March 24, 2007

Know Your Biomes V: Deserts

From a human perspective, deserts, like tundras, seem barren and desolate, inhabited by organismal oddities, pressed into their respective niches by patch of bad luck, or a salt flat, as it were. But thinking beyond our prejudice, seeing through the eyes of a camel or transpiring through the stomata of a saguaro cactus, some conception of deserts as biologically viable and diverse regions of the planet can be gained. Life may not be particularly abundant in most of these areas, but it is varied, unique and beautiful.

For the most part, deserts occur in a consistent band at 30 degrees north and south, with some exceptions on the coasts of North and South America. Dry, subtropical air robs these areas of moisture as it descends, circulating it to more temperate zones. Not all deserts are as parched as the Sahara in Africa or the Atacama-Sechura in Chile and Peru, which receive less than an inch of rain per year - essentially nil. The Sonoran Desert, for example, receives as much rain as the lower threshold of a temperate grassland, about 300 mm per year. The Sonoran remains a desert because of this cardinal rule of being: evaporation exceeds precipitation.

Temperatures are typically hot during the day and freezing at night because of the lack of cloud cover, though in areas of the Gobi Desert in Mongolia, the average temperature is only about 3.6 degrees C annually, with temperatures dropping well below zero C.

Soil has a low concentration of organic matter, so much so that it is often classified as lithosols, or strictly inorganic soils. This is especially evident in aged, undisturbed soils, where a special limestone horizon called a caliche is formed (because of its inherent low level of disturbance, ecologists can use this layer to accurately age a desert). Great salt flats are common in desert areas, where pools of accumulated water from heavy rains evaporate, leaving crusts of salt crystals spanning large areas, making it more difficult for organisms to extract water from their environment.

But extract they do, in various specialized ways. Desert perennials like the prickly pear have evolved a thick, waxy cuticle capable of retaining water more effectively year round. The stems have low surface area exposure, its "thin" parts facing into the sun. Roots extend horizontally below the surface, increasing their area of absorption. Their leaves, which would have been a liability in the extreme sun and moisture sapping aridity, have become reduced to non-photosynthesizing, defensive spines. Even their cycle of photosynthesis is different than most plants, closing their carbon dioxide absorbing stomata during the day, when the potential for water loss is greater (CAM).

Annuals are a different story. Given the extreme rarity of significant rainfall, these plants grow rapidly when water is available, producing seeds that can lie dormant for years, until the rains come again. Some of these plants keep a death grip on their seeds until the touch of water hydrates the cellulose of their seed pods, opening the pod releasing the seed.

Some plants, called halophytes, have even adapted to thrive in the salt flats. Atriplex is an extraordinary example. It can maintain higher levels of salt within its cells in order to extract water from the salt flats. Some of these cells burst, coating Atriplex with a defensive layer of salt, making the plant a dangerous meal for any water conserving herbivore.

One species, however, has found a way around this problem: The red vizcacha rat (in the same family as the chinchilla) has evolved a series of teeth that can remove the outer salty layer from Atriplex, making it just edible for the resourceful animal.

Other animals have similarly adapted to the heat and aridity; in fact, some, like the camel, have independently evolved the same measures as plants for keeping cool and hydrated. The camel faces into sun, keeping a slim profile, reducing its surface area exposure. It maintains a store of fat in its hump in order to produce metabolic water. A thick coat of hair, much like cactus spines, covers its body, reducing heat absorption. As the prickly pear keeps its stomata closed during the day, the camel does not sweat, reducing its own loss of water.

Most desert animals, however, are nowhere near the size of the camel, preferring to hide in burrows during the day, emerging to hunt and/or forage at dusk.

I could go on indefinitely about this biome. The extremity of the climate has produced fascinating characteristics in desert wildlife, and they are so different from region to region that they deserve descriptions of their own (especially areas like the Madagascar succulent woodlands, home of the bizarre Didiereans, pictured at the beginning of this post). Perhaps I will return to these areas of interest at a later date.

Next we'll look at the bread baskets of the world - the temperate grasslands.

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