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Invasive species can wreak havoc on an ecosystem, as well as a region’s economy. In Europe, some of these invasive species are already affecting certain areas and researchers have identified these species, including the golden apple snail that is putting the agricultural sector in the Ebro river basin at risk as well as water hyacinths that are threatening to destroy the natural ecosystem of the Guadiana River. What can be done?

What is Happening?

Pablo González Moreno, an expert on invasive species and a member of the ERSAF (Assessment and Restoration of Farming and Forest Systems) group at the University of Córdoba, who collaborated on the study, says regarding the feasibility of eradicating different invasive plant species,”It would be ideal to eradicate all the invasive species but financial and labor resources are limited, even more so now, when we are dealing with other priorities.”

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While it is natural that species will venture northwards to escape the warming effects of climate change, invasive species may nevertheless seriously damage ecosystems in which they venture to, making studies like these vital, not only to help manage these invasive species currently found in Europe, but also to have a future management plan in case these species are able to get to Europe.

Featured image by: Flickr 

The climate crisis and invasive species are often studied as two independent threats to global biodiversity. However, studying the relationships and feedback loops between them can create a more complete analysis of how Earth’s natural systems are being changed and shaped by anthropogenic forces. An invasive species is defined as a population of species that have become established, permanent residents, in an environment that is not their native one, causing ecological and economic harm. Species very rarely ‘invade’ by themselves and are usually introduced by humans, often by accident. Despite only around 10% of introduced species becoming invasive species, the total cost in the US as reported in 2005 by ecologist David Pimentel is nearly $120 billion USD a year.

Why are Invasive Species Becoming More Common?

Since the mid-1800s world trade has grown over 140-fold. This increase in aerial transport and container shipping has increased the amount of biological matter getting transported around the globe. This is coupled by an overarching trend of poleward migration and emigration by multiple species, fuelled by an increase in surface water temperatures that are currently climbing by 0.18 °C per decade. Even if warming is not further exacerbated, that still suggests an increase of over half a degree by 2050.

Why are Invasive Species A Problem?

The impacts of invasive species are more widespread than just economic damage. One of the most studied cases of a singular invasive species and its impacts is the brown tree snake (Boiga irregularis) in Guam. It was most likely introduced by military traffic in the 1950s following World War II. By 1968 it was widespread across the island and has since resulted in the extinction of 10 native forest bird species. The consequential impacts of this extinction have been cascading ecological effects affecting native invertebrates and pollinators and has led to a decline in native plant species. From a social perspective the fear of snake bites must be considered alongside the loss of productivity resulting from multiple power-outages as snakes interrupt power lines. 

What is the Situation in Guam Now?

As the situation in Guam unfolds this example can tell us a lot about how an invasive species might be impacted by the climate crisis in the future. Currently the brown tree snake population on the island is declining. This is suspected to be the result of the population of Brown tree snakes on Guam exceeding the carrying capacity (the maximum population size of the species that the environment can sustain indefinitely), meaning the population is not stable. In addition to this, a study published in the Biological Conservation Journal in 2005 suggested that the stress of overcrowding and competition for food resources limits reproductive ability. Even with this developed understanding, it is still unclear whether this will result in the stabilisation of the population or if it could create an opportunity for Guam’s naturally existing predator populations to recover, even temporarily. From an economic perspective, the annual costs of Guam’s brown tree snake research and management efforts are USD$7 million. 

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The Threat to Our Oceans

This is not only a threat affecting terrestrial biodiversity. Although increases in global mobility have increased the likelihood of introduced species becoming established on land, the link between the climate crisis and an increase in the number of invasive species might be most apparent in aquatic environments. Warming oceans allow species to create corridors across water bodies that may previously have been too cold for them to survive in. If the ecosystem they inhabit is vulnerable to a new predator and the majority of species are specialists (species that have evolved and adapted to be suited to a certain environmental niche), then it is likely to have large ecological consequences. For example, in recent years there has been a pronounced expansion of invasive thermophilic species to the central and western basins of the Mediterannean Ocean. This expansion has been intensified since the 1800s by the construction of canal pathways, in this case the Suez Canal.

Future of Invasive Species 

The uncertainties of the climate crisis leave a lot of questions about the future of invasive species and whether ‘invasive hotspots’ identified in studies as areas vulnerable to increased numbers of invasive species will be realised. It is also possible that a warmer climate might eradicate some invasive species, particularly in isolated locations where they might be unable to migrate to more favourable climates.

As species migration surges as a result of the climate crisis, questions have been raised concerning the classification of invasive species. If a native species population migrates or emigrates further poleward to higher altitudes where they used to be absent, is this considered an invasive species? The consensus by scientists at this stage is that they will still be considered native unless they cause discernible damage to their environment, whether through seasonal migration patterns or by displacing the pre-existing species that occupied a specific ecological niche.

The dichotomy between “native” and “invasive” species similarly creates a schism in the conservation field. While the terms seem objective and fit-for-purpose when identifying those species that ‘need’ protecting, they are becoming increasingly redundant as the climate crisis forces the migration of many flora and fauna globally. 

