A few weeks ago when NASA announced it had a big discovery (which turned out to be bacteria that can build themselves with arsenic – see below), it reminded us all how cool it would be to colonize space. Maybe we could start small with Mars. As Dirk Schulze-Makuch and Paul Davies suggested in an October article in Journal of Cosmology, we could make it financially feasible by sending people on one-way missions there.
That was kind of the idea behind building Biosphere 2 in the early 1990’s, as a mock-up for a self-sustaining colonial ship to pace. And as we found out, that’s not so easy. And since University of Arizona agreed to take it over a few years ago, we have found out more about how the big biosphere, the one we live in, works as well.
This inspired the topic of the lecture I gave last weekend out at Biosphere 2. People are still counting how many species have survived the missions, the periods of disuse, Columbia University’s tenure, and the various management regimes throughout that. Ty Taylor, who is a grad student in my department (EEB), censused the plants left in the rainforest. He’ll publish that later, so this is just a teaser – he has cool results.
So how many species could last indefinitely in the different biomes of Biosphere 2? There are a lot fewer of some kinds than were originally introduced there in the 1990’s. Other species, like cockroaches, have moved in since it has been kept unsealed since then. We can imagine it like an island that has been cut off from its continent. Perhaps it used to be a penninsula, connected by a narrow bridge (or in this case, an open door). Then that last bit of rock, buffeted by waves, collapsed. Or in the case of a Biosphere 2 type rocket ship, it was sealed up and blasted off.
Two very famous ecologists, E. O. Wilson and Robert MacArthur, wrote about the Island Theory of Biogeography. This idea is that within a group of organisms – for example, trees or grasses or mammals – the larger the area, the more species will live there. It’s kind of an obvious point, right? You can have more types of rainforest size trees in 100 acres than you could in a quarter acre. On an island, new species show up periodically. They fly there or float there on logs. If there are enough of them surviving to make babies and their babies can make babies, a new population takes hold there. Because they all come from just a few individuals and the island habitat might be different from the environment where they evolved, eventually they look different than their ancestors. This is called adaptive radiation.
But these new populations can go extinct, too. If there are only a couple of individuals to start the population, their offspring have no choice but to mate with cousins or siblings – yuck! Their offspring are inbred and do not survive as well. Or maybe the island is hit by a tsunami or a really cold winter and a whole population dies.
The Island Theory of Biogeography is the idea that larger islands can support more individuals, and so can support more species because their rate of arrival is higher than their rate of extinction. Islands that are closer to continents also have more species than islands the same size that are further away, simply because they get more new arrivals, regardless of their extinction rate. Robert Ricklefs and Eldredge Bermingham recently published a review of this idea.
So… an island the size of two and half football fields is pretty small for many Amazon trees to survive – and even smaller when you consider the rainforest is only part of that. Heck, it’s pretty small for human-sized animals to persist. And if you’re on Mars, there are pretty few new colonizations going on.
Would you go if offered the chance? What if you could take your family? What would you want to know before you did?
I’ll post next about how many species there are back on the mainland (Earth) and what this tells us about biodiversity here.