This is the first in a series of posts in which I plan to examine one of the fundamental concepts in ecology - the species-area relationship
Species conservation has always been intimately linked with the idea of habitat conservation. While habitat quality determines the amount of habitat required to protect a viable population of a given species, it’s only a modifier - the determining factor is area. Habitat quality can determine whether you need more or less area, but area is still the critical factor. While protected areas can be set aside for specific species, more commonly protected areas seek to protect as many species as possible. More land is likely to protect more species, but there are other factors that influence conservation decisions like the cost of land acquisition and the competing interests such as agriculture, mining or housing development.
One way to maximise the number of species in a protected area is to include as many habitat types as possible. If you include a forest, a meadow, a marsh and a lake in your protected area, you are likely to get a lot more species than you would if you only had forest habitat. The heterogeneity of the area increases the number of species. (After all, you don’t find a lot of fish in a pine forest, or field mice in a lake.) But again, this overlays a simple factor of area. A larger tract of forest will probably have more species than a smaller tract of forest. A larger section of a marsh will probably have more species than a smaller section of marsh. This fact, known as the species-area relationship is fundamental in both ecology and conservation biology. The existence of a relationship between species richness and area is obvious to anyone who has taken the time to think about it, but it is still interesting enough that it has attracted the attention of generations of ecologists.
The fact that species richness increases with area is apparently to anyone who has taken a walk through a park. If I step outside my door and count the number of bird species I see, the number of species will increase as my sample increase. If my sample unit is “number of birds” (i.e., I sit on the front step and count birds) I will be compiling a species-individual curve. If my sample unit is area, I will be compiling a species-area curve. Since individuals exist in space, these two relationships are closely related (especially for things like plants, which are nice enough to stay put and wait for you to come and count them).
So lets say I start out from my door. The first thing I will see are sparrow - lots of sparrows. If I go north I am almost certain to see a robin on the lawn. Go a little further and I will see starlings and cowbirds, maybe some grackles. If I’m lucky, I will see some scissortail flycatchers. As my sample area increase, my species list can either stay the same, or increase. The smallest sample area - within a few feet of my front door - will have one species of bird. Increase that area to include the nearest patch of lawn, and I have two bird species. Keep going and I will add a third and a fourth. As long as I include everything I’ve seen already (what is called a nested species-area curve) there’s no way for my species list to decrease.
So we have deduced the existence of a species-area relationship. Seems a bit too trivial to be science, doesn’t it? Well, you’re right. There’s much more to it than that, but it all builds on some extremely basic ideas. But like any good idea in science, there’s plenty of predictable behaviour, and then there are the things that are a bit more difficult to figure out. In future posts I will look at some of these expected - and unexpected - behaviours, and how they helped make conservation biology into a scientific discipline.