LIFE CAN BE MORE COMPLICATED THAN YOU THINK
you like margaritas, you are beholden to a plant in Mexico known as
the blue agave, the source of tequila. Blue agaves are grown as an agricultural
crop in the state of Jalisco, Mexico, which means that this cool-looking
plant persists with the help of another species, humans. How do other
species of agaves manage to survive in the harsh environment of the
southwestern deserts of Mexico and the United States without human assistance?
of desert agave is impressive, considering the soil surface temperatures
of more than 150 degrees that must be tolerated in the Sonoran Desert.
The roots of an agave seedling remain within three inches of the surface,
a very hot spot for a seed. How does a young agave plant manage to survive
summer in such an environment? For at least one species, part of the
answer is known. A study by researchers Augusto C. Franco and Park S.
Nobel from UCLA provided evidence for a phenomenon in which one plant
(bunchgrass) positively affects the survival of another (desert agave)
in a subtle but previously unsuspected manner.
studying the phenomenon found that nearly all the young desert agave
seedlings were growing in association with another plant known as desert
bunchgrass. In fact, most agave 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. Desert bunchgrass serves as the nurse plant on which
many desert agave depend for their 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. Even a tiny shift
in favor of long-term survival against the overwhelming odds of near
certain early mortality can be crucial in a desert environment where
only a limited number of perennial plant species are able to survive
and thrive. In essence, without bunchgrass, there might be no agave
in some areas.
have described more than 150 species of agaves, which include century
plants and others from which mescal is produced. (Commercial tequila,
by Mexican law, can only be made from the blue agave in selected localities;
mescal can be produced anywhere from many varieties of agave.) Bunchgrass
is nursing along agaves naturally in the desert without our help.
have discovered many complicated biological relationships, and each
time a new one is revealed it is enlightening. Hence it is gratifying
to learn that the same investigators subsequently discovered that the
impressive saguaro cactuses of the Sonoran Desert also benefit from
bunchgrass as a nurse plant during their seedling stage. Considering
the massive grandeur of these quintessential cactuses of movies, cartoons
and southwestern photography, it is sobering to think that many of them
owe their existence to help from another plant when they were seeds.
is a relatively simple ecosystem in terms of the fauna and flora 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 about delicate yet vital interactions among species
give us a clear message that the world and its environments are far
more intricate and interconnected than one might think. We need to be
increasingly careful in our manipulation and management of natural systems.
We ought to understand how all the pieces work together before we start
making changes to the environment.
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