Unwelcome Guests: Alien Animal Invaders

Monday, 14 October 2013 - 1:00pm
Museum of London


Human dispersal around the world has been accompanied by a range of other species that have established themselves outside the limits of their natural distribution. These invaders, non-native, exotic or alien species, are changing the natural world as these species, when once established, are very difficult to eradicate.

Part of the Mondays at One Autumn Series

Transcript of the lecture

14 October 2013
Unwelcome Guests:
Alien Animal Invaders
Professor Tim Blackburn
Welcome everyone – thank you very much for coming out to hear me speak this lunchtime.
I am going to start in a slightly different place; I am going to start out by thinking very briefly about native biodiversity. Our planet is home to an absolute fantastic richness and diversity of organisms, but not all places are equal in terms of this wealth.  
So here, for example, I have a map of the global distribution of bird species, and essentially what you can see is that, in the higher latitudes, they have very few species and there are some areas towards the Tropics that have many more species, and some areas in the Andes can have almost a thousand bird species in an area of a hundred by a hundred square kilometres, so an incredible wealth and diversity of species. 
There is an enormous range of interactions and processes that people study to try and understand why we see this distribution of biodiversity, but essentially what it comes down to is, ultimately, there are only four processes that drive variation in the numbers of species in different parts of the world, and these four processes are: speciation and immigration, and these two processes can increase the number of species in an area; and then extinction and emigration can decrease the number of species in an area.
One of the big things that we are concerned about in the world today is the fact that the growth in the human population and the increasing pressures that humans place on the natural environment means that this particular process, extinction, has increased dramatically over the sort of background natural environmental rate, and people spend a lot of time worrying about elevation in extinction. But, in fact, I would argue that a much greater concern, and a much greater elevation, has been in this particular process here, species immigration, and in fact, this process can have knock-on effects on these other processes too.
Immigration is changing because humans are essentially increasingly driving this process, and immigration of animal species has changed a lot over the last few tens of thousands of years because humans have started to move around. It is widely accepted that humans evolved in Africa, modern humans around about 150,000 years ago, and then they have subsequently spread out and colonised every other ice-free land mass in the world, and as humans have moved around, they have taken species with them.
The first animal species that we definitely know to have been moved by humans is a little species of mammal called the grey cuscus. That was moved from its natural distribution in New Britain to the island of New Ireland around about 20,000 years ago. These are the islands off the east of Papua New Guinea. We know it was moved then because, if you study the fossil record of New Ireland, there is no grey cuscus in the fossil record until you start to get evidence of humans arriving, with charcoal appearing in the fossil record, and then, after that, you get the fossils of the grey cuscus, and that is strong evidence that humans actually moved the grey cuscus.
So, some species, like the grey cuscus, have been moved deliberately, but many other species have just hitched along with humans for the ride, and humans are just increasing in the amount of movements that they make across the world.
For example, we see a very simple depiction of the intensity of global shipping routes in any given year, so you can see that there are some routes that are particularly high in the numbers of ships being moved, but essentially, most of the ocean is being criss-crossed over the course of the year by many, many vessels, and many of these vessels can take species with them accidentally as hitchhikers.
I should really, before I go too much further, define what I mean by an “alien species” because that is really what I am going to be talking about today. An alien species is a species that is not naturally present in the flora or fauna of a location but is a species that has been moved beyond the limits of its normal geographic distribution by human actions. Some of those movements are deliberate and some of them are accidental, but nevertheless, those human agencies are moving species around. So many of these alien species may subsequently spread in the new environment into which they are moved and, in that case, they become invaders, and although I will generally talking about alien species, there are many synonyms that we have in biology for alien species – exotic, non-native, introduced, just to name a few – and I will inevitably, through the course of this talk, slip into using some of those other terms instead of alien, but essentially they all mean the same thing.  
One of the key issues with alien species is that, in fact, most species are not aliens, so there has to be some process by which a species goes from being a native species in its natural distribution, essentially minding its own business, to becoming an alien species in somewhere where it does not naturally occur, potentially spreading across the environment and, as we will see later, potentially causing problems. In fact, the process by which a species goes from being a native to an alien is a sequential one. There is a series of stages that a species has to go through and only if it successfully passes through each of those stages will it ultimately become an alien invasive species.
