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THE ENERGY CHOICES WE MAKE TODAY ARE resulting in changes in the amount of carbon dioxide emitted to the atmosphere that will be seen in the composition of the atmosphere for hundreds of thousands of years. Climate change is here; it’s happening and going to be with us for thousands of years.

The last two decades have seen an upsurge in research into the potential synergies between invasive species and climate change, with evidence emerging of increased invader success under climate change. All stages along the naturalization-invasion continuum are likely to be affected, from the introduction and establishment of alien species to their spread and transition to serious invaders. A key question is whether alien plants will have a relative advantage under climate change conditions. So far, evidence for differential responses of alien invasive and native species to climate change drivers (elevated CO2) and outcomes (increasing temperature, changing rainfall patterns, changes in disturbance regimes) is mixed. Although alien invasive plants appear to be more responsive to elevated CO2 than many native species, plant response to elevated CO2 and other climate change components is dependent on environmental conditions and resource availability. Similarly, correlative modelling of species-climate relationships has not revealed clear evidence that invasive plants are likely to be able to increase the extent of suitable habitat under future climates any more than their native counterparts. We suggest that the most important driver of a shift to alien-dominated vegetation under climate change will be the superior capacity of alien invasive plants to take advantage of colonisation opportunities arising from climate change, such as extreme climatic events, changes in disturbance regimes, and widespread reduction in vegetation resilience as range margin populations decline. There are substantial challenges ahead for managing invasive plants under future climates. Weed risk assessment and management approaches must incorporate consideration of future climatic conditions. Most importantly, we will need a shift in management approaches away from a focus on the control of undesirable alien plant species to building resilience of resident vegetation assemblages, in association with targeted monitoring and early eradication of alien plant species.

Disturbances are a primary facilitator of the growth and spread of invasive species. However, the effects of large-scale disturbances, such as hurricanes and tropical storms, on the broad geographic patterns of invasive species growth and spread have not been investigated. We used historical aerial imagery to determine the growth rate of invasive Phragmites australis patches in wetlands along the Atlantic and Gulf Coasts of the United States. These were relatively undisturbed wetlands where P. australis had room for unrestricted growth. Over the past several decades, invasive P. australis stands expanded in size by 6–35% per year. Based on tropical storm and hurricane activity over that same time period, we found that the frequency of hurricane-force winds explained 81% of the variation in P. australis growth over this broad geographic range. The expansion of P. australis stands was strongly and positively correlated with hurricane frequency. In light of the many climatic models that predict an increase in the frequency and intensity of hurricanes over the next century, these results suggest a strong link between climate change and species invasion and a challenging future ahead for the management of invasive species.

This report is targeted at policy-makers, particularly those responsible for developing climate mitigation and adaption strategies that address issues like conservation, ecosystem services, agriculture and sustainable livelihoods. It focuses on the primary linkages between invasive species and climate change, as well as the secondary and tertiary interactions of their corresponding impacts. Finally, the enclosed recommendations are intended to provide guidance on the best ways to integrate invasive species prevention and management into the consideration of climate change responses across a range of sectors. Building on a review of existing scientific and conservation literature (which is frequently centered on well-studied invasive species in developed countries), our research has reaffirmed that there are significant gaps and questions about the intersection of these two major drivers of change. The case studies included below highlight key relationships and questions related to invasive species, climate change and the role of ecosystem-based adaptation. The three key messages that can be distilled from this report are: 1. Climate change will have direct and second order impacts that facilitate the introduction, establishment and/or spread of invasive species. 2.Invasive species can increase the vulnerability of ecosystems to other climate-related stressors and also reduce their potential to sequester greenhouse gasses. 3.Using an ecosystem-based adaptation approach, these pressures on ecosystems and their ability to provide important services can be offset by preventing the introduction of new invasive species and by eradicating or controlling those damaging species already present.

In September 2003, Hurricane Isabel caused unexpectedly high levels of wind damage to an 80-to 100-year-old forest in the Piedmont of Maryland. The storm had decreased in intensity from landfall by the time it reached the study site—sustained winds were moderate and maximum gusts recorded in the area were only 62.7 mph (28.1 m?s-1). Midsized gaps (up to 1 ha) were created in forest that historically had only small or single-tree gaps. Isabel created the opportunity to determine whether natural disturbance facilitates the spread of exotic invasive plant species. Exotic invasive species populations were sampled in 400 5 x 5 m quadrats in a heavily damaged 1-ha, long-term forest study plot and in 160 5 x 5 m quadrats in 0.4 ha of a nearby, less-damaged forest between mid-October and mid-December 2003. Light levels (quantum flux density of photosynthetically active radiation) in the heavily disturbed Permanent Plot and the Less Damaged control plot were surveyed in October 2003 and 2004. The fall 2004 resurvey for exotic plants has also been completed. Based on a random sample of the fall 2004 exotics data, exotic invasive plant species responded strongly to the increased light levels in patches of forest damaged by Isabel. Collectively, the mean increase in percentage cover of exotic plants was 47.8% in high-light canopy gaps versus only 4.8 % in low-light non-gaps and 4.2% in the less-damaged forest. Several individual exotic species—Polygonum perfoliatum, Polygonum positive responses to higher light levels. The shadeloving caespitosum, and Lonicera japonica had significant biennial, Alliaria petiolata, changed significantly in the opposite direction, decreasing in the high-light areas and increasing in the lowlight areas. The authors are also investigating the interaction of exotic plants with native plants, forest regeneration, and white-tailed deer (Odocoileus virginianus) in damaged areas. Study areas and exclosures for these projects were set up in 2004 and will be resurveyed beginning in 2005.