The Convention on Biological Diversity (CBD) has a glossary of terms for both “native” and “invasive” species, and others that vary between organisations and institutions. The Bern Convention defines a native species as “a species that has been observed in the form of a naturally occurring and self-sustaining population in historical times.” The definition puts emphasis on the time established in a specific area. 

Invasive species were first introduced as a concept by Charles Elton in the 1950s, and are now defined by the CBD as “alien species whose introduction and/or spread threaten biological diversity.” The two key characteristics of these species are that they were introduced, either by humans (intentionally or unintentionally) or naturally into an ecosystem and they cause harm to it.

While there are several nuances between this dichotomy – “exotic,” “non-native,” “alien,” “naturalised” – Emma Marris, author of Rambunctious Garden: Saving Nature in a Post-Wild World, argues that despite these nuances, proposing invasive species as bad and natives as good is well-publicised, as demonstrated by the public’s response to native and non-native flora. She says, “…citizen groups rip out non-native plants. Native gardens have become increasingly popular, both as ways to celebrate the unique flora of each region and as tiny hot spots of diversity. Native trees provide food for native bugs, which feed native birds…we’re even going native in the kitchen, with fine restaurants increasingly focused around locally hunted, foraged and grown ingredients.” 

The original intentions of these definitions were to clearly identify those species which are most in need of protecting. However, they are now, arguably, too rigid for today’s rapidly changing environment. 

How can global warming influence invasive species activity?

The climate crisis is putting intense pressure on flora and fauna in a myriad of ways. As temperatures increase, species are forced to migrate to higher latitudes or altitudes to find conditions that support their survival. As a larger number of species migrate into cooler environments to survive, the argument that they should be labelled as “invasive” and removed is becoming redundant as removing these species is to assist in their extinction. 

For some species however, the shifts in climatic zones cannot be solved by migration. In alpine environments, flora are slowly moving upwards as temperatures become warmer, but will eventually run out of space to migrate and will be out-competed as conditions favour lower-altitude plants. Similarly, along the Boreal forest, climatic zones are moving northwards ten times faster than trees are able to migrate, meaning the change in temperatures and competition from more temperate plants will make them vulnerable to extinction. 

While the International Union for the Conservation of Nature’s (IUCN) discourse surrounding invasive species and climate change is one of fire and brimstone, it should not be used to label all migrating species as invasive. The hostility towards invasive species comes from the potential increase in range of disease-carrying insects, such as mosquitos, and crop and plant pests. However, this blanket approach of labelling all migrating species under climate change as invasive, being extremely wary of their impact and being ready to eradicate at any second, is too simple for the effect that the climate crisis is having on ecosystems and the migratory response of many species. As these species shift their natural ranges into new territories, should we eradicate them? Under these rigid definitions, these species would be considered non-native, but some are likely to earn the badge of invasive as they pressure native flora or fauna.

In a changing world, the terms’ validity is questioned. The concept of the “tens rule” which emerged in the 1990s weakened the polarising effect of the terms in the sense that it suggested not all alien species are damaging, stating that only “10% of alien species establish themselves in new habitats and only 10% of those are likely to cause unwanted harm to economies, ecosystems or human health.” However, this rule from invasion biology has been repeatedly doubted and has been reassessed in different studies which show that there is reason to be wary of introduced species. 

This caution, more specifically, comes from the impact of invasive species on endangered native species. The National Wildlife Foundation says that 42% of endangered or threatened species in the United States are at risk due to invasive species. The pattern that is observed is that unless the species is endangered, authorities and scientists are less likely to care about the alien species. This creates a hierarchy when it comes to conservation – the endangered-common species hierarchy – that circumvents the native-invasive classifications. If the rigid categorisation can be overruled by this hierarchy, then a classification based on different values, such as contributions to overall biodiversity or ecosystem service value, can replace it.

While introduced species that cause harm will, seemingly, always be labelled “invasive,” the reception for “non-native species” is warmer. Martin Schlaepfer, a conservation biologist at the University of Geneva, Switzerland, argues that non-native species often provide ecosystem services. He argues that there are some urban areas where non-native species make up almost half of local species, contributing extensively to biodiversity, citing examples of Berlin, Germany and across central Europe. On Ile aux Airgrettes, an island off the coast of Mauritius, the introduction of alien Aldabra tortoises helped to restore the native ecosystem by replacing the native giant tortoises that went extinct there. These tortoises provide a seed dispersal service which encourages tree growth and provides habitat and food for local native birds. While the introduction of an alien species to an island is easier to control, there are other examples of non-natives that have contributed to ecosystem services. For example, the common rabbit in the UK was found to contribute positively to biodiversity through the grazing of chalk downland, which provides suitable habitat for rare butterflies and insects.

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native invasive species
The “alien” Aldabra tortoise that was introduced to Ile aux Airgrettes island to replace the native tortoises that had gone extinct there. These tortoises provide valuable ecosystem services (Photo by: Flickr). 

