Climate change and invasive alien species
Likely effects of invasive species are evaluated by assessing the vulnerability of regions and ecosystems to invasive species under climate change, taking into account any management actions that may be available. Outcomes of invasions depend not only on the attributes of the invasive species but also on the vulnerability of the invaded habitat. There is therefore a need to give at least as much attention to understanding the recipient ecosystem as to the alien species for the prediction of the outcome of an invasion. The impact of climate change on each of the three elements of invasion – the source location, the pathway and the destination – needs to be assessed as part of the process of estimating vulnerability to a given invasive species.
Tools for analysis of risk from invasive species under climate change
Climate determines the suitability of temperature and moisture for population growth in favourable seasons. Both types of effects have been described in CLIMEX, a climatic matching model, which can be used as a very efficient tool to analyse such a risk.
Impacts of climate change on invasive processes
Sources, pathways and destinations of invasive alien plants can be affected by global change.
Sources: area freedom (to which a commodity may only be exported if it can be demonstrated that the invasive species is absent from the growing area), is at serious risk from climate change. Similarly, the spread of many species with climate change will challenge the surveillance systems as invasive species continue to encroach on areas that were previously designated as outside its endemic range.
Management responses to climate change will result in changes in the pattern of production and trade in commodities.
Pathways: the risk of contamination of pathways is increasing with increased trade and tourism and the development of a global economy. Climate change per se is likely to have limited direct effects on the movement of invasive species along trade routes. While the main source of transfer of pests appears to be trade, another accelerating source is the massive displacement of people from countries affected by political unrest. Refugees may bring their livestock with them, forming a mechanism by which parasites could be moved around. The associated food and other materials such as seed provided in these cases could also act as vectors for invasive alien species. These risks are greater in Africa, Asia, and South America, were political boundaries are poorly supervised.
Climate and wind systems affect the long-distance migration routes of some pests. Changed conditions that affect the spread of migratory pests, waterborne weeds and pathogens are likely to provide more natural pathways.
Destinations: the impact of invasive species at destination depends on (1) their initial success in establishment, (2) their direction and rate of spread, and (3) their population dynamics and geographical distribution.
- Initial establishment: given the extent of human disturbance, the likelihood of establishment of weeds under climate change will be increased.
- Spread: Interactions between climate change and other factors such as accelerating transport may increase the spread capacity of a pest. Climate change may also alter the frequency and intensity of extreme events, which are important in many dispersal processes. Wind and water transport is affected by climate, with storms, prolonged rainy seasons, and flooding aiding the dispersal of many invaders. Isolated events such as major floods can allow invasive species to escape from areas that were secure for decades, as shown by the example of Mimosa pigra in Australia.
- Population dynamics and geographical distributions: the potential for population growth each year depends on the duration and suitability of the season. Longer growth season will allow species with few generations to undergo an extra generation or part of one, causing larger population increases. Perennial species such as woody weeds could also increase their biomass rapidly and pass through the vulnerable seedling stage more rapidly. As climate change and enhanced CO2 will affect soil moisture availability, they will also affect plant population dynamics. The outcome of the competition between C3 and C4 species is likely to change. Moreover the projected increase in the amount of foliage produced by plants under elevated CO2 is likely to alter the microclimate by retaining more moisture in the canopy. This is likely to favour the propagation of invasive fungal diseases.
Climatic effects on species interactions
Climate has effects on interactions between invasive species and their host plants, predators and competitors in addition to direct effects on species. These interactions have pervasive effects on ecosystems and need to be taken into account when making a risk assessment for global change impact.
Climatically induced stress on plants can reduce their ability to resist invaders. Vegetation, made vulnerable by the stress of a changing climatic environment often falls prey to insects or pathogens, or succumbs to competition. For example, drought kills native plants, leaving gaps in vegetation that are quickly occupied by invasive species such as Lantana camara in rain forests. Drought and freezing reduce the resistance of trees to insect attack and each phenomenon is likely to change in both frequency and intensity under climate change.
Different species at the destination may limit the population growth rates of an invasive species through a variety of mechanisms. Conversely, an invasive species may out-compete an established species and displace it from a large region. Similarly, the balance between predators and their prey may vary with the season. Climate change will therefore alter the effectiveness of biological control of some species in some regions. However, the greatest impact of climate change is likely to be through the impact of extreme climatic events on establishment rates of small populations of introduced biocontrol agents rather than through the effect of changed average conditions on the size of equilibrium populations of the agents.
Interaction between climate change and other global change factors
Climate change is only one of several global change factors, land use, fire, cultivation, or deforestation, that disturbs natural vegetation. The resulting fragmentation of the landscape reduces the ability of resident species to resist invasion by opportunistic invasive species. Corridors for movement of beneficial species can also provide access for noxious organisms. Disturbance from multipurpose use of protected areas carries high risks of providing opportunities for invasion by pests.
A few examples
Many native plants and forests are susceptible to the root pathogen Phytophthora cinnamomi. A 3°C rise in temperature, an intensification of the hydrological cycle, and the increased occurrence of extremely dry or wet weather events could lead to more outbreaks of the disease.
A warming of temperate areas will enable entire colonies of European wasps (Vespula germanica) to overwinter, as they have done in Australia. On the other hand, the wasps do not appear to thrive in hot and humid environments, and their range could retract somewhat in the subtropics.
Adaptive management
There is limited potential in the natural systems to adjust to impacts of climate change. In agriculture, changes in regional climates will affect current, ecologically based management practices to contain outbreaks of pests. Monitoring of climatic trends and their biological consequences will need to be coupled with modelling to tune adaptive management measures progressively.
Sources
Sutherst R (2000) Climate change and invasive species: a conceptual framework. In Invasive species in a changing world (Mooney HA, Hobbs RJ, Ed.) 211-240.