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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.

Nestled between the hills north of Hong Kong and the breathtaking skyline of Shenzhen lies Deep Bay, semi-encircled by the Ramsar site of Mai Po wetland, a location of significant ecological interest home to numerous species of birds.  

Since the opening of China’s economy in the 1990s, Shenzhen has grown exponentially into a city of almost 13 million people. Deep Bay faces severe environmental threats including water pollution, rising mudflat levels from intense urbanisation and land reclamation on the Shenzhen side of the Bay. Mangrove forests have been cut down and the natural coastline converted into concrete sea walls. But threats to the natural order extend to the sky. The high-rise buildings in Shenzhen Bay are threatening the bird populations that find sanctuary at the Mai Po wetland during the winter season.

Mai Po Bird Species

Every winter, around 90,000 migratory birds seek refuge in the marshes and mudflats of the internationally-acclaimed Mai Po Nature Reserve in Hong Kong. Of the 380 species of birds that inhabit the reserve, 35 are of global conservation concern, including the Saunders’s gull and the black-faced spoonbill. 

Other critters such as otters, fiddler crabs and mudskippers also call the area home, and are the main food source of the waterbird. Hong Kong is situated beneath two major bird migratory pathways, the East Asian-Australasian Flyway and the West Pacific Flyway. Birds with breeding sites in North Asia and Siberia fly to Hong Kong every winter to rest and stay over winter and they fly back to their breeding site in spring. The nature reserve is managed by World Wildlife Fund (WWF) and a bird survey is conducted every winter by the Bird Watching Society (BWS). 

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Figure 1. Number of overwinter waterbirds surveyed by the Bird Watching Society

A clear correlation exists between the construction boom that has engulfed Shenzhen and declining bird populations. In particular,  the 2019 BWS annual report shows that the number of birds has decreased since 2008, coinciding with the development of Nanshan and Futian districts in the west and north of Shenzhen Bay, adjacent to the Mai Po Nature Reserve. 

Overall Shenzhen, one of China’s premier shipping and manufacturing centres, is home to 223 completed skyscrapers higher than 150m, giving the city the dubious honour of having the third largest concentration of such infrastructure in the world, after neighbouring Hong Kong and New York City. 

Shenzhen’s construction boom is impressive. In 2008 alone, eight skyscrapers taller than 150m were completed and in 2017 the fourth tallest skyscraper in the world- the Ping An Finance Centre- was completed, standing at 599m tall. More skyscrapers are scheduled for construction, including China’s would-be tallest skyscraper. At 700m tall, the project is expected to be completed by 2023. But the concrete jungle represents a major flight hazard for migratory birds.

How many birds are killed by skyscrapers?

Bird-skyscraper collisions are decimating populations. This phenomenon is known as towerkill; the dynamics of which have been studied in Toronto and New York City. It is estimated that nine million birds die each year due to them mistaking reflective windows for open sky or being dazzled by the bright lights from the skyscrapers at night. 

There are no official studies of towerkill in China but it is estimated that the number of birds killed would be much higher than New York City as the skyscrapers in Shenzhen are more concentrated and the urban area is in close proximity to the wetland that the birds rely on.

Studies show that using ecologically-friendly architectural designs such as specialised glass, window film and external shutters could reduce the glaring from skyscraper windows. Legislation on light pollution would also help reduce the collision of birds. 

Active participation by all could ease this problem. Interactive tools are available that allow citizens to report bird deaths, the data gleaned from which is used to map the relationship between skyscrapers and bird collision and assist urban planners to design more eco-friendly urban areas for birds. 

Rarely has the scramble for resources been an orderly affair. Climate change adds a new dimension of strife that upsets geopolitical balances, engulfing fragile nations and forcing people to flee poverty. Conflict and migration are becoming ever more interlinked with changes in climate. A new study has finally grounded these correlations in data and fact.

How has climate change affected human migration?

While droughts, food shortages and climate-related stressors have long been assumed to be “push factors” for instability since Biblical times, scientific evidence for these phenomena has been circumstantial. Migrations resulting from man-accelerated climate change  have been subjects of debate in international fora, with numerous United Nations agencies taking an active role devising contingency and mitigation plans as part of the wide-ranging Post-2015 Development Agenda.

It is of course a political hot button in Europe, where over 2.3 million illegal migrants entered the EU’s borders in 2015 and 2016 alone. Yet, the human origins of climate change are seldom mentioned as an igniting factor behind large-scale human migration in public discourse.

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Syrians and Iraq refugees arrive at Skala Sykamias Lesvos, Greece. Image: Wiki Commons

Climatic conditions have been blamed for creating political unrest, civil war and subsequently, waves of migration.

“Changing weather, floods and droughts in many places increasingly threaten people’s safety and livelihoods. That is leading a lot of families to have to consider whether they can stay where they are, or try to live somewhere else,” said Koko Warner who leads the migration section of the Institute for Environment and Human Security at the UN University.

“For example, if you’re a farmer and the rains fail you for several years in a row, you may all of a sudden lose not only your access to food, but your entire source of income, and the well-being of your entire family can become very precarious,” explained Warner.

The study indexed a number of drought indicators on a scale of severity, including measurements of temperature and rainfall in countries where most migrants originate from.

Innovatively, the results were correlated with socio-economic and geographic data on migrants themselves, including distance between the countries of origins  and migration destinations, population sizes, migrant networks, ethnic and religious demographics. Data on conflict and civil unrest was analysed using data on battle-related deaths.

Focusing on the decade from 2006 to 2015, the team found that human-driven climate change can cause and exacerbate conflict, leading to an increase in migration.

“Climate change will not cause conflict and subsequent asylum-seeking flows everywhere. But in a context of poor governance and a medium level of democracy, severe climate conditions can create conflict over scarce resources,” said an IIASA team member.

The Middle East and Africa are prime examples of this dynamic.

In Syria, long-running droughts and water shortages caused by climate change resulted in repeated crop failures, with rural families eventually moving to urban areas. This internal migration led to urban overcrowding and unemployment, factors that fuelled widespread resentment and primed the country for the ensuing civil unrest.   

There is little evidence that environmental pressures in the Middle East or Africa will ease anytime soon. The Intergovernmental Panel on Climate Change (IPCC) warns that those regions will experience some of the worst water shortages as the century progresses.

Africa alone is projected to have 250 million people living in regions with food and water insecurity by the end of the century, which will likely increase the flow of climate migrants.

Further research is needed to fully understand migration flows and climate-change related displacement, but IIASA scholar Raya Muttarek says their work “contributes to the debate on climate-induced migration by providing new scientific evidence”.


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