Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 150

leave offspring. Interestingly, however, ecologists also see
that avoiding competition can actually be advantageous
for a species, and that doing so has probably played a key
role in the development of species diversity.
Without actually studying the mechanisms of interfer-
ence that are involved in competition, and identifying the
removal process from the environment that leads to it, we
can only assume that competition occurs. Agroecosystem
management requires a more detailed determination of
competitive interactions; otherwise, the farmer is left with
no other option but to overload the system with excess
resources.

The relationship between mistletoe and various spe-
cies of trees is an example of this kind of removal inter-
ference (Figure 11.1). The mistletoe plant actually pene-
trates and taps into the vascular system of the host tree,
drawing its water and nutrients from the host. If the
parasite becomes too abundant on the host tree, the tree
is stunted and often deformed, and can become subject to
debilitating attacks from other pests. Farm and range ani-
mals are especially susceptible to parasites; these include
ticks that attach externally to the host, screwworm flies
that lay eggs in the flesh of the animal, and stomach
parasites ranging from bacteria to worms.
Under natural conditions, parasitism probably repre-
sents something of a compromise between the host and the
parasite. They have evolved together over time, with the
host being tolerant of a constant low-grade infection, and
the parasite depending on the continuity of the host's life
for its own reproductive success. In agricultural situations,
however — especially the human-maintained conditions
of concentrated monocultures — heavy parasite loads
become a serious form of disease that puts the entire crop
or herd at risk of developing secondary diseases and dying.

Parasitism

As described above, parasitism is an interaction in which
two organisms live together, with one (the parasite) deriv-
ing its nourishment from the tissues of the other (the host)
without killing it. In interference terms, the environment
from which removal takes place is the body of the host.
Parasites are physiologically dependent on their hosts, live
much shorter lives, and have a high reproductive potential.

FIGURE 11.1 Parasitic mistletoe on a guava tree, Monteverde, Costa Rica. The guava branch is so heavily infested by the parasite
that only the red-orange flowers of the mistletoe are visible.