Has Humanity Carved the Earth into Many Islands?

In 1967, biologists Robert MacArthur and E.O. Wilson developed a new ecological theory: Island Biogeography. It wound up becoming crucial to the way we understand how animals adjust to living in a world that has been completely altered by humans.

Islands are surrounded by water, and this water is typically pretty inhospitable to the animals that live on islands. It is a sort of closed system, different from mainland ecosystems where plants and animals can easily migrate. So, special theories were needed to explain the number of species living and dying on islands at any given time (sidenote: the experiments necessary to test this theory, which involved fumigating entire small islands in the Florida Keys, could probably never be conducted today).

Perhaps the most ecologically important kernel in this theory is this: The number of species on an island is determined by how distant it is from other islands/the mainland (further away = fewer species), and the rate of extinction on islands is determined by the size of an island. So, on tiny, distant islands, there are fewer species, and they can go extinct rapidly, and on larger, closer islands, more species are present (since it is easier to get there) and extinction takes more time.

This difference in extinction rate is because large islands have larger and more varied habitat types, making it less likely that any given habitat or species will be wiped out by some chance event. For example, imagine a forest fire destroys all the forest on a distant, small island, plus all the forest creatures, forever and ever (or, until some other poor species blows onto this island, which is a low-probability event). Contrast this with a forest fire on a nearby, larger, more varied island with more forest habitat. In this scenario, the animals can run screaming and smoking to the next forest patch, and keep on eating, humping, and dying while new species from nearby colonize the burned patch. Circle of life.

This theory works pretty well for islands, and so, over time, ecologists started applying it to lots of other types of systems where an "island" of one habitat is surrounded by another, different habitat. Most often, this theory is applied to landscapes that have been altered severely by humans, like a patch of uncut tropical forest surrounded by a deforested area that has been converted to agriculture. This theory has also heavily informed conservation; we must preserve larger, more interconnected "islands", to stave off extinction and loss of species, so big reserves with corridors connecting them are the best strategy.

As it turns out, a vast, inhospitable ocean is not really all that analogous to human-altered habitat between patches of unaltered habitat — unless that human-altered habitat is a uniform cement slab or something, which it isn't. A recent paper in Naturesuggests that we should probably stop treating the "countryside" (human-altered landscapes), as though it were an inhospitable ocean, because research shows that animals can use the countryside, and in some cases, the countryside even creates unique assemblages of plants and animals, because humans create novel opportunities.

Biologist Chase Mendenhall and colleagues looked at bat communities on actual islands (Panamanian Islands surrounded by water), and "islands" of Costa Rican tropical forests surrounded by coffee plantations and pastures to determine whether "islands" behave like actual islands in terms of the number of bat species they can support, and the rate of local extinction of bats. The researchers focused on bats not just because bats are totally bad-ass in basically every way, but they are also really sensitive to habitat changes and loss (so, actually, not very badass in the "coping with humans" department).

The researchers captured almost 10,000 bats in a 2 year period on both the mainland and the island systems, and compared the number of species and variety of species found in "islands" with those found on actual islands. Not surprisingly, the actual islands behaved like Island Biogeography theory would predict. The bigger an island, the more species it had, and the further away an island was from other islands, the fewer species it had. Islands also had less "even" species diversity, meaning some areas had few species, and some areas had more.

This wasn't the case on the mainland, as the "evenness" of bat species was about the same in both forests and countryside. Finally, the researchers looked at bats over the "oceans" of plantations and pastures. And they found that some bats were more common in the countryside than in the pristine reserves. Contrary to the view of human-occupied land as an inhospitable ocean, some bats actually preferred the human-altered landscape.

The researchers found this pattern to hold true at a global scale using meta-analysis of 29 papers on bat diversity. Bats on islands follow the Island Biogeography rules (presumably, they have read the book), while bats in the countryside don't. The authors argue that an island is terrible way to think about a forest fragment embedded in a countryside, and we need to take a new approach to looking at and conserving these novel, human-altered ecosystems.

In the case of bats, forests, and farmers, the story isn't all bad. Bats aren't driven uniformly to extinction by shrinking habitat fragments, and some bats even prefer hanging out with humans. Although preserving pristine habitat is unarguably critical, maybe future conservation strategies can incorporate making human-altered landscapes even more useful to sensitive species.

Read the full scientific paper at Nature

Photo by EricksonSmith via Flickr