PLANTS HELP OTHERS SURVIVE
by Whit Gibbons
April 17, 2005
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.
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.
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.
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.
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
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.
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.
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
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