These contributions to ecosystem services and biodiversity as well as increased species migrations, coincide with a change of outlook in the field of conservation. The field has been moving away from the preservation tactics of the 20th century and is now looking at the ‘resilience of nature and “services” that nature contributes to human well-being’, as Schlaepfer argues. As the school of thought changes, conservationists are increasingly distancing themselves from the restrictive terms, to consider the multiple benefits that new species provide. Especially as 15% of mammals and 10% of birds that have been introduced in non-native habitats are under threat in their home ranges, it is important that the rigidity is relaxed to protect more species.

However, Professor David Simberloff at the University of Tennessee, argues that the possible inclusion of non-native species into biodiversity assessments, as rallied by Schlaepfer, is ‘misguided and would hinder our ability to achieve international conservation and development goals’. He argues that while the establishment of a non-native species into an ecosystem may initially contribute positively to ecosystem services and biodiversity, it may take years or decades to determine their full effect on the ecosystem, but by then, it would be too late to remove them. 

The arguments surrounding this issue are complex and interlinking, making it one of the most difficult debates for scientists and conservationists to resolve. The solution will always be dependent on subjective criteria based on what we find most valuable. Is nativeness enough to warrant the most protection, or do we look at biodiversity as a whole and the service value each species can contribute? With many species on the move due to climate change, is it time to put up the walls and keep all vulnerable, migrating species out or do we let nature take its course? In a system with which we have meddled so extensively, can we walk away from local ecosystems that need our intervention? It seems that even if the answers to these questions will never be unanimously decided, the awareness of the limitations and redundancy of “invasive,” “non-native,” “native” and all the terms in between is making us rethink our attitudes towards conservation and to be more effective by thinking holistically with entire ecosystems and not at individual species level.

As the world becomes more connected, invasive species spread further posing greater threats to our ecosystems, agriculture, and even food security.

Invasive bugs are wiping out millions of trees across the United States devastating forests and causing release of more than six million tons of carbon– equivalent to the emissions of five million vehicles–into the atmosphere each year. A study published this week in Proceedings of the National Academy of Sciences reveals that US forests are under constant threat from at least 15 invasive species– pests that come from foreign lands.

The menace of Alien Invasive Species (IAS)– non-native animals, plants, fungi or microorganisms that grow and spread quickly endangering native organisms–is not just confined to the US. Hundreds of forests and habitats across the globe are ravaged by them, and the ramifications are truly alarming. Invasive plant species frequently alter ecology by driving native crops to extinction, destroying terrestrial or aquatic vegetation, and polluting the native gene pool through cross breeding.

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How do invasive species spread?

In the past few decades, globalisation has enhanced international trade and the movement of people, goods, food products, and animals resulting in the mobility of various species outside their native habitats. Extreme weather events like floods, droughts, and hurricanes have further accelerated the pace of this mobility. Climate-induced changes on land cover and atmosphere also boosted their proliferation. A study from the Stanford University finds that increased nitrogen concentration in the atmosphere, which favours fast-growing plants, has allowed alien grasses to invade California’s native nutrient-poor serpentine grasslands.

In 2016, the European Commission revealed that more than 1800 species of plants, animals, fungi, and microorganisms in the European Union were identified as invasive, and their numbers have been growing rapidly. Those like American bullfrog, African curly waterweed, and Indian house crow are major threats to the EU’s ecosystems.

In Asia, a biodiversity research conducted in Nepal concluded that the country suffers an annual loss of $1.4bn due to crop failure caused by invasive bugs. Researchers identified at least 26 invasive species, and predicted that majority of them would spread quickly across the nation. 

In Africa, a study revealed that as many as 775 invasive species have found their way to the South African forests harming the native vegetation.

Why are invasive species a problem?

Invasive species have caused the extinction of at least 142 native species and endangered as many as 500 species globally, according to a study from The University of Tennessee. In the US, where 4,300 such species have been cataloged, they are the second biggest cause of plant and animal extinction. Zebra mussel–a native to freshwaters in Eurasia–has endangered at least 30 freshwater mussel species in the lakes and rivers throughout the Midwest and the southern regions of the country. In Hawaii, Argentine ants have been displacing native ant species that are vital to pollination and seed-dispersal on the island.

In England, feral cats have been killing 25-29 million native birds every year. Ship rats–natives of Indian subcontinent–have been causing catastrophic declines in bird population on many islands across the world.

Besides biodiversity loss, invasive species cause significant loss to the world economy. The International Union for Conservation of Nature (IUCN) estimated that invasive insects alone cost the global economy more than $70bn every year. A study from Stanford University found that in the US, crop and forest production losses from invasive insects and pathogens have been estimated at almost $40bn every year. The UK economy suffers losses of more than £1.7bn annually as food production affected by alien bugs.

Invasive species can also cause more frequent wildfires in different parts of the world. In the US, invasive grasses that burn more readily than native plants have increased the frequency of wildfires in southern California shrublands. As fire clears swathes of native shrubs, these invasive plants often fill in the space left behind, creating a positive feedback loop.

The impacts of invasive species are increasingly compounded by climate change. So the policy responses addressing these issues need to take the links between climate change and invasive species into account. Mitigation policies should incorporate prevention and control of invasive species by establishing effective biosecurity measures to tackle alien plants, animals, and microorganisms before they become invasive.


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