by Whit Gibbons

April 17, 2005

Intricate ecological relationships have been documented among many animal species. For example, some frogs and insects have been shown to alter their mating calls and behavioral strategies in order to avoid predation. Male frogs and katydids attract females by announcing their location and availability through song. Unfortunately, many species of bats eat insects such as katydids and some large tropical bats eat frogs. So advertising by sound may attract not only a mate but also a hungry predator.

Equally complex relationships are known for a variety of other animal species. For example, scorpionfly males will pose as females in order to trick other males into relinquishing a meal planned as a courtship offering; certain species of small marine fish can swim, unharmed, in and out of the venomous tentacles of the potentially deadly Portuguese man-of-war jellyfish; honeyguide birds of Africa lead badgerlike mammals called ratels to trees where beehives are located. The ratel breaks open the hive to eat the honey while the honeyguide eats the beeswax.

Thousands of such interactions exist in the animal kingdom, and the only factor limiting the known number is their discovery by ecologists. Similarly, many complicated relationships have been discovered between particular animal and plant species. However, relatively few complex ecological interactions between species of plants have been documented. This is partially because very few plants have behavior patterns that can be observed, so that most relationships are passive and subtle.

Nonetheless, a study in the Sonoran Desert provided evidence for a phenomenon in which one species of plant positively affects the survival of another in a manner previously unsuspected. This can be very important in a desert environment that has only a few plant species that are able to survive and thrive. In the Sonoran and Mojave Deserts of the American Southwest, a common plant is the agave. Agave is well-known by many as a common garden plant.

The persistence of agave in the desert is impressive, considering the temperatures that must be tolerated. Soil surface temperatures in the Sonoran Desert commonly reach temperatures greater than 150? F, yet all the roots of an agave seedling remain within three inches of the surface. How does a young growing agave plant manage to survive its first full day of summer in such an environment?

Ecologists studying the phenomenon found that nearly all the young agave seedlings were growing in association with another plant species known as desert bunchgrass. In fact, most seedlings occurred only in the center or on the north side of the bunchgrass plant. The shade provided by the bunchgrass gave the seedlings protection from the desert sun. Ecologists use the term "nurse plant" for a species on which another plant is dependent. Thus, desert bunchgrass serves as the nurse plant on which agave depends for its survival during early development.

The study also revealed that the soil nitrogen, a critical element for growing plants, was significantly higher around the base of a bunchgrass clump than in open soil areas. Thus, agave plants benefited from an enriched soil situation. One detrimental aspect for a seedling agave associated with bunchgrass is that the amount of water available to the roots is reduced, compared to other sites on the desert floor. But, the value of the shade that is essential for early survival presumably outweighs the water loss that must be endured by a young agave. In essence, without bunchgrass, there might be no agave in some areas.

The desert is a relatively simple ecosystem in terms of the number of plants and animals and their interactions. Imagine how complex the ecological network is in an oak-hickory forest or a tropical rain forest with its rich fauna and flora. Findings such as these about the subtle yet vital interactions among species give us a clear message that the world and its environments are far more intricate and delicately balanced than we might think. We need to be increasingly careful in our manipulation and management of natural systems and make sure we first understand the way all the pieces work.

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