are likely to become invasive aliens in a world of climatic change are not dissimilar to those which are invasive aliens today. Many life history traits that have been identified in current invasive aliens, such as short generation times, will remain important, and many existing invasive aliens will be further advantaged by climate change. However, it is important to emphasize that the factors controlling which species become invasive are not only intrinsic and climatic variables. Any prediction of which species will become invasive will also require careful consideration of local processes, including the identity of competing species and any human intervention through habitat disturbance and fragmentation. However, in a world of climatic change which will involves range readjustment for some species to survive, any concept of an 'invasive alien' which rests upon political or relict ecological boundaries will need serious re-examination. Perhaps it is time to assess species in terms of their impact rather than their origin, and to move from perceiving species as having a rightful place to considering their potential ecological space. ...read more.

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