Here we have a very complex diagram that tries to summarise the sort of biology that underlies this process, but we can simplify this substantially just by thinking of one particular example. I will think of the example of bird species introduced to New Zealand.
New Zealand is an archipelago that I am going to be talking about on and off throughout this talk, and one reason why it is of interest to me is that it has a large number of non-native species, and in terms of birds, around about 120 bird species have been introduced into the New Zealand environment by humans, so species that are not naturally occurring there, and around 34 of those species have established viable populations on the island. But there are around about, give or take, 10,000 bird species in the world that do not have populations on New Zealand and have never been introduced to New Zealand, and one of the main reasons for that is simply that most of those species have never been transported to this archipelago, so there are many species around the world that have simply never sort of set out on that process of being moved to this set of islands.
But there are a sub-set of those species that have been moved to New Zealand – many of those have been moved to New Zealand but remain in captivity, so they have never actually made it out into the New Zealand environment, and obviously a species cannot become an alien invasive species, spreading across these islands, if they have never actually made it out of captivity. Where I work, in London Zoo, for example, we have lots of species there that have been moved beyond their native distributions but do not make it out of the cages and start spreading across the environment, and considering we have got things like lions and tigers, that is perhaps no bad thing.
Some of those species do make it out into the environment – they are either introduced deliberately or accidentally - but they may nevertheless not establish a viable population there.  So, essentially, they get out into the environment but, for whatever reason - and there are all sorts of potential reasons why a species might get out into an environment but then the populations die out – they do not actually establish.
There are some species that do get transported to New Zealand, they do make it out of captivity and into the environment, and they do establish a viable population. An example is the laughing kookaburra here, which has a small population up in the far north of the North Island, but we do not consider it an invasive species. We consider it an alien species in New Zealand, because it is not naturally occurring there, but we do not consider it an alien invader because, although it is established, it has not really spread very far beyond the release point, so it is established but it is not invasive.
On the other hand, there are species, like this Dunnock, which will I am sure be familiar to many of you from your London gardens or whatever. This is a species that has been taken from the UK to New Zealand, it has made it out into the environment, it has established a viable population and it has spread, and basically, there are very few parts of New Zealand now where you cannot go and see Dunnocks. Dunnocks are not native to that region or indeed to anywhere near it.
We do not have to go to New Zealand to think of examples of alien species, many of which are invasive. You can go out into the local environment and you can encounter many yourselves. I am sure anyone who lives in London will have encountered the ring-necked parakeets. I often encounter them about 5am in the morning when I am trying to get some sleep!  
Canada geese – this photograph was taken in Regent’s Park a couple of winters ago when the park lake froze.
You will have heard in the media about the oak processionary moth, a non-native insect that has established viable populations in Richmond Park and the region and is now potentially spreading out from there.
The grey squirrel obviously needs no introduction, and the alpine newt, a species of amphibian that has been introduced to the UK from Continental Europe.
Even the celebrity homes of Hampstead are not immune from the vagaries of invasive species, as seen by this report in the papers a few weeks ago.
There is a very brief background to alien species and the processes that get them to becoming invasive, but what is the big deal because, after all, invasion – synonym immigration – is a natural process. I mean, it is one of the ways in which the biodiversity of areas changes and increases. Very few islands around the world would have any species on them at all if it was not for the process of invasion.  
There are many dramatic examples of invasions happening in nature. So you may be familiar with the great American interchange that occurred around about three million years when the isthmus of Panama, here formed as South America finally met up with North America, and when this land bridge was formed, there was a huge exchange of fauna between the two continents, and still today, we see many examples in North America of species that are from groups that evolved in South America and then spread from south to north, through this isthmus, and many other examples of species that we think of as South American but in fact derive from groups that evolved in North America and have only colonised South America in the last three million years.  So, North American species like the possum and the armadillo come from groups that evolved in South America; and South American species like the llama and the jaguar that we think of as iconic South American species are actually from groups that were not present in South America until three million years ago. Hundreds of species made this change.
