Have you ever gone abroad on holiday and done something totally out of character? My 60-year old parents went to visit my sister in New Zealand, and the next thing I know they’re calling me to tell me they’ve Bungy-jumped off the Auckland Harbour bridge. Or take the case of the famous British tourist. A Ryanair flight to Benidorm seems capable of turning the most shy and retiring Brit into a rampant force of nature, carousing bawdily into the small hours. Hear what the Independent has to say about my research here, and read the paper here.
Well it’s not just humans that can undergo a transformation when they travel abroad. Over the course of human history we have transported thousands of species around the world, and many of these species have established populations where we’ve taken them. Some examples of these in the UK are horse chestnut trees, rabbits, and pheasants. It’s generally expected that these species will continue to inhabit the same kind of environments when they are introduced, as they do where they are native. Yet scientists still don’t have conclusive evidence to support this. This is surprising because asking if species live in the same environmental conditions at home and abroad, gives us the chance to ask a fundamental scientific question: what controls where a species can and can’t live?
Now answering a good old Fundamental Scientific Question is the kind of thing that gets scientists all goose-pimply, but there are also very good practical reasons we need to know about what controls species distributions. Firstly, some of these introduced species can become nasty ‘invasive species’. They can eat or out-compete native species as mink have done to water voles, they can over-run habitats as Giant Hogweed does, or they can damage our health as Common Ragweed does to hay fever sufferers. In order to control the spread of invasive species, we need to understand the conditions in which they could thrive, and focus our efforts on preventing the species from reaching areas that have those conditions. Secondly, our planet is facing a sixth mass extinction, of which climate change is a major cause. One of the reasons we expect this to happen is because we think that climate is one of the most important factors controlling where a species lives. If climate changes in the places where a species currently lives, we need to know where, if anywhere, that species can move to in order to survive.
In both of these cases, scientists calculate the climate conditions in which the species can survive based on the climate conditions the species currently lives in in its native range. But if we’re wrong about our basic assumption that it’s climate that controls where species live, that spells big trouble for our predictions about where invasive species might invade, or which species are threatened by climate change.
To test this, myself and Dov Sax studied 50 species of plants from Europe that had been introduced to the USA. To my surprise, I found that many of these plants were living in parts of the USA with climate that is completely different to their native range. What’s really strange is that the species that seem to thrive most in completely unexpected places, are rare species that are limited to really small ranges in Europe. To go back to my British Tourist analogy, it’s like you took a plane-load of Brits to Benidorm, and it’s the most timid and unassuming member of the group that goes absolutely crazy and has to be bailed out of a Spanish police station.
Take, for example, the Balkan Catchfly, Silene cserei. This species grows sparsely in the Balkan states and Ukraine and yet in the USA, you can find the plant growing in the North East, through the Mid-West to the Pacific Mountain west. In the USA the Balkan Catchfly thrives in places with far colder winters and with hotter, drier summers than anywhere in the species’ native range.
Potentilla anglica, aka Trailing Tormentil (I think, though definitive ID is tricky until the fruits set!), in it’s native habitat in the UK. This species likes to live in cool, moist parts of Europe, but has naturalized in much hotter, drier areas of California and Utah.
What this tells us is that climate is only partly responsible for controlling where species live in their native ranges. So what else could be involved? It boils down to four main possibilities. It could be that in Europe these plants are ‘trapped’ by mountains or oceans, or perhaps they can only grow in places where the soil is just right. But when you look at where these species live in Europe, these explanations just don’t seem likely. It could be that since these species were introduced to the USA they have evolved very quickly. But that wouldn’t explain why it’s the rare species in particular that have spread so much more widely in the USA than expected. My best bet is that it’s other species that are the problem. Insects that eat the plants, fungi and viruses that attack them, and other plants that compete with them. Individually, most of these other species have such a small effect that we can’t normally observe it. But I think hundreds of these kinds of interactions with other species can add up to play a major role in limiting the distributions of the plants I studied, and likely many other species as well.
So does this mean that we all got worried about climate change for nothing? If species’ ranges are not really that strongly affected by climate then it’s not a big deal if climate changes, right? Unfortunately not.
Take the example of the Balkan Catchfly. If climate change makes conditions worse for a bug that’s eating the Balkan Catchfly, there will be fewer bugs around to eat the plant and its population might grow. But if climate change makes conditions better for that bug, or climate becomes suitable for another bug to turn up and start eating the Balkan Catchfly, that could be catastrophic – potentially causing the plant to disappear entirely. To make the situation more complicated, these threats could come from any of the fungi, viruses, insects, and other plants that are interacting with the Balkan Catchfly. And scientists are only just getting started on understanding how these interactions might be affected by climate change.
So climate change is just a big deal as we thought it was, but it turns out we’re lot further than we thought from being able to predict its effects. Luckily, figuring out a better way to do this is going to be fascinating.