22 March 2010
Turning up the volume on nature
I made some calculations in a recent paper in the New Zealand Journal of Ecology showing just how far New Zealanders have turned up the volume on species invasions. Not surprisingly, it's a lot.
This is usually tricky to do because it's hard to know the rates with which new species have naturally invaded isolated places like New Zealand. Many, probably most, some suggest even all, of New Zealand's plants and animals are descendants of dispersal events from Australia and beyond long after New Zealand left Gondwana. Invasions have always been part of New Zealand nature. But how much?
The Chatham Islands, off the eastern coast of mainland New Zealand, offers a useful opportunity to figure out just how much people have turned up the volume on invasions in the New Zealand region. The Chathams archipelago has most recently been above the ocean for about 3.5 million years (the precise age is still being debated by geologists, but it's likely between 2–4 million years). All of the terrestrial species currently on the Chathams therefore had to invade since then. The nearest land is >650 km away on the New Zealand mainland, which is a very long way to swim, fly, or drift. Still, 3.5 million years is a long time to play Lotto and so prior to human arrival the Chatham Islands had accumulated a lot of lottery winners: 392 plant taxa, 64 breeding bird species (including recent extinctions), and 283 beetle species (references for these numbers are in the paper).
The first people arrived in the Chathams about 500 years ago and changed the rules of the lottery. The Chathams have since accumulated a further 396 wild plants, 16 breeding birds, and 39 beetles, and more keep arriving. Since the great bulk of the new species arrived after European settlement 218 years ago, this represents an increase in the successful invasion rate of >16,000 times for plants, >4,000 times for birds, and >2,000 times for beetles. Note that these estimates exclude unsuccessful invasions, both before and after human arrival. Note here that I am comparing the rates at which species arrive and establish.
My pre-human estimate of natural invasion is complicated by several factors. Three million years is a long time and spans some substantial climatic changes in the region, including the Ice Age Pleistocene epoch. The current known list of Chathams species most likely misses some species that arrived, established, and went extinct. This will mean that I am underestimating the natural invasion rate. Any species that arrived, went extinct, then re-invaded from mainland New Zealand again, perhaps many times through the cycles of glacial maxima and minima, are counted as one invasion. This will also mean I am underestimating the natural invasion rate. In contrast, any species that have arrived, established, then speciated into several species will result in my overestimating the natural invasion rate. While important details, none of these uncertainties are likely to alter my estimate of natural invasion by anything close to a thousand-fold.
A common justification I often hear from people who don't want to alter their lifestyles to benefit their environment and future generations is that it's all natural anyway. Why should we remove known weeds from our gardens or stop releasing exotic fish into rivers? All these new species are just nature running its course. Let them be.
That argument is clearly not valid. An increase in plant invasions of 16,000 times is simply not natural and neither are the disruptions they are causing to the invaded ecosystems.