But while invasion is a natural process, I would argue, and have argued, that alien invasions are actually very different, and they are different for a number of reasons.
The first thing is that the numbers of species that are being transferred around the world is simply staggering. Tens of thousands of species have been moved by human activities to places where they do not naturally occur, and I will come back and then just give one example here from the islands of New Zealand again.
New Zealand has a native plant diversity – and I am digressing into plants a little bit here, but New Zealand has a native plant diversity of around 2,500 species. 25,000 plant species have been introduced to New Zealand by humans, and essentially, most of those introductions have occurred over the last 250 years. So these are plant species that are essentially planted out in gardens or botanic gardens, some of them have spread out into the wild, and around about 2,500 of those 25,000 species now have naturalised alien populations, so essentially you could think of them as being established alien species in that environment. There are as many alien plant species that are living wild in New Zealand as there are native plant species, but ten times as many aliens as natives have been brought into that New Zealand environment, and essentially from pretty much every plant family that there is.
Not only are the numbers staggering, but the rate at which these invasions occur is staggering as well. So, New Zealand split off from the rest of the land masses that form the Antarctic Plate around about 80 million years ago, so you could see that that native plant diversity of around 2,500 species is the result of about 80 million years of evolutionary processes.
There are about 2,500 species of exotic plants established there, and humans have been on these islands for around about seven or eight hundred years, so that is seven or eight hundred years for 2,500 species, versus 80 million years for 2,500 species – that is a hugely elevated rate.
We can do more precise calculations with another example. This, down here, this tiny little dot in the middle of the Atlantic Ocean is St Helena. It is a small island, and this island is thought to be about seven million years old, so emerged from the ocean around about seven million years ago.  When it was first discovered and the first human set foot on St Helena, it had 22 native species of bird. This first discovery, this first footing on St Helena was about 500 years ago. So essentially, the fauna of this island had developed over about seven million years of its existence to become 22 native species. There are now 35 alien species of bird on this island which have colonised and established in the course of 500 years. If you do this comparison, this is a vastly elevated rate of colonisation since humans have started to move things around. In fact, the rate of colonisation driven by humans of birds on this island is more than 50 times the natural rate, and that is even if we assume that 99% of the native species that have made it naturally to the island have subsequently become extinct - so if we assume that these 22 natives is the end result of over 2,000 species actually arriving, but most of them subsequently going extinct. So, many more species are being moved around these days and the rates are much faster.  
Those rates, I would say, are not slowing down either. Here we have some data for invertebrate introductions into Europe and mammal introductions into Europe. We have the year running on the x-axis, and the number of species per year here on the y-axis, and basically you can see that the numbers are increasing. So, we have been moving species around quite a bit now, but we still continue to move more and more now.
Alien invasions are also different because alien species can essentially come from anywhere in the world. If you think about natural colonisation, native species normally come from areas very close to the land mass that is being colonised. If we come back to consider New Zealand again, most of the native New Zealand species are either descendants of groups that were present, or were likely present on the island when it split off from the rest of Antarctica, or the Antarctic Plate, so things like moa and kiwis, ancient, iconic, flightless species, and then most of the other species are related, or most closely related, to groups that are present on Australia or, in some cases, on other islands in the Pacific, but most of the colonists have essentially hopped a very short distance. Actually, since humans have arrived on these islands and started changing the habitats, around half a dozen bird species have colonised naturally these islands because they have not been directly moved – they have simply been able to move in because humans have changed the habitat, and all of those species have come from Australia, and that is what you would expect in terms of natural colonisation.
If you look at the alien bird species that have been introduced to New Zealand, essentially you have species from every single continent on Earth. So, yes, you have some species from New Zealand’s near-neighbour Australia, like the fairy wren here that was introduced to New Zealand, but you also have Muscovy ducks from South America. Colonists to New Zealand introduced scarlet tanagers from North America. They have introduced Indian mynas from, perhaps not surprisingly, India. They have introduced guinea fowl from Africa, and they have introduced species like the Dunnock that we saw earlier and the yellow-hammer from Europe. So, essentially, the pool of potential colonists from New Zealand has gone from species in this area to every species in the world.
