Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 151

Herbivory for the associated organisms, or when it excludes them

from the area entirely. In other cases, the impact of the
addition interference can be positive for the associated
organisms, as when they use the added substance or mate-
rial to improve their own standing in the community,
or when the exclusion of intolerant organisms from the
habitat allows them to occupy it. Ultimately, associated
organisms benefiting from the addition may develop a
dependence on the organism making the addition, creating
a relationship of coexistence or even of symbiosis.

The interference relationship between a herbivore and the
plant it consumes — like that of parasite and host — is a
very direct one, with plant tissue being the part of the
environment that is removed. Beyond the scope of the
individual plant, however, herbivory is removal interfer-
ence in an even broader sense in that biomass and its
associated nutrients are removed from the environment.
The consumption of plant material reduces the return of
biomass to the soil, and if the removal is too intense and
takes place over an extended time frame, it can lead to
depletion of nutrients in the system.
From an agricultural perspective, herbivory can have
three types of negative impacts. First, herbivory removes
photosynthetic surface area that may be of importance in
the development of the crop plant. Second, if the plant
part that is consumed were going to return to the soil as
crop residue, herbivory is reducing this input to the sys-
tem. Third, if the herbivory damages a part of the crop
that is intended to be harvested and sent to market, the
product's sale value may be reduced.
The effects of herbivory, however, are not always nega-
tive. In some pasture or range situations, for example,
grazing can be beneficial to the productivity of the forage
species. Removal of excessive plant material can stimulate
the production of new biomass, or even allow certain plant
species that are suppressed by old or excessive plant cover
to germinate or become more predominant in the pasture
mixture. The evolutionary role of such removal interfer-
ence has been well documented for the Serengeti Plains
in Africa (McNaughton, 1985), where it has been shown
that the highest productivity and species diversity of both
plants and animals has developed under cyclical patterns
of multispecies grazing. Good range managers know that
periodic rotational grazing promotes the most production
in pasture systems.
In natural systems as well, herbivory plays an impor-
tant role in removing excess biomass, directing energy
flow, and recycling nutrients. These processes have the
potential for playing important and positive roles in agro-
ecosystems, but humans have tended to view herbivory as
wholly negative, a constant challenge to be overcome.
Further research needs to be focused on how the pressure
of such removal interference can be directed away from
the economically valuable parts of the agroecosystem and
concentrated in parts that stimulate other components of
the system in ways that contribute to sustainability.

Epiphytism

When one organism lives on the body of another without
drawing any nutrition from it, addition interference is
occurring because the host is adding a physical structure
to the environment that is providing another organism with
a habitat. When the two organisms are plants and the
habitat is a trunk or stem, the perched plant is called an
epiphyte; when the habitat is a leaf, it is called an epiphyll .
In Odum's terms, epiphytism is a form of commensalism.
Epiphytes and epiphylls do not obtain water or food
from the supporting plant, nor do they have connections
to the soil. Water is derived from precipitation, and nutri-
ents from wind-borne particles, the decay of the support-
ing plant's bark, and minerals and organic compounds
dissolved in raindrip. Most epiphytic plants face frequent
drought conditions in their aerial environment, even in the
moist habitats where they are most common. Algae,
lichens, mosses, and a few ferns are the most common
epiphytes in cold and wet environments; a wide variety of
vascular plants have evolved the epiphytic lifestyle in
warm and wet climates, especially ferns and species
belonging to the families Bromeliaceae and Orchidaceae .
A large number of species in these two families have taken
on considerable economic importance in horticulture and
floriculture, and are raised on artificial perches in green-
houses and lathhouses for commercial sale.
An epiphytic plant of considerable economic impor-
tance in agriculture in several tropical countries is vanilla
( Vanilla fragrans ). Vanilla produces long whitish aerial
adventitious roots at each leaf that adhere firmly appressed
to the trunk or branches of the host plant. Sometimes roots
climb down the trunk to the ground, but only ramify in the
humus or mulch layer. Capsule-like fruits up to 25 cm long
(called beans in the trade) form on the aerial stems, and
are dependent on hand pollination for successful formation
in many parts of the world into which the crop has been
introduced from its native Mesoamerica (Figure 11.2).

A DDITION I NTERFERENCES

Symbioses

Many organisms in the course of their daily life processes
add something to the environment that impacts associated
organisms. These impacts can be negative, such as when
the addition causes a reduction in growth or development

When two organisms make additions to the environment
they share so as to benefit each other, they form a symbi-
otic relationship. If the relationship is nonobligatory and