Desert plants have made a number of adaptations to survive in the extraordinarily rough desert climate. In addition to dealing with the lack of water, the extreme temperatures and the hot sun, they also have to deal with animals, insects and birds that recognize them as a source of precious water. Fortunately, many have made adaptations for these problems, too.
Many desert plants contain alkaloids, which are compounds that contain salts and nitrogen. These plants--in almost any growth stage--are highly poisonous to animals and can cause violent, nervous system reactions and death when even the smallest amounts are consumed. More than 100 separate species of desert plants containing alkaloids have been discovered, including senecio, crimson spot rockrose, threadleaf groundsel, desert tobacco and locoweed. Some plants contain alkaloids in a smaller amount, making their toxicity lower. However, these plants can still be dangerous if enough plant matter is ingested. Such plants include peyote (which, in addition to its properties as a hallucinogen has also been known to cause paralysis, vomiting, tremors and difficulty breathing).
Plants such as greasewood, Russian thistle and Gambel oaks contain the most common kind of poison found in desert plants--organic acids. These acids cannot be digested by animals that consume them, and cause colic and finally kidney failure. Russian thistle is a common plant in the southwestern United States. Native to southeastern Russia and parts of Siberia, it was brought to the United States by immigrants, when it became more commonly referred to as tumbleweed. An extremely hardy plant, measures have been taken to contain the growth and reproduction of tumbleweed. In addition to strangling other plants growing near it, it has also been known to cause severe allergic reactions to people who have been exposed to it.
There are a number of desert plants who defend against insect infestation by consuming their would-be predators. The byblis, a desert plant native to Australia, with long, slender leaves, secretes a substance that traps insects on its leaves. The byblis is similar to the American sundew, which has leaves that wrap themselves around the captured insects before digesting them. Some of the bromeliads--common desert plants with thick, fleshy leaves--are carnivorous as well (e.g., Catopsis tillandsioideae).