The final way in which alien invasions are different is simply that humans select the characteristics of the species that are doing the moving, that are doing the invasion or the immigration. So, natural colonisation, natural immigration, is only really possible for species that have certain sets of characteristics – they have to have characteristics that make them good colonisers: they have to be able to disperse, they have to be able to cross large distances, and they have to be able to survive and be able to generate new populations when they come into new areas. Much of that is changed when you actually have humans that are driving the process because, essentially now, any species can be moved to a place, and all that really matters is that it has characteristics that humans want to move.
So, if we think about birds again, actually the reasons why people move bird species around are relatively few. Actually, most of the species that have been moved around historically have been moved for purposes of hunting – they are species that are good to eat or are fun sport. They have been moved for the processes of ornamentation, so people want to make the environment more beautiful or more attractive in some way, or, for example, they have a big country estate back in the UK and they want to impress their neighbours with the wonderful, beautiful wild fowl, for example, that they have swimming on their lake. Many bird species have been moved for bio-control because many of them are insectivores and many farmers think that having insectivorous bird species around is a good thing. Increasingly, as we have got more towards the present day, species tend to be moved for unplanned reasons, if you like. So, we tend to get a lot of releases or escapes, and these often relate to species that people want to keep as pets or in cages, for example, and which subsequently get out into the environment.  
Most of the bird movement by humans historically has been in what we term the great European diaspora, so this is essentially from the middle of the nineteenth century, where there was a great rash of colonisation out from the European countries into the neo-Europes of the colonies. The British were particularly involved with this, and so the colonies of Australia, New Zealand, the United States and so on were subject to a lot of movement, both to and from.  
Also, because people are selecting species on the basis of these reasons, we would expect that the species that are being moved are not a random set of birds in a variety of ways. We would expect certain groups of species are more likely to be moved. The species that are moved should have certain characteristics, and the locations to which they are moved to and they are moved from should also be very different from natural colonisations.  
This complex picture here is essentially the family tree of birds, so all the different bird families across the world, and the ones that I have just highlighted here in red and blue are the families that are more likely than you would expect by chance to have had species moved to new areas by human activities. What you can particularly see here, if we just look at the red ones, which are the five groups that have had the most species moved relative to the number of species that they actually have, you have pheasants and other game birds, you have ducks, geese and swans, you have pigeons – so, essentially, you have three groups that are actually very tasty and, in many cases, also very pretty so nice to have in the environment. Then you have parrots have been moved around a lot, and you have passerids, which are sort of weavers, sparrows and also little mannikins. Essentially, what you have here are two groups of species that are moved around for the caged bird trade and then essentially have made it out into the environment, either deliberately or by accident.
The characteristics of species are also not random. So, it is perhaps no surprise, given that two of the main reasons that people are moving species around are for eating them and to look pretty, what you tend to find is that larger species tend to get moved around because, you know, larger species are a better meal if you are out shooting and also, if you are ornamenting the environment, you probably want to have species that people can see, and so it tends to be larger species moved around. If you think about the average bird in the world, the average bird in the world weighs about 50 grams, so it is about the size of this dunlin here, whereas the average species that has moved around is around about three times that size, so around about the weight of this tern species here.
Species are also not random with respect to the extent of their distribution. There is a large element of availability in terms of what people move around. So essentially, if you are a more common widespread species, you are more likely to get moved simply because you are more available in the environment for anyone that is moving birds around to go and collect and to move on.  
For example, the average size of a geographic range of a bird species across the world is around about the area of Yemen, here, whereas the average range size of a species that has been moved by humans and established a population is around the size of Saudi Arabia, so much larger species distributions.
Where species come from and also where they end up are also very different under human-mediated invasions than they are under natural colonisations. Here is the picture I showed right at the start, showing the distribution of bird species and showing that most bird species are found in the Tropics, and actually in the Tropics associated with mountain ranges. But here we have the distribution of exotic bird species, or at least as it was sort of round about twenty years ago – we are improving the data instead of seeing an expansion of this map here. The scales are very different, so here we have up to around 900 species indicated by the red marks here, and here, it is only up to about 40 species, but nevertheless, you can see the distribution of areas where there are lots of alien bird species is very different to where there are lots of native bird species.   So, in all these ways, human-mediated invasions are different to natural ones.
