Invasive species & climate change

In consequence, under changing environmental and climatic conditions and due to natural selection, new species are established. These are extremely slow processes that take many millennia and centuries to happen.

During the last few centuries however, people started moving over large distances at an accelerating pace and shipping a larger volume of commodities to far-away destinations. This ever-increasing massive and rapid movement of people and goods has been facilitating the transport of various plants, animals and organisms to completely new, non-indigenous environments. Upon encountering new ecosystems, many of these intentionally or accidentally brought organisms may perish, not being able to survive in foreign environmental conditions. Some of them may be able to survive only if they are deliberately cultivated. Finally, some of them will become invasive, establishing their presence and spreading over a non-native environment.

Invasive species are most commonly defined as a non-native plant, animal or other organism that dominates the encountered ecosystem and impairs its function and structure. Invasive species displace or damage native fauna and flora, often posing serious threats to local biodiversity and causing adverse environmental, economic or public health effects. The lack of a natural competitor in this new ecosystem allows invasive species to be successful and resistant enough to survive in a foreign environment.

Invasive plants comprise about half of the flora of New Zealand and Hawaii.  Entire ecosystems in Northern California have been simply replaced by an alien counterpart. Florida, with its warm and sunny climate, as well as important transportation and trade hubs, is regarded as the US capital when it comes to species invasions. Nonnative plants that grow and propagate fast in the absence of their natural enemies cover thousands of acres of Florida’s forests, wetlands and waterways, posing a serious burden to the state and national economies.

A striking example of how invasive species can dominate a particular habitat is Cabo da PhotoRoca in Portugal, the westernmost tip of Europe. Flora autonomous to cliffs in this region has been subjected with fierce competition for some decades from hottentot fig (Carpobrotus edulis), an exotic shrub from South Africa introduced to Portugal as an ornamental plant. It has developed extraordinarily quickly in the absence of natural competitors, colonizing the space of other species and overwhelmingly dominating the cliff ecosystem.

It is important to remember that biological invasions are a fundamental and integral aspect of nature and have always been present in the history of life on Earth. What is of concern however, is the extraordinary rate at which the invasions are now taking place. This requires global attention and action.

Climate change and the host environment

Both climate change and invasive species pose extraordinary ecological challenges to the world today. The impact of climate change and rising average world temperatures can have a profound influence on species’ geographical ranges that are often set primarily by climate and consequently the host environment.

Climate change is altering vital aspects of the environment like temperature and precipitation, the frequency of extreme weather events, as well as atmospheric composition and land cover. The temperature, atmospheric concentration of carbon dioxide (CO2) and available nutrients are the key factors that will drive species survival; changes in these factors will most likely stress the ecosystems and the chances of invasions. The process of establishment of species has received wide research attention. Many scientists agree that climate change will alter destination habitat and increase vulnerability to invasion because of resource scarcity and increased competition among native fauna and flora.

It remains uncertain whether increasing concentrations of CO2 in the atmosphere will generally favor non-native plant species over native plant species. Some research is suggesting that elevated CO2 concentrations might hinder the pace of recovery of some native ecosystems after a major disturbance, like flood or fire. This could potentially lead to increased dominance of invaders in some regions. Furthermore, some organisms’ photosynthetic rates increase with elevated CO2, creating a new advantage: they can use water and nutrients more efficiently and therefore are more resistant to extreme conditions. Changes in the atmospheric composition could decrease the rate at which plants transpire and result in a higher availability of moisture in the soils underneath. In that scenario, it could be expected that in ecosystems where growth is limited by water availability, species that could take advantage of extra water could become more abundant.

Another effect of climate change could be increases in nitrogen deposition driven by fossil fuel combustion. In general, higher nitrogen concentrations favor fast-growing plant species. In some regions of the world, for instance in Europe, many plant ecosystems are adaptive and responsive to changes in nitrogen, and therefore changes in its concentration will not make them more vulnerable to invasions. However, in other regions (including parts of North America), many of the most adaptive plant species are alien grasses that are likely to proliferate and hinder the survival of native species. Thus, impacts of nitrogen deposition on invasions are likely to vary by region.

Finally, changes in precipitation and temperature will have the most profound impact on the geographic ranges of many species. Currently Florida and Hawaii are the two states most affected by invasive species, in part due to their mild climates. Changes in precipitation could cause water-loving or water-resistant species to outcompete one another. Rising temperatures would allow the spread northward of some species currently restricted in their northern ranges due to, for example, the probability of an early freeze. Increasing temperatures can enhance winter survival of some invasive organisms that would not survive otherwise. Habitat for some fish species in North America that favor cool and cold waters may decrease and warm fish species’ habitat might expand. Shifting species habitats can also result from natural climate fluctuations and geographical species migration. One good example is in the Western United States where a warming climate will cause various species to migrate to higher elevations. Some native species that will not be able to migrate upward at the same pace can be lost as a result. Such stressed ecosystems may have more available resources that could facilitate the successful invasion of non-native plants.

