For most of evolutionary history, species moving from one place to another is a natural process. In contrast, the movement of species from one place to another in the modern world can occur as a consequence of human activities. For instance, the high vessel traffic within the Great Lakes brought about by international trade is one major factor that led to the introduction of non-native aquatic species in the said body of water. The Great Lakes is said to be the freshwater ecosystem with the highest rate of introduction of non-native species. According to a 2006 study published in Diversity and Distribution, one new non-native species is discovered in the Great Lakes every 28 weeks.
All invasive species are non-native species. However, not all non-native species that are intentionally or unintentionally release into a new environment become invasive species. Non-native species are considered invasive if they cause harm to the economy, environment, or human health. Studies prepared for the former Congressional Office of Technology Assessment revealed that 31% of non-native insects, 28% of non-native fishes, and 8% of terrestrial non-native species were not invasive and were actually beneficial for humans and various industries. There are invasive species that are damaging in one area but are neither beneficial nor detrimental in other areas, some are even beneficial. For instance, the Asiatic clam is beneficial in the Potomac River because it increased water clarity which in turn encouraged the growth of aquatic vegetation and attracted various aquatic birds. In other places, Asiatic clams can clog intake pipes, damage industrial water systems, and alter habitats. When the Asiatic clams alter a habitat, their influence can affect the entire ecosystem. They can threaten the very existence of native species. In fact, several invasive species were linked to extinctions. Despite having several studies on the subject of invasive species and reaching a deep understanding of what makes environments suitable for invasion and determining characteristics of species that are capable of invasion; predicting which species will become invasive and when they would invade is highly difficult.
Non-native species may become invasive when they are introduced to areas that have climates that are similar to the climate of their original habitat. Climate change can change ecosystems, this complicates managing native, non-native, and invasive species.
Loss of native biodiversity, altered habitats, changes in water chemistry, altered biogeochemical processes, hydrological modifications, and altered food webs, are the ecological impacts that are caused by Aquatic Invasive Species.
Among the different kinds of water bodies, wetlands, including estuaries, are some of the most invaded habitats in the world. Zebra mussel, purple loosestrife, tamarisk, Asian carp, Caulerpa or marine green alga, and the green crab are aquatic invasive species that are of great concern.
How Climate Change Affects Invasive Species
Climate change may cause alterations in ecosystems that change the conditions of the area making them suitable for introduced non-native species. Climate change could also enable the spread of invasive species through range expansion. Increased atmospheric carbon dioxide, modified precipitation regimes, warming ocean and coastal currents, increased ambient temperature, and altered nitrogen distribution are several changes that are brought about by climate change. All of these can increase the chances of invasive species to get into and dominate a certain habitat. There are several studies that corroborate this, climate change may enhance the climate impact of invasive species.
There have been several studies that investigated the effects of increased atmospheric carbon dioxide on enabling species invasions. Despite this, predicting invasions is still difficult because most of these studies are done on a small scale. With regards to increased carbon dioxide levels in aquatic ecosystems, their effects on invasive species are not well understood. However, it is well understood that increasing carbon dioxide in aquatic ecosystems leads to acidification which in turn can damage coral reef ecosystems. There are also studies that revealed increasing carbon dioxide in aquatic environments led to alterations in macroalgae and microalgae and plant dynamics. One of these studies revealed that increasing both carbon dioxide and temperatures leads to an increase in the growth rate of the invasive aquatic plant dioecious hydrilla. Since climate change can increase both carbon dioxide levels and atmospheric temperatures, climate change can increase the growth rate of invasive aquatic plants. A different study investigated how elevated carbon dioxide can impact plant invasions in various ecosystems. In this study, it was found out that increasing carbon dioxide levels can create favourable invasion conditions by increasing resources, plant production, soil moisture, and nitrogen uptake.
For Aquatic Invasive Species, increased temperatures and changes in precipitation patterns are more likely to affect the rate of invasion than increasing carbon dioxide levels. Emergent macrophytes in lakes are able to increase their biomass greater when there is an increase in temperature compared to when there is an increase in carbon dioxide levels. The establishment, spread, and impact of aquatic invasive species are influenced more by variables such as water temperature and hydrological regimes than by carbon dioxide.
Due to increased storm intensity which is brought about by climate change, some areas would experience increased droughts while others would experience increased floodings. Droughts and floods are consequences of climate variability. Having sufficient knowledge of climate variability can also improve one’s understanding of how ecosystems respond to the stress of altered hydrology.
A significant portion of invasive species are ecosystem pioneers and most ecosystem disturbances can encourage most of these pioneer species to increase in numbers. Ecosystem disturbances can come in the form of changes in precipitation patterns which are caused by climate change. Basically, climate change can cause changes in precipitation patterns which encourages an increase in the population of invasive pioneer species. This would ultimately affect their establishment and dispersal within a habitat. An increase in precipitation may push species downstream. Thus, upstream species could get displaced downstream. This may turn the displaced upstream species into an invasive species in their new downstream habitat.