That is essentially a very broad-brush introduction to what alien animal invaders are, but the second part of this talk is why are they unwelcome, and so what I want to do now is spend the second half of the talk just essentially telling you some of the reasons why biologists, and indeed other people, get a little bit worked up by alien animal invaders.
One reason why non-biologists get very worked up about it is that alien invaders cause economic problems. Now, there have been a number of attempts to estimate exactly how much alien species cost to economies around the world, and I think it is fair to say that all of them are very bad estimates, and they are bad estimates for a variety of reasons, but all of them share this characteristic of involving very large numbers.  
One of the early estimates was by a guy called David Pimentel and he estimated that alien invasive species cost around about $138 billion to the US economy every year, and this was through things like the cost of agricultural pests and controlling them, the cost of things like human diseases, but also he tried to estimate the cost of the impact of alien species on the environment and that is something of a nebulous thing to try and quantify.
More recently, in fact just a couple of years ago, the EU went through a similar exercise, and they came up with a number of around about €12 billion per annum that it costs the EU for the impacts of invasive species. It is broken down in a number of ways, so they estimate that the cost of damage by alien species to agriculture is around €5.5 billion, damage to fisheries and aquaculture around €240 million, and so on and so on. But what is interesting about these numbers is that actually the cost of epidemic animal and human diseases are excluded from these estimates, and obviously these are some of the key ways in which aliens can impact on economies.  
In fact, if you think about it, many of the diseases that we have most concerns about in the modern world could be considered alien invaders. So, species like HIV, SARS, West Nile Virus and so on, these are diseases that have originated in one small part of the world and they have become problems because they have been transported outside of those regions by human activities.
It is not only the disease organisms themselves that get moved around, but it is also the vectors.  So, here we have a particularly lovely species, the Asian tiger mosquito, which you may be familiar with if you have ever been to the Far East – I am certainly very familiar with this species.  But it has been introduced into Southern Europe – it was introduced into Italy, I believe, and it has now spread quite widely throughout Italy and is moving down into Spain and to Greece. You can see that it is restricted to certain climatic regions, the warmer parts of the European Continent, but obviously, as the climate changes, so the area that is going to be suitable for this mosquito is going to expand, and this mosquito is a vector of around about twenty human pathogens, including Yellow Fever, so a number of nasties, so we should probably be a little bit worried about this.
The main reason why biologists are concerned about alien invaders is because they cause significant problems to the natural world. In fact, when we think about some of the major causes of extinction and population declines around the world, we count alien invasion species as one of the Four Horsemen of the Apocalypse. So the main ways that human activities are driving species declines and extinction at the moment are through over-exploitation, through things like hunting and fishing, through habitat destruction, and particularly its conversion for agriculture to feed an ever-growing human population. Pollution is a major issue, and particularly the pumping of ever-increasing amounts of carbon dioxide into the environment, which is changing our climate, and also, the fact that we are introducing many species to areas of the world where they do not naturally occur.
These alien species can drive extinction through a whole variety of different processes, by eating native species, by spreading disease to them, by competing with them, and also by hybridising with them.  
So, I am going to start by talking again about this wonderful island group down here, New Zealand, which is a great experiment in terms of species introduction and also invasion and extinction. New Zealand is a really great place. Essentially, it is the last major ice-free land mass to have been colonised by people, and the evidence now suggests that the first human colonists, the Maori, only made it to New Zealand around about seven or eight hundred years ago, so only a few hundred years before the first Europeans discovered New Zealand.  
It was a Polynesian colonisation – they colonised all the different islands across the Pacific, and they finally made it down to New Zealand about seven or eight hundred years ago. When they got there, they found an archipelago that had no native terrestrial mammal species at all. So, it had a few species of pinnipeds, seals and so on that lived round the edge, and it had a few species of insectivorous bat, but no native mammals. What it did have was an incredible, unique bird fauna, so well over a hundred species of birds, most of which were found nowhere else in the world.  The recency of this colonisation means that actually we have a pretty good idea of what was there at the time of first human colonisation. There has been a lot of archaeological work, palaeontological work, that has enabled us to reconstruct what that avifauna looks like and therefore that has allowed us to track patterns of extinction in this fauna after humans arrived.
