A perspective on climate change and invasive alien species
It is not an easy task to predict how climate change will affect biodiversity because of the difficulty to predict species’ responses and the complexity of interactions. It is considered that the problem of biological invasions will worsen due to climate change. It is expected that more non indigenous species will cross frontiers because human activities will promote species movement. The alteration of sites and new climatic conditions could favour the reproduction and spread of alien species. Nitrogen deposition, increasing CO² concentration in the atmosphere, global warming, changes in fire frequency and precipitation patterns, together with land use modification are likely to play an increasing role in the success of invasive alien species.
Impact of climate change on plants
Climate change could affect the dynamic of plant invasions in two different ways:
- by causing alterations in native ecosystems leading to the establishment and spread of invasive alien plants
- by favouring individual traits of particular IAS
Alterations in native ecosystems
Changes in temperature and increasing disturbance elements such as fires may stress native species, decreasing the resistance of natural communities to invasions. Native communities could be affected through particular species being limited or favoured, or through the alteration of inter-specific relations at all levels. The loss of keystone species or functional groups of plants could profoundly influence the degree of vulnerability of native communities to invasion. Effects of climate change have been projected for the distribution of 1350 European plant species for the late 21st century. Results show that the worst scenario leads to a mean species loss of 42% and turnover of 63%, thus predicting a profound alteration in communities and ecosystems. Changes in temperature, precipitation, moisture, level of CO² and nitrogen deposition could act as factors for the selection of plants, unbalancing ecosystems by changing the dominance equilibrium, and the interactions between species and their environment.
Favouring individual species traits
The response of species to a changing environment will be individualistic, highlighting the importance of producing predictions at species level.
The response of plants to the increased temperatures seems to be mainly phenological compared to those of animal species where range shifts have been clearly detected. However, the spread of shrubs into the tundra has been reported, as well as shifts in the upward tree-limit in Sweden and Russia. Nevertheless, range shifts of plants are slower than animal shifts. A longer growing season could influence species’ reproductive capacity (increased seed production and biomass) and higher temperatures could improve plant fertility resulting in increased population sizes. Animal pollinated invasive plants could benefit an increased insect activity due to higher temperatures and longer summer periods, leading to an increase in fruit and seed set. However, increasing asynchrony in insect-plant systems or predator-prey could have the adverse impact. Fewer winter frosts and fluctuations in water levels may cause the expansion of aquatic invasive alien plants.
In experiments, invasive plants grown individually respond positively to high levels of CO², but their response changes in the presence of other species. Species using the C3 photosynthetic pathway used CO² even more efficiently than species using C4 and CAM pathways. Among plants using C3 pathway, species in symbiosis with nitrogen-fixing microbes respond strongly to elevated CO² in both conditions. Separately, C3 plants respond more positively than C4, but species’ responses change in mixed C3-C4 communities depending on other factors such as water, nutrient and light availability, temperature, the efficiency of species using resources, making the prediction of which species will be the most favoured difficult.
The combination of rising temperatures and CO² that stimulates plant growth and litter accumulation could lead to an increase in fire frequency. Additionally, extreme events such as floods, storms, heat-waves, droughts, acting as disturbance elements, could increase the risk of new invasions.
Impact of climate change on insects
Insects are strongly influenced by climate, especially temperature: life cycle duration, voltinism, population density, size, genetic composition, etc., can vary in response to the change in temperature. The distribution of many species is limited by summer heat availability rather than the lethal effect of extreme temperatures. Therefore, predicted climate changes are expected to take part in the range of expansion/contraction of insects, affecting their phenology and altering their rates of growth and development.
The responses of insects to climate change are expected to be complex and diverse, depending on the life-history of the insect and host plant growth strategy. Generalist feeders, cosmopolitan species, multivoltine species, phenotypical plasticity, etc. might be traits predicting future invasive success. Opportunity for colonization, and dispersal suitability of the habitat and the host community are factors that also play an important role, making predictions of invasive species response to climate change a real challenge.
Insect traits
Diet breadth
Generalist feeders have a higher probability of finding a suitable host plant than those species that are specialist and restricted to one or a small number of host plants. Specialist feeders will have to move and stay on the single host species in order to survive. Cosmopolitan species (species that have a broader host range and species found at more than one latitude) are more likely to find suitable host plants under climate change.
Phenological plasticity
With climate change, springs occur earlier and the growing season is expected to extend. The majority of herbivorous species rely on close synchrony with their host plants to successfully complete their life cycles. Phenological uncoupling will take place when climate change has different impacts on insects and their host plants. This will be unfavourable to herbivore species such as Lymantria dispar (Lepidoptera: Lymantriidae) that are tied to a specific window of time. Expansion of the growing season will be beneficial to multivoltine species since they could produce a larger number of generations in an annual cycle.
Lifecycle strategy
Many researchers have predicted that increasing temperatures will lead to increasing winter survival and increasing numbers of generations per year, thus greatly increasing pest pressure. Species may also increase their range. There is evidence of new invasions of migratory insects, such as Lepidoptera in London, as a result of the rising temperatures. Moreover, non diapausing species, frost sensitive species and species able to overwinter in their active stage show an increase of winter survival in warm winters, and are therefore expected to increase their population densities and expand their geographical range.
Dispersal potential
Large scale shifts in the geographical patterns of agricultural and forest production are expected as they adapt to climate change, and additionally, the origin of the respective products plus the way in which they are transported are likely to change. This will allow a whole new collection of potential invaders. In addition, changes in atmospheric circulation patterns could lead to aerially dispersed insects reaching new areas.
Changes in resource/niche availability
The occurrence of intense storms, late frosts and severe drought will increase, and may lead to detrimental effects on native species, providing opportunities for non native species establishment.
Sources
Capdevila-Argüelles L, Zilletti B (2008) A perspective on climate change and invasive alien species. Council of Europe. Convention on the conservation of European wildlife and natural habitats. T-PVS/Inf(2008)5. 26 p.