Desert Survivor

An immigrant to the Southwest, the creosote bush is a Desert Survivor that dominates the landscape and engineers biodiversity

A drive from Los Angeles to Las Vegas is all one needs to be impressed by one of nature's great success stories. The vast plains separating mountain ranges are covered everywhere by a species of brownish green shrub called creosote bush, a South American invader known as Larrea tridentata. Its common name suggests a key to its success in some of the world's harshest habitats: stems and evergreen leaves covered with a sticky resin that smells like--but doesn't contain--the wood preservative creosote. Lending the plant its brownish cast, the resin screens leaves against ultraviolet radiation, reduces water loss, and poisons or repels microbes and most plant-eating animals.

The creosote bush monopolizes soil nutrients, starving competing grasses and other plants. Its taproot can extend fifteen feet into the soil, while lateral roots--whose chemicals repel neighboring plants--often fan out into more than fifty square yards of surface soil. Adding to its power, the bush is a frequent self-pollinator and an opportunistic bloomer; it is able to grow and reproduce even when water seems virtually absent. Such advantages have enabled this Argentine shrub to spread extensively across North American deserts after arriving (possibly in the tail feathers of migrating plovers) sometime during the last glacial episode, between 10,000 and 14,000 years ago.

Sheer numerical dominance makes the creosote bush a major influence on nearby plant, animal, and human communities. In the driest deserts, it may be the only plant that can survive, but elsewhere scores of native plant species grow beside it or under its canopy. Many, including the saguaro cactus, begin life in the shade of a creosote plant "nurse bush." Side-blotched lizards, desert iguanas, snakes, toads, and termites burrow beneath the shrub, and the chuckwalla feeds on its flowers and fruits. Ten or twelve species of small mammals depend on it for food, nesting sites, and refuge from the elements. Kangaroo rats consume its seeds, and jackrabbits prune its branches. Local birds, such as the verdin, black-throated sparrow, and black gnat-catcher, as well as scores of migratory visitors like the yellow warbler, frequent stands of creosote bush for seeds and insects, while roadrunners prowl the vicinity in search of snakes and lizards. Generations of Native Americans have used the extracts of creosote bush as antiseptics and treatments for arthritis and rattlesnake bites. Its resin was once used to waterproof baskets and fix arrowheads to shafts. Today, some of the resin's more than fifty components are thought to be effective in shrinking cancerous tumors; others are consumed as dietary antioxidants or used as food preservatives.

As almost the only species in its lineage to reach North America, the creosote bush is distinctive in both chemistry and appearance. As a result, it hosts several dozen insect species that, without leaving the bush, can nourish themselves and hide from their enemies, and reside nowhere else. Presumably, these species did not exist in their current form before the creosote bush arrived. The causes of such narrow adaptation, which is typical of insects, have been hotly debated by evolutionary biologists for decades.

Since all of these creosote bush insects are of North American origin (not having migrated from South America along with the bush itself), their specializations presumably evolved as a response to the introduction and eventual dominance of this new resource. Despite settling in comfortably on their new host--and, in some instances, losing the ability to consume other local plants--the insect population that eats creosote bush rarely consumes even a noticeable amount. Given the tremendous adaptability of insects and their ability to generate large populations in a brief time, the question "Why does the world remain green (or brown, in the case of the creosote bush)?" joins "Why are there so many kinds of insects?" as another of the major issues in ecology.

In today's ecological jargon, the two likeliest answers to both questions are "top-down" and "bottom-up" forces. Top-down forces--predators, parasites, disease--exert pressure on herbivorous insects to develop specialized defense tactics at the same time that these insects are being consumed faster than they can consume the plants. Such pressures, the theory suggests, can generate diversity among the insects while keeping their numbers low. Bottom-up forces, on the other hand, are essentially plant defenses, including such traits as spines, tough tissues, and noxious chemicals. According to this argument, herbivores fail to devour the entire green world because much of it is inedible--or even downright poisonous--to any given species of herbivore. Creosote bush and its resident insects offer an exceptional opportunity to address these questions: the plant's small size allows us to identify its well-known insects and exhaustively count (even manipulate) their numbers, as well as to alter the top-down forces by restricting predator access to individual plants.