Following the Maori arrival, and in the period between the Maoris’ arrival and the first European arrival, 42 New Zealand native bird species went extinct. What you can do is you can plot the probability that a species went extinct against its body size, and you can do that for different groups of birds. When you do that, what you see is that, for most of these groups of birds, the probability that a species went extinct increased as it got larger, and the reasons for that are pretty obvious. The Maori encountered many native bird species that were very large, that had never seen an alien predator, that were incredibly easy to hunt. There were, for example, about ten species of moa living on New Zealand 800 years ago, and within a couple of hundred years, they had all gone extinct. These were so abundant and so easy to hunt that the Maori did not even bother to take the whole carcass back to camp when they killed it. They would just hack off the legs. The legs, as you can see, was where most of the meat was, and these are gigantic drumsticks. They would just carry the legs back. So, the middens that you encounter are basically stuffed with moa leg bones, but very few other parts of the skeleton.
What you also see, however, in New Zealand, is that there is a very high probability of extinction in a group of very small-bodied bird species. These are ground-dwelling and sea-birds laying eggs less than 60 millimetres in length. An example would include the flightless stout-legged wren, a species that was found nowhere else other than New Zealand, and in fact, in a whole bird family found nowhere else other than New Zealand. The probability that these went extinct was also very high. The reason for that is that when the Maori arrived, they were not the only mammal to colonise New Zealand because they brought with them a second mammal species, the Pacific rat or kiore.  
Now, this is a very efficient predator of small, ground-dwelling vertebrates and invertebrates.  Experiments have shown that it can open eggs up to about 60 millimetres in length. Any bigger than that, the shells get too thick for this rat to be able to handle. But, essentially, what you have here is a major extinction event in species susceptible to predation by this Pacific rat. So, these species essentially have been driven extinct by this species transported with humans.
One of the great things about New Zealand is that there were two stages of colonisation: so, the Maori arrived, seven or eight hundred years ago; and then, around 500 years later, Europeans turned up. Europeans brought with them a whole extra suite of new mammalian predators. In addition to the Pacific rat, they also brought the black rat and the brown rat, and they brought the house-mouse as well. They brought their domestic cats. They brought stoats, ferrets and weasels, and they brought pigs. They essentially brought a whole range of new predators, and as it turns out, they brought a range of predators ideally suited to eat all the species that had managed to survive the first wave of colonisation because they were too big to be eaten by the Pacific rat and they were too small to be of interest to the Maori. So, if you now plot probability of extinction against body size for the species that went extinct after European arrival – and this is 21 more species went extinct in that period – essentially what you see is the probability of extinction was much higher for medium-sized species. The Pacific rat had eaten out all the species at this end of the body size distribution, and the Maori had eaten out the species at this end, and then the cats and other rats and stoats and ferrets and whatever ate out everything in the middle, and actually, what you see is that the second effect is that the higher the level of endemism of a species, the more likely it was to go extinct. So, in other words, essentially, the longer a species had been associated with the island of New Zealand, the longer it had had to forget what a mammalian predator did, the more susceptible it was when all of these other mammalian predators came in.
This is the Field Guide to New Zealand birds, so I mean this is an indication of how recently these species have gone extinct, that the guys have considered it worth putting the pictures into the field guide just in case there might be a few individuals hanging around somewhere, but the very famous huia, where the female and the male have different size and shape beaks, the piopio, the New Zealand thrush, two species in a family only found in New Zealand, and the laughing owl, all went extinct in the last hundred or so years.
What that means is that, if you go to New Zealand now and you want to see native bird species, the best places to do that, and in fact the only places you can see many of the native species that are still being predated by these new mammal arrivals, is in mainland reserves, like this one just outside Wellington. This side is the Karori Reserve, this side is native bush outside the Reserve, and this is the predator-proof fence that the New Zealanders have put in to stop mammal predators getting from this bit of habitat to this bit of habitat. I mean, they put this fence up, and then they essentially trapped out all the mammals from this area – there is no scale on this, but there are fences up to about twelve feet tall – and essentially they have a very intensive monitoring system to make sure that nothing gets into this reserve, and bird diversity is fantastic here, and not so good here.
