Tuesday, August 28, 2018

There Is No Such Thing As a Desert

Or a forest, or a grassland, for that matter. But I think I can illustrate my point most vividly using the example of deserts.

Many different kinds of habitats get lumped together in the category we call “desert.” Unfortunately, once our human minds have created a category, we assume that everything within that category is the same.

The habitats we call “deserts” are very diverse. The most obvious way they differ is that the “deserts” of the different continents are the homes of almost entirely separate sets of species. When you see desert shrubs in California, in the Middle East, and in western Asia, they are not the same species. Leafless, succulent plants in American deserts are usually cactuses, whereas in Africa they are spurges or milkweeds. South Africa has many species of succulent, leafless plants in the Aizoaceae family, different from all the others. The animal inhabitants of the deserts on different continents are also distinct.

But even within one “desert” there is a lot of habitat diversity. Consider the southwestern deserts of North America. The areas that ecologists refer to as the Mojave Desert (e.g., southeastern California) are covered with widely-spaced shrubs. The areas that ecologists call the Sonoran Desert (e.g., in the Phoenix area) has shrubs (but not the same species) and many species of cactus. In fact, the shade cast by acacia bushes is important to the survival of young cactus seedlings. The areas that ecologists call the Chihuahuan Desert (e.g. near Big Bend) has shrubs and succulents (though usually agaves rather than cactuses), but the space between them, unlike the other deserts, is filled with grasses. This makes sense, as this kind of desert intergrades into the high plains grasslands. (No, that’s not where chihuahuas come from.)

Above: Chihuahuan desert in Big Bend National Park; below, saguaro flowers in the Sonoran desert near Tucson.

But even within one of these subtypes of desert has a lot of habitat diversity. The Chihuahuan Desert grow on the tops of hills as well as on the slopes and lowlands. At the tops of some of these mountains in southern California, Arizona, New Mexico, and Texas, and in northern Mexico, there are some small oak forests. These forests are the remnants of oak lands that used to be widespread in the countryside back when the climate was wetter. As the climate became drier, the forests survived only as little patches on somewhat cooler and wetter hilltops. Most of these oak species are found nowhere else in the world; some of them live only in one forest. If you scan your eyes over the entire Chihuahuan Desert, you might not even notice these forests. Of course, the oak-patch animals are a lot different than those of surrounding desert land. See here for a link to a 2013 blog entry.

Maple trees in a desert? In Big Bend National Park, maple trees grow in an isolated moist spot near the base of the pinnacles.

Another example is desert arroyos. Arroyos are stream beds that are dry most of the time but which can be filled by flood waters during heavy rain. The soil under the arroyos is usually wetter than in the surrounding desert. Not surprisingly, the species of plants that live there are almost entirely different than those of the surrounding landscape. In southwestern North America, one of the shrubs that grows in arroyos, but not away from them, is the desert willow Chilopsis linearis, which is not really a willow.

An arroyo is the same as the middle eastern wadi, although the species found in wadis are almost completely different from those in arroyos.

All this does not include what scientists sometimes call “polar deserts” in which there is too little snowfall to allow soils to support plant life even during the brief summers.

I find the word “desert” to be a useful category. It is one of the entries in my Encyclopedia of Biodiversity. It refers to a place with too little water to support a thick growth of plants. But I recognize that “desert” is an artificial category that exists only as a way of us making sense of the world. As I explain in my soon-to-be-published ScientificallyThinking, humans tend to lump things into categories, which is find unless we then homogenize everything within the categories.

The human tendency to lump things into categories may seem harmless enough when we lump different habitats into the category “desert.” This makes us overlook much beautiful diversity. But when we start lumping people into racial categories, we start assuming they are all alike, characterized usually by the worst people in each race.

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