Species disperse, this is an universally accepted part of ecology and evolution. Eventually animals will spread from one area to another, and if the conditions in this new habitat are different enough that they favor unique adaptations, evolution will occur, possibly resulting in the creation of a new subspecies. When animals naturally disperse, we tend not to have a problem with it, at least in most cases. Take for instance the Cattle egret. This small Heron has rapidly spread to colonize most of the world, while even just a century ago it was mostly restricted to Africa. This seems to have been completely by its own doing, and thus we have allowed it. Another example of this is the Turtle dove, which rapidly spread from western Asia to most of Europe within a few decades, though it is now declining again.
The Cattle egret, conqueror of the world
However, when a species does not disperse by itself, or rather, when it is spread via Humans, we have quite a different attitude. These species are labeled "aliens", in order to signify their non-native status. More often than not, the word "invasive" is tacked on. These so-called "invasive aliens" are a mainstay of environmental coverage. They are often referred to as one of, if not the single greatest issue in conservation, the cause of nearly all problems, from habitat destruction to extinction and the spread of disease. If you can think of it, it has been blamed on alien species. In several cases this is completely apt, as many of these introduced species are indeed a major threat to local fauna and flora. An example of this is the Cane toad, which, upon being introduced(deliberately) to Australia, proceeded to wreck havoc on local predators. Another example is the case of the Brown tree snake in Guam, which single handedly wiped out almost the entire Avian fauna of the island. But these are still only a couple species, out of the tens of thousands that have been transported around the world by humans.
The vast majority of these, over 90%, never manage to become established. Of the 10% that do, another 90% are completely reliant on human activity, only persisting due to a constant stream of escaped individuals bolstering the population. Only the remaining 10%, or rather, 1% of all introduced species, actually manage to become established and self sustaining in the wild. And yet even of this 1%, the vast majority are not a problem, often confined to single sites, and even when they do spread, most species have little to no negative effect on biodiversity. It really is only the 1% of the 1% that go on to become true horror stories, like the Cane toad. These should not be ignored, and we must remain vigilant whenever a new species is spotted, but to condemn all introduced species despite 99,9 percent having no negative effect is just not reasonable. Unfortunately, this is exactly what we are doing. Many conservation bodies simply assume that any introduced species must be harmful, and so almost every single website mentioning any, and I do mean any introduced species, from flowers to frogs to birds, will invariably say something along the lines of "it causes significant damage to local species, and must be exterminated". It does not matter that the vast majority of introduced species have never been studied, and that of those that have, most do not have a demonstrably negative effect.
The Little owl, introduced to the UK by Humans, yet beloved by many
And so we come to the subject of this post, Australia. Australia, being the world's only(populated) island continent, is undoubtedly also the one most affected by "invaders". Today, it is populated by an astonishing diversity of introduced species, from Rabbits to Camels. Many of these, like Cats and the aforementioned Cane toads are indeed a bad thing, but as I will argue here, many are not, and in any case, we are stuck with them, and will just have to get used to it. First some context to set the scene:
Australia was once populated by a menagerie of beasts unlike any other. Terrestrial crocodiles, marsupial lions, echidnas the size of boars, it was a land of monsters. Then Humans arrived, approximately 50,000 years ago, and all of the species listed above were wiped out, either at the end of a spear, or by the fire that we spread across the land. When the dust from this first colonization settled, almost all of the megafauna was gone, with only the Kangaroos, Emus, and Thylacines remaining as reminders of what once was. Fast forward to 4000 years ago, when another wave of colonists arrive. This time, they bring with them the ancestors of what we now call Dingos, and shortly after this, the Thylacine and Tasmanian devil both disappear from the mainland. This was the Australia that the first Europeans encountered, a continent devoid of almost any Megafauna, populated by only a single large terrestrial predator. It was the perfect habitat for new species to arrive in. When we look back at Earth's past, we see that generally, some of the greatest migrations and diversification occurs in the aftermath of a catastrophe. These devastating events wipe out many local species, allowing new arrivals to settle without competition. This is essentially what happened in Australia. Without its prior megafauna, the continent was open to invasion by new large animals, and invade they did. Today Australia is now populated by Camels, Water buffalo, Pigs, Goats, Horses, and Donkeys. Along with these large herbivores came small ones, such as Rabbits. The one thing that did not immediately arrive was the predators, as they were held at bay by the presence of the Dingo. However, a single species can only exert so much dominance over a fauna, and in the absence of the Devils that would have otherwise helped control smaller introduced predators, carnivores began to arrive. The biggest factor that allowed for the arrival of larger predators however was not the preexisting conditions of Australia. Europeans did not much like the Dingo you see, and it was accused of all manner of horrible things, from killing sheep to eating children. Thus, it was heavily persecuted, and effectively wiped out in many parts of the continent. With it gone, there was nothing left to prevent invading predators from become established, and so they did.
