Building from Boredom
In the mid-1980s, a scientist named George Stevens looked upon Alaska and found it, well, boring.
This was a professional judgment, not a social one. Stevens was, and is, an ecologist. He taught courses for the biology department of the University of Alaska Fairbanks during the summers of 1985 and 1986, and during the winters he did fieldwork in Costa Rica. After Alaska, Costa Rica looked biologically rich indeed. Other people who have a similar geographic spread in their work have been struck by the difference that caught Stevens's attention; as an acquaintance of mine put it, "You can find more species clinging to your socks after an hour's hike in Latin America than you can see if you walk all day in Alaska."
That's a robust if irreverent way of stating a biological principle known for a century or so: as you go from the equator toward the poles, the number of different kinds of living organisms to be found in a unit area declines. Formally, this is known as the species-richness gradient. It is easily observed, but it is not so easily explained.
Naturally, that means there are about a dozen explanations for the gradient, but none of them are fully satisfactory. Building on earlier ideas and on his observations from high and low latitudes, Stevens came up with another one. He thinks the underlying mechanism is the adaptation of species to the physical conditions of their natural habitat.
At high latitudes, survival takes a lot of flexibility. Successful organisms have adapted to widely varying conditions, from daylength to--especially--temperature. Interior Alaska's tough plants and animals have to endure annual temperature variations that can easily exceed 100 degrees Fahrenheit, while those in a tropical rainforest confront only very small changes. What that means, says Stevens, is that species at high latitudes can occupy broad geographic ranges. Those in the tropics evolved to succeed in more subtly differentiated habitats, which means they deal best with very narrow geographic ranges. A single hillside in Costa Rica may offer dozens of distinct habitats, while a dozen hillsides at high latitude may amount to only a portion of a single habitat.
If the living landscape at high latitudes can be seen as a kind of patchwork of big blocks, then that near the equator can be taken as a mosaic of many little patches. Each patch is occupied mainly by the things that live and reproduce best in the conditions predominating there, but lots of less well-adapted organisms from neighboring patches are always moving in. They can't out-compete the better-adapted locals, but neither can the locals exclude them definitively because the supply of intruders is always being replenished from the nearby areas where they are at home and thriving. This too has a formal name: it's the rescue effect.
The rescue effect doesn't play a very important role at high latitudes because each species has a much wider range---a bigger home patch. From wandering mouse to airborne seed, each creature has a better chance of ending up in a homelike habitat. The continuing influx of struggling colonists is small, and their chances of successful invasion even smaller.
Stevens put the rescue effect together with habitat definition and came up with a very good description of the species-richness gradient. In the tropics, the environment is stable, the habitats are very subtle but very numerous, and the rescue effect is very strong. The result is that each unit area supports a great number of species. As latitude increases, so does environmental variability, but habitat diversity, the strength of the rescue effect, and the number of species in a given area all decrease.
It's a nice argument, but of course it hasn't convinced everyone; explanation number fourteen may be along shortly. Stevens may even be the person who comes up with it. He's based in Minnesota now, which is far enough from the equator to keep him productively bored.
