Wednesday, 15 January 2014

Invasive Species

I thought I’d settle on invasive species as an initial topic for Science Behind the Curtain, as they are both an interesting and important topic, certain to generate plenty of material for an ongoing series of posts in future. It’s a subject most New Zealanders are familiar with to some extent, as the impacts of introduced mammals such as rats, ferrets and possums on our native bird species are well documented and publicised, but it’s also a broad, global topic, as humans spread new species across geographic barriers, whether deliberately or not, with unintended and often unfortunate consequences.


Before the arrival of humanity on the scene, species were able to spread only according to their own ability to disperse. Whilst some might have seeds able to weather long journeys by wind or tide, and others can fly over such over such obstacles, for the most part, geographic barriers such as mountain ranges, deserts or oceans prevent dispersal. However, humans simply don’t take geographic barriers seriously, and we provide a powerful vector for the dispersal of organisms into new ecosystems, whether we do so deliberately or the organisms simply hitch a ride as we move about the globe.

There are three things a species requires to become established in a new region. Firstly, it requires a hospitable climate – adverse temperatures, even over only part of a year, can affect organisms’ survival or breeding success. Secondly, it requires an ecological niche to exploit – a lack of suitable habitat or food will also impact survival. Finally, it requires a vector by which to enter the new region, which can be either through natural spread or as a consequence of human activities.

Invasive species are distinct from introduced species, in that they have the potential to cause significant ecological or economic damage to those countries they spread into. Given their impacts on both host environments and economies worldwide, a great deal of money and effort has gone into combatting non-indigenous invaders, whether by taking measures to prevent their establishment in a country (by far the cheaper option), or mitigating the spread and impacts of already established invaders.

Ecological impact

Most New Zealanders are familiar with invasive species to some extent. Essentially, if it’s a carnivorous or omnivorous mammal, we tend to regard it as a furry death machine (much as we may love the family pets). As an isolated pair of islands, New Zealand has been a difficult country for new species to colonise. The oceans have posed a significant barrier to dispersal, to the point that the only native mammals we have are bats. Consequently, without mammals to predate upon them or occupy key habitats, our native bird species have evolved in isolation and are often very vulnerable to introduced predators.

Each continent or region has its own unique ecosystem, but what they have in common is that each ecosystem is a finely balanced web of plants and animals which have evolved in concert for millions of years. Natural checks and balances mean that no one species is likely to disrupt the ecosystem, or overpredate, smother or otherwise crowd out other species. However, divorced from that context, lacking the usual controls on their actions, and in the presence of other species which may lack the behaviours necessary to deal with them, introduced species have the potential to run riot in their new environments.

Economic impact

The ecological impacts of invasive species do have the potential to damage tourism earnings, if they displace key native species, or alter environments which act as tourist attractions, but generally when people think of the economic impacts of invasive species, they’re considering damage or disruption of commercial crops or farmed animals. There are two forms this can take.

Firstly, in a similar way to the ecological impacts above, invasive species may feed upon or parasitise species which may not have evolved to cope with them. Secondly, as many commercial crops and animals are themselves introduced species, if some creature or pathogen from the original environment is subsequently introduced which in some way interacts with the commercial species, there is the potential for the yields farmers have come to enjoy in their absence to be reduced. Indeed, selective breeding for improved yields at the cost of other features which confer resistance to ancestral pests may exacerbate the problem. In either case, costly control measures are required in order to mitigate their impact on crops.

This means that farmers have a keen interest in biosecurity controls, and the relevant authorities keep a close eye out for particular invasive species which have a history of successful invasions and subsequent economic impacts elsewhere. Even if a species does get an initial foothold in a country, prompt and decisive action may still see it eliminated.

Examples of invasive species

Of all the mammals introduced to New Zealand, the Brush-tailed Possum tends to stand out in peoples’ minds. I help run trap lines to keep them out of a local reserve myself. As with many of our pest species, it was deliberately introduced, in hopes of establishing a fur trade. Native to Australia, the possum evolved in concert with the native vegetation, which is unpalatable in comparison to our own. The possum is a very good example of a species which is not a problem at all in its native environment, but which runs amok in a different ecosystem. Left unchecked, possums are able to do a great deal of damage to our vegetation, which lacks the defences possums evolved to cope with and, being omnivorous tree-dwellers, they will also devour the eggs and chicks of our native birds. As if that weren’t enough, they transmit bovine tuberculosis, impacting on farm livestock. It’s difficult to put a price on the damage they do, but control operations to keep them in check cost millions annually and represent a significant drain on funds available for conservation work.

Public enemy number one: The brush-tailed possum.
Image licensed under Creative Commons 2.0 Source

The Great Lakes of North America are a particular hotspot for invasive species. A considerable fraction of the world’s shipping passes through these waters, taking on and discharging ballast, as well as transporting fouling organisms on their hulls. The aquarium trade has also played its part in introducing species into the lakes. Perhaps the best known invader of the Great Lakes is the Zebra Mussel. Native to the Aral, Black and Caspian Sea regions, in the Great Lakes, this invader not only smothers native clam species but also colonises and clogs water pipes and treatment facilities, necessitating costly procedures to control and remove the mussels. Damages and preventative measures to deal with the Zebra Mussel run into the billions of dollars. Unfortunately, as with most invasive species, eradication is simply not possible once they’ve become established.

For those interested, the Global Invasive Species Database provides a listing of invasive species worldwide, with their top 100 list outlining species of particular concern.

Update: Serendipitously, I see a very interesting guest post over on SciBlogs from NIWA on aquatic invaders (both weeds and fish) in New Zealand. Well worth a read for anybody who's interested in learning more about the aquatic side of things.

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