Some studies have shown that a short-term increase in changing conditions may facilitate the long-term establishment of alien plant species. Accelerating anthropogenic climate change might disadvantage species that cannot quickly extend their range and that have long generation times. Species that are able to shift ranges quickly might have an advantage. Species that are spread over a large latitudinal distance and thus tolerate a wide range of climates tend to be the most successful invaders.

Climate change and the transmission mechanism

When it comes to trade and the transmission of invasive organisms, changing climatic conditions influence three fundamental elements of invasion mechanism: the source location, the pathway and the destination.

There are a few possible scenarios of the climate change impact on pathways and destinations. From one point of view, it can become a limiting factor. Increased variability of extreme climatic events can potentially hinder the exports of agricultural goods that won’t be able to produce expected yields due to extreme climatic events like drought or flooding.  Overpopulation, and consequently water shortages, can disrupt the fragile balance of food supplies. In that sense, climate change can have a negative impact on the pathways and decrease the risk of the interception of invasive species in different countries. From another perspective, extreme events are likely to result in an increasing volume of migrating climate refugees. Displaced people are likely to bring goods, food and livestock that can be contaminated with larvae or an organism that may invade the new habitat where they relocate. Climate change and melting ice in the Arctic can also facilitate the process of opening new trade routes to ship goods and consequently invasive pests hitchhiking with transported commodities.

Historically, botanical and ornamental gardens have also been the sources for many plant invasions worldwide, once preadapted species escaped. Those plants that can survive only due to intentional cultivation might be able to disperse if the climate shifts in their favor. Some species can also disperse in areas that receive artificial treatment such as artificial irrigation.

Impacts

Many invasive organisms have an adverse impact on human health. Nonnative species can cause a disease and they can also vector a disease, which itself can be either native Mosquitoor non-indigenous. Many centuries ago in North America, smallpox was an invader from the Old World, as Black Death invaded Europe from the Far East. Modern trade and movement of commodities are greatly facilitating the dispersal of disease vectors. One recent case is the Asian tiger mosquito (Aedes albopictus) whose larvae were imported to the US on container ships carrying used car and truck tires. This mosquito is an efficient vector for many viruses including dengue, yellow and West Nile viruses and therefore its dispersal raised big public health concerns at various global destinations where it was introduced and was able to become established.

It is important to remember that today it would most likely be impossible to feed the population of the United States with only native plants. According to the US Census Bureau, in 2001 introduced species provided about 98% of the US food system, including crops and livestock. However, some dangerous invasive organisms can be accidentally introduced with crop seeds, soil or with various imported plant materials. Many of them, mostly weeds and plant pathogens, are causing several billions of dollars annually in losses to the US economy from crops, pastures and forests. It is estimated that losses in crop yields are about 65% attributable to plant pathogens and about 12% to weeds. Approximately 73% of these weeds are estimated to be alien.

Finally, invasions alter a wide range of ecosystem functions including productivity, hydrology and nutrient cycling. Invaders change the rules of survival for all species in a given ecosystem and tremendously affect their function. The most irreversible impacts are genetic extinctions. Other possible effects include reduction in biodiversity, major genetic changes and production of an entirely new, highly invasive species.

Management

Invasive plant species can be both the result of global changes and the drivers of change. They may be harmful to crops, industries, the environment and public health. Ecologically, species invasions are very problematic for biodiversity conservation and may lead to native species extinction. Invasions are of high concern also from the economic perspective as their mitigations pose very high costs, especially when protecting agriculture.

Preventing the spread of invasive species into more susceptible environments requires an awareness of the types of species that pose a threat to a particular ecosystem. They must be distinguished from various native plants forced to migrate in order to survive. Fast and uncontrolled propagation is a key issue in invasive species, because most of them disperse rapidly. If climates change, then new invasive species may disperse into novel climate regions. Ideally, various managers should be alert to new infestations and they should target populations that are likely to spread significantly.

It is also important to know which ecosystems are especially vulnerable to invasion in the face of climate change. As it is still hard to project the final result of these interactions, the best option for management at this point seems to be monitoring, early detection and rapid response. Once invasive species become established, strategic approaches for control and management become necessary. In US each year billion of dollars are spent on pest control.

Ecologists are now beginning to understand how species and ecosystems respond to single drivers of climate change, but they are still uncertain how these factors interact with each other and how the intensity of these phenomena will vary across different regions. There are many questions remaining to be answered.