Ann Reid's picture

The Occasional Evolutionist

I recently returned from a trip to Australia, where I was lucky enough to get to visit the Great Barrier Reef. My partner and I spent 3 days on tiny Heron Island on the southern end of the reef. I’m not kidding about the tiny part; you can walk around the entire island in less than half an hour. The island is essentially a pile of sand (made of crushed coral) that rises above the surrounding ocean by, oh, maybe 20 feet at its highest point. It takes two hours to get there by boat from the coast of mainland Australia–about 45 miles.

Now everybody knows that evolutionists love them some islands (cf. Darwin and the Galápagos). Isolation, challenging environment, the allure of a biological blank slate – what’s not to love? I found it impossible to explore the island without coming up against evolutionary questions at every turn.

Here’s an example. Heron Island is almost literally covered in birds. It is literally covered in bird droppings. Ironically, the dominant bird species are not herons (as far as I can tell, there are no herons there; and, yes, I do wish I’d asked someone why it’s called Heron Island), but two less familiar birds: white-capped noddy terns and wedge-tailed shearwaters. According to the resident island naturalist, there can be up to 120,000 terns on the island and up to 30,000 shearwaters during the breeding season (that would be now). This year was a bumper year for both birds, so she estimated that the numbers might even be a bit higher. Hmmmm. How about a little math? 150,000 birds on 44 acres. That’s 3,500 birds per acre. That's a bird for every square yard! OK, so that’s spread out over several months of a breeding season, but still, that means every breeding pair on the island is being supported by an area about the size of your average dining room table.

The island is also home to rather dense vegetation, dominated by a tree called Pisonia grandis. I will have a lot to say about this frankly rather sinister tree in a moment, but for now, please just picture the interior of the island (that would be everything beyond the reach of high tide) as a reasonably dense forest of scrubby 20-30 feet tall trees and assorted shrubby undergrowth.

Now how in the world can a pile of sand support a dense forest and 12 birds per square foot? There’s an evolutionary question for you, the answer to which is not to be found on the island itself, but in the surrounding water. The primary producers of the island ecosystem are the surrounding corals. Without the Great Barrier Reef, the island would probably have remained just a pile of sand. Oh wait. Without the Great Barrier Reef, there would probably not be the pile of sand, which is made of crushed coral. So, no reef, no island.

It turns out that the birds are the mechanism for capturing some of the reef’s productivity and bringing it back to the island, in the form of, well, poop (formally known as guano). Both the terns and the shearwaters are marine birds; they eat fish, which in turn feed on the myriad organisms that ultimately depend on the photosynthetic organisms called zooxanthellae that live in mutualistic symbiosis with the coral polyps themselves. What is astounding is that this underwater partnership supports not only the vast reef ecosystem, but also the entire above-water island ecosystem.

The birds eat the fish and poop on the island. The poop nourishes the plants. The plants provide nesting sites for the birds. All one big happy ecological success story. However, while islands made of guano are found in other marine habitats, they are not universally covered with lush greenery. So how did the dominant Pisonia trees of Heron Island get there? Remember how I said those Pisonia trees were sort of sinister? Well, they don’t get to the island because their seeds float like coconuts, or because birds eat their fruit and deposit the seeds in their excrement. The seeds don’t float on the wind either. Pisonia has evolved seeds that are sticky, like burrs. So the seeds stick to birds’ feathers. But the seeds don’t detach easily from the whorls of twigs they form on; the whole twiggy bundle sticks to the birds and makes it very hard for them to fly. If they can’t fly, they can’t feed. So they starve to death. And the seeds sprout using the bird’s body as its first nutrient source.

That’s not the only way Pisonia sprouts – when a tree falls down, or a branch falls off (which they often do; Pisonia wood is very spongy and fragile), new trees will sprout off the fallen wood. But to get to the island in the first place? Yeah. They capture a bird and feed off its carcass. Quite a dramatic evolutionary adaptation, wouldn’t you say?