The other place where you can see native birds in New Zealand is some of the offshore islands, which essentially either never had predators or have subsequently had predators cleared. So, this is Hauturu, or Little Barrier Island. It is in the Hauraki Gulf, just north of Auckland, and this is a fantastic now predator-free nature reserve, where the native birds and insects and plants as well are doing fantastically in the absence of the mammal predators.  
However, obviously, conservation measures like this are not without their risks. This is one of the original island nature reserves, off the coast of New Zealand. It was predator-free, and this was one of the islands that was the last resting places of the bush wren, the New Zealand snipe, and this species of bat. In the ‘60s, unfortunately, a consignment of black rats managed to get onto this island, and four native New Zealand species went extinct following the rat arrival. So, the rats got in, and the bat and the snipe were never seen again. The bush wren, the Department of Conservation in New Zealand managed to get a handful of individuals off the island before the rats got to them, but unfortunately none of the individuals survived the translocation. So, these three species were extant on this island until the 1960s and are now lost for good.
I have talked about New Zealand, but New Zealand is not the only island in the world and it is not the only island that has had exotic mammals introduced where there were none before, and it is not the only island that has had large numbers of extinctions. We can analyse extinctions across islands in the historic period, so following European colonisation of islands around the world, and essentially what you find is that the likelihood that a species of bird is going to go extinct on an island around the world increases with the number of exotic mammal predator species that have been introduced.  So, if you have a lot of mammal predators introduced on an island, then it is very likely that your bird species will go extinct.
I have talked about New Zealand because it is a really good study system for extinction, but actually it is very far from unusual as an island. Some work I have done with some colleagues in New Zealand recently has looked at extinction rates for, or estimates of extinction rates for bird species on a whole range of islands across the Pacific. So, this graph simply shows a list of Pacific islands – it is islands in the tropical Pacific, with the probability that a bird species on those islands went extinct. So, as you can see, many of the islands in East Polynesia, for example, and also in West Polynesia, have had very high extinction rates, so they have lost probably between 70 and 90 percent of their native bird species in the period following first human colonisation.  New Zealand, as we saw, has lost at least 60 bird species to extinction over the last 700 years, but that actually gives it a relatively low overall extinction rate relative to most of these islands, so most islands in the Pacific have actually lost much more of their diversity than New Zealand, despite the fact that New Zealand has lost horrific amounts. In many cases, we simply do not know many of the species that have gone extinct here, and we do not know the reasons for it, but these are all islands that have had Polynesians spread across them, Pacific rats spread across them, and so it is not unlikely that those are two of the major drivers of those extinctions. We estimate that at least 1,300 bird species have gone extinct in the tropical Pacific over the last four or five thousand years, or about one in twelve of all the bird species in the world.
It is not only islands where extinctions have happened. There is another well-known extinction event in Lake Victoria that happened when Europeans introduced Nile perch to the lake. This is obviously a nice game fish and they thought it would be great to have Nile perch. Unfortunately, this lake was home to a huge radiation of cichlid fish, found nowhere else in the world, and, as you can see, these are censuses of the cichlid fish in the lake over a period of about fifteen years, and the Nile perch in the same period, and essentially many of these cichlids have disappeared and Nile perch have become super-abundant as they have gone through, just munching their way through the fauna.