Wetland invasive plant species have seeds that are easily dispersed by water which is why they can invade wetlands with ease in the first place. Flooding and rainfall are also important factors that affect the dispersal of invasive weed species. There are data that suggest that the amount of rainfall in the previous wet season affects the size of the area that is colonized by invasive plant species which substantiates the notion that rainfall is an important factor in plant species invasion.
There are new species invasions that are caused by the rising ocean temperatures. For instance, the population of aquatic sessile invertebrate species were compared with temperature data in one study. The study’s data shows that winter water temperatures are becoming warmer that the recruitment of introduced ascidians is happening earlier than usual. However, the recruitment of native ascidians was not affected. These conditions allow the introduced ascidians, which are able to settle first, to dominate and out-compete the native ascidians. Another thing that is revealed by this study is that introduced species grow faster in warmer waters compared to the native species. Aside from encouraging the growth of invasive species, the warming waters may shift coastal currents northward which could introduce invasive species towards that direction. A separate study which was conducted in California revealed that shore water temperatures rose 0.75°C on average and 2.2°C in summer. Furthermore, invertebrate species from the south became more abundant as a result of the rising temperatures while the northern invertebrate species decreased in numbers. This observation may not entirely be a direct result of the increase in water temperatures but could also be due to cyclical tidal fluctuations. Essentially, these changes in species composition are not simply caused by temperature changes.
Mytilus galloprovincialis is one of several aquatic invasive species that invaded northward of California as a consequence of the warming waters of the northern area. This may have to do with their relatively high tolerance for warm water temperatures and increased salinity levels compared to the native blue mussel. Although M. galloprovincialis hasn’t replaced the native mussel species up north, they have replaced them along the southern and central California coastline. Despite the rising temperatures, the northern coastline of California is still dominated by cold-tolerant native species. However, if the temperatures continue to rise, the possibility of the invasive mussel invading the northern areas increases.
There are some studies that investigated how increasing temperatures brought about by climate change could enable certain species to invade certain areas under specific circumstances. There are also some studies that deal with the potential of a species to invade the Great Lakes. In one of these studies, 58 species were evaluated to determine their potential to invade the Great Lakes. This evaluation was done by comparing the 58 species to 11 recent invaders. Among the 58 species that were included in the study, 27 have the potential to invade and establish in areas of the Great Lakes if the temperature warms up due to climate change. Several studies that re-evaluated the conditions in which certain plant species thrive revealed that monoecious hydrilla and Phragmites australis or the common reed, are actually adapted to higher temperatures than previously known in scientific literature. Another process that can increase the rate of spread of plant species as a consequence of warming temperatures is the increasing ratio of the sexual to asexual reproductive periods for plant species.
Management Responses to Climate Change
In a 2007 study, it was revealed that zebra mussels have already invaded, aside from the Great Lakes itself, 473 other smaller lakes, and several rivers in 23 states. It has even reached aquatic ecosystems in Nevada. They tend to form dense aggregates on hard substrates and thus they can clog pipes of water systems. Moreover, they can consume native phytoplankton and other species in the water which can disrupt the ecosystem. It is estimated that zebra mussels and quagga mussels may cause $1 billion in damages and costs annually.
Sadly, zebra mussels are difficult to deal with when they would establish in a water body and there are no methods to selectively eradicate them once this happens. The only way to deal with zebra mussel invasions is to prevent them from invading and or establishing a body of water. To do this, a sufficient understanding of zebra mussel invasions is needed. Zebra mussels are spread by passive transport. They can hitch a ride in the ballast and or bilge tanks of ships. They could also attach to boat hulls and other aquatic equipment. By inspecting and washing boats properly and dumping live bait and bilge water onto land, aquatic invasive species such as zebra mussels are barred from invading certain environments. Aside from that, educating people who are likely to spread the aquatic invasive species such as recreational boaters and anglers could also help with preventing aquatic invasive species from invading ecosystems and damaging them. Fortunately, in places where zebra mussel invasion has occurred, or are vulnerable to it, extensive efforts are being made to prevent further or future invasions. These efforts can come in the form of boat inspections, news releases, presentations, education of divers, and training of port of entry personnel.
A study published in the Canadian Journal of Fisheries and Aquatic Sciences revealed some interesting findings regarding zebra mussel invasions. As climate change continues to increase the water’s temperatures, waters in the southern latitudes of the United States may become increasingly unsuitable for the zebra mussels. This is due to the fact that as temperatures rise, the zebra mussels’ metabolic rates rise as well. This species of mussels are sensitive to metabolic changes brought about by temperature changes and are not well adapted to them. Thus, if the water’s temperature increases too much, the zebra mussels may die off. Aside from increasing temperatures, an increase in turbidity could also kill zebra mussels. The increase of indigestible particles can clog the gills of the zebra mussels and suffocate them. With these findings, it is predicted that climate change will make bodies of water in the high latitude areas of Canada and the U.S. become more suitable for the zebra mussels which encourages invasion while the low latitude areas would become less suitable for them which could lead to massive die-offs.