Foxes and Cats are the principle "invasive" predators that Europeans brought to Australia. They rapidly spread across the continent, eating their way through the local faunas, and have had a devastating effect on Australia's wildlife. Several species have gone extinct possibly solely due to them, and many others are endangered because of them. But. And there is a But. In the places where Dingos still persist in large numbers, Foxes and Cats are noticeably less successful. That is not to say that they are not present, they are, but their numbers are quelled by these apex predators. The result of this is that Dingos have a tremendously positive effect on local faunas, as they heavily control the populations of Foxes and Cats, while being predators of larger prey themselves. This larger prey incidentally includes other introduced species, such as Camels and Horses. And this is where we get to the crux of my point. Many, if not most of these introduced species are not completely incompatible with the local flora and fauna of Australia, as long as they are regulated by some other force, chiefly large predators such as the Dingos and Tasmanian devils. Foxes and Cats, while definitely major predators of small marsupials, can coexist with them, as long as Dingos and Tasmanian devils are present. This is very, very important, as this brings us to the other part of my point. We cannot rid Australia of these introduced animals. Foxes and Cats, along with Rabbits and Camels, are here to stay. Sure, we can manage their populations, with some species to a higher degree than others, but they will always endure, and eventually, once said management stops, their populations will bounce back. Attempting to control introduced species via fences or culling is not only unnecessary, it is also completely and utterly futile.
A Fox in Australia. They are here to stay
But I am not just here to talk about how introduced predators can coexist with native faunas. Even when they do coexist, they obviously still have a suppressing effect on said native animals, and we would all prefer if they were not here. No, what I also want to talk about is the introduced species that do not have a negative effect, and yes, they do exist, in fact, there are quite a lot of them. Remember the Camels, Water buffalo, Horses, and Donkeys I mentioned before? Well, this is the part where I anger almost everyone reading this by claiming that they are in fact a good thing. Yes, you read that correctly. You see, while these large herbivores are often accused of many devious deeds, from overgrazing to the destruction of habitats, this view is, simply put, wrong. More-so, it is based on a lack knowledge of the continent and its ecological history. Back before Humans arrived, when Australia was still pristine, the Megafauna played an enormous role in the continents landscape. They trampled vegetation, preventing the development of monocultures, knocked over trees, creating clearings and openings, and formed temporary wetlands and ponds where they sunk the ground with their footsteps. Most vitally however, they caused natural disturbance. Because of the constant damage they dealt to the local vegetation, the environment was ever changing, always adapting to new conditions. This created a mosaic of habitats, allowing for a far greater diversity of life. In fact, there is evidence that, upon their extinction, as they no longer grazed the continent's foliage, dead twigs and leaf litter built up on the forest floor, and it was this that caused Australia's climate to dry out, as these huge piles of leaves began burning, and wildfires raged across the continent, reducing much of the once verdant vegetation to the dry, fire-resistant scrubland we know today. In other words, the past extinction of Australia's megafauna was the reason why it turned from quite a lush and verdant continent to the dry and often inhospitable place it is today. Theoretically, the reintroduction of large grazers and browsers could reverse this process, as they might prevent the buildup of leaf litter, thus allowing it to turn green once again.
This, coupled with the proven benefits of natural disturbance, serves to provide a compelling argument for the reintroduction of large herbivores to Australia. The problem is, unlike in Europe and the Americas, all of the Australian megafauna is gone for good. There are no close relatives, no species filling the same ecological niches, who graze, browse, and create disturbance in the same way. There are no Indonesian Diprotodons that can simply be lifted over to Australia. Thus, a reintroduction in the typical sense is impossible, and any large herbivores introduced to the continent will be very different from its original inhabitants. And yet, a fauna persisting of some sort of megafauna, even if it is a different one, would still more closely resemble that of pristine Australia than the depauperate and empty one many view as ideal. While they would not provide the same kind of disturbance, introduced megafauna would still provide disturbance in some form. And they, the "invasive" Horses, Camels, and Water buffalo that now populate the continent, do provide disturbance, they do cause change and diversity in the environment, and they do, I will argue, increase biodiversity. This is a controversial view, which has never to my knowledge been tested, and as such I must admit to it being to a large extent speculation, but at the end of the day, there are two main things we do know: 1. Australia was originally populated by a megafauna, one which provided disturbance and alteration in the continent's ecosystems, thus increasing biodiversity, and 2. The newly introduced species, while very different in ecology than the extinct megafauna, do in fact provide disturbance and alteration in some form, even if it is not the same as that provided by the extinct megafauna.
A herd of Camels in Australia. Like it or not, this is their home now
In conclusion, Australia's situation with introduced species is very complicated, and there are no clear answers. Not enough research has been conducted to properly determine exactly what effects many of these species actually have on their environments, and much of what we say is based purely on assumptions. What we can say for certain though is that, however much we may like or dislike them, these so-called invaders are here to stay. We have no hopes of removing them, and in the long term, they will go on to form the ecosystems of the future. The best we really can do is make sure that the wonderful Marsupials and other unique animals do in fact have a place in these future ecosystems, so that their descendants may still pass on these ancient lineages a million years from now.