Predation is probably the main reason why alien mammals have caused extinction, but there are other reasons as well. The Hawaiian Islands are a particularly nice example of disease causing a major extinction, again in the birds. Around about the end of the nineteenth century, biologists in the Hawaiian islands suddenly noticed that all of the sort of native passerine bird species in the lowlands were disappearing, and over the course of about five or six years, essentially every native bird species disappeared from the lower elevation regions of all of these islands and essentially moved so were only then found in the higher elevation regions. The reason was that the Hawaiian Islands had no native species of mosquito. Unfortunately, a species of mosquito was introduced and this allowed avian malaria, a disease of bird species around the world that was also not present in the native fauna of the island, it allowed it to be spread from migrants coming into the island to the native fauna, which caused them to die. Not only that, but Hawaii also has many non-native bird species which are not susceptible to avian malaria, so they were able to act as large reservoirs for this disease and maintain it in the environment to allow it to be continually spread into the native species. Essentially, what you have here is a situation where, as mosquitos spread round the lowlands, and they can only live up to a certain elevation so it is only lowland birds that were affected, but essentially, the entire lowland avifauna of these islands was wiped out over a very short period by this malarial disease.  
One of the main reasons, it is now thought, that grey squirrels are replacing red squirrels in the UK is because grey squirrels are carriers of a disease, a parapoxvirus to which they are immune, or largely immune, but to which the red squirrels are highly susceptible.
If we look worldwide, the RUCN, one of the main conservation NGOs, has documented around about 700 recent animal extinctions. If you look at the causes given for those extinctions, around about 170 have causes associated with them, and invasive species are listed as one of the causes of extinction for just over half of those losses, and alien species are the major driver of extinction for birds, for North American fish, and actually other fish around the world, and also for mammals too. So, this is one reason why we worry about them.
There is another reason too, which I will just very briefly touch on: this is the issue of homogenisation. Essentially, what people are doing when they are moving species around the world is they have the potential to make environments more similar. It tends to be the same few species that get moved all over the place, and they get moved to areas that have their own distinct native faunas already present. For example, if we look at the United Kingdom and New Zealand, 800 years ago, they essentially shared no species in common at all, whereas now, if you go to New Zealand and you go bird-watching, you can see goldfinches, yellowhammers, skylarks, song-thrush, chaffinch, starling, blackbird, house-sparrow - a whole range of our native bird species are now widespread across New Zealand. What this means is that these two areas which formerly, a few hundred years ago, were completely different are now rather similar, and this is a process that is happening all around the world.
I will not really go into any detail here, but all I will note is that, if we look across the Hawaiian Islands, all of these islands, all these main islands in the Hawaiian chain, have lost native species to extinction. For example, Kaua’i, up here, has lost fourteen bird species to extinction since humans first arrived, but it has had 27 alien species introduced. The Big Island has lost seventeen species to extinction, but it has 34 non-native species established on this island. So what this means is actually the diversity of these islands overall has essentially gone up because there are more bird species living there now than there used to be, but unfortunately, these extinctions all relate to species found nowhere else in the world, whereas the introductions relate to species that are common and widespread all over the place.
This graph here simply plots the similarity – it is a statistical way of plotting the similarity in the faunas of the islands, and essentially, the islands started out like this, widely separated, meaning they are very different in their faunal similarity, but now we are at the circles here and the islands are all very much more similar in their faunal similarity, and if we actually lost all the species that are threatened with extinction to extinction, then essentially all of these islands would more or less be homogenised to be exactly the same. This is often referred to by biologists as the “McDonaldisation” of native faunas, for obvious reasons.
I will conclude by saying that not all species are aliens and not all aliens have impacts, but aliens can be bad. They can be bad for us, they can be bad for the environment, and they are in fact a primary cause of environmental change. If we look at the other causes of environmental change, the other ways in which we are driving species extinct, it is very easy: we can stop hunting and we will not be killing any more animals; we can stop destroying habitat and the remaining habitat will stay there; we can even stop pumping carbon dioxide into the environment and the amount of carbon dioxide in the environment will not go up – I mean, the climate will continue to change for a while, but eventually it will stop.  
If we stop introducing alien species, that is not going to make the problem go away – you know, these species are out there in the environment and they have phenomenal capacities for reproduction. You may be aware of the famous mouse plays that they have in Australia. Here we have a pile of mice. Brown tree snakes in Guam, they can reach phenomenal densities, and things like this kudzu vine can just grow over and destroy everything in an area. So, even if we stop introducing species that are not meant to be in environments around the world, if we stop it today, the problem is not going to go away, and in fact, it is only going to get worse, so we have essentially committed ourselves to a problem for all eternity really.
© Professor Tim Blackburn, 2013