This is a tropical aquatic plant that is native to Brazil but has been unintentionally introduced to many countries. Due to this, they became the most problematic weeds in the world because they are highly invasive. Since they are floating weeds, water hyacinths grow quickly, faster than any other saltwater, freshwater, or terrestrial vascular aquatic plant. Water hyacinths can rapidly overtake water bodies and block waterways by forming thick mats.
Efforts to control the rapid growth of water hyacinths are intensive and expensive. For instance, Florida spends $15 million a year just to keep the water hyacinth’s population in check. They have also utilized various control efforts including biochemical and chemical control measures which have proven to be effective. Using weevil species as a biocontrol method was reported to be successful in other parts of the world. Still, prevention is highly preferable and is more economical. To prevent water hyacinth and other aquatic weed invasions, early detection and regional coordination are essential.
Water hyacinths are fairly tolerant to cold temperatures and can even survive in open waters but cannot withstand winter temperatures in the northern latitudes. The impacts of climate change may enable both the spread and establishment of water hyacinths in areas where they usually would not survive. The dispersal of water hyacinths can increase as a result of increased floodings, rainfall and hurricanes. Water hyacinths are tough enough to survive these harsh conditions and can even go both up and down the stream systems. Other causes of ecological disturbances can make certain habitats unsuitable for the native species but appropriate for invasive species such as water hyacinth. In some areas in the northern latitudes, water hyacinths would invade certain bodies of water annually on warmer months. If climate change continues to warm up the waters in the northern latitudes, water hyacinths may spread further north at some point. Non-native plants escaping from gardens is a common occurrence and they are a major factor with regards to the spread of water hyacinth.
The common reed, scientific name Phragmites australis, is prevalent on the Atlantic coast and is rapidly spreading westward and northward. There are certain strains of the common reed that is native in some regions in the United States. The invasive strains were believed to have been introduced from Europe in the late 1800s. A study which was conducted between 1945 and 1999 mapped changes in Phragmites coverage by using aerial photos of the Great Lakes region. In this study, it was hypothesized that the Phragmites expansion will continue quickly through the Great Lakes. Aside from that, shoreline development may increase nitrogen levels within the area which can facilitate the expansion of Phragmites.
Wetland restoration projects should include Phragmites control activities. The methods for controlling phragmites may include biocontrol, flooding, non-specific herbicide control, cutting, and/or burning. Aside from that, other management techniques, such as mechanical removal, burning, and induced tidal flooding are carried out aside from herbicide application. In some states in the US, aerial herbicide application proved to be effective in controlling phragmites. Although the factors that lead to its success are not fully understood, other states are considering this method.
The effectiveness of Phragmites’ control methods may be affected by climate change. Phragmites cannot tolerate saline water. As sea levels rise, saline water may intrude into areas where phragmites grow. This process would kill the phragmites. Thus, the rising sea levels could help in controlling phragmites’ population which increases the chances of success of restoration efforts on coastal wetlands. Although climate change may work in our favour with regards to controlling phragmites populations. Other areas that do not experience saltwater intrusion may experience significant growth of phragmites population as the ambient air temperature rises beyond average.
Ballast water and other shipping vectors; pet, aquaculture, and aquarium releases or escapes; the opening of seaway canals between water bodies; and, to a lesser extent, research activities; are some vectors that introduce invasive species into marine ecosystems. Carcinus maenus, or the European green crab, is an invasive predator in various places including the coasts of the U.S., South Africa, and Australia. The green crab can damage coastal ecosystems by competing with the native species for the native bivalves as a source of food. Green crabs are difficult to control and manage. Biocontrol agents for controlling green crab populations is currently unavailable. However, developing and implementing this kind of biocontrol method is encouraged.
Green crabs aren’t adaptable to cold temperatures. Water bodies present in the northern latitudes are too cold for the green crabs. As global warming increases the temperature in these water bodies, they become increasingly suitable for green crabs and thus encourages invasion and establishment. In other words, global warming can make it easier for green crabs to invade bodies of water that used to be cold. Climate change may reduce native bivalve populations which makes it easier for invading green crabs to spread and establish. As green crabs can damage ecosystems, damage done by climate change may exacerbate the ecological impacts of green crabs. For instance, commercially important native aquatic species that are already stressed by green crabs and other invasive species may easily succumb to additional stressors brought on by climate change. These additional stressors could come in the form of temperature changes, altered precipitation regimens, and altered patterns of wind and water circulation.