Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 152

FIGURE 11.2 A plantation of the epiphytic vanilla orchid in Tabasco, Mexico. The vanilla plants ( Vanilla fragrans ) grow on the
shade tree Glyricidia sepium .

nonessential for the survival of either organism, the result-
ing relationship is called protocooperation . An example
of protocooperation is the relationship between the Euro-
pean honeybee ( Apis mellifera ) and the plants it pollinates.
The plant a bee visits is adding pollen and nectar to the
environment, serving to attract the pollinator. The actual
gathering of the nectar or honey by the bee is removal
interference, but then the pollen is added back into the
environment when the bee deposits it onto the stigma of
another flower — this is the point at which the positive
effects of the interaction are realized. Honeybees visit a
wide range of plant species, most of which are visited by
other pollinators as well, making the relationship between
the honeybee and any particular plant species nonobliga-
tory. In many agricultural landscapes, however, the dra-
matic reduction in biotic diversity that has accompanied
the expansion of monocultures, heavy use of pesticides,
and fencerow-to-fencerow farming has created an artificial
dependence on honey bees that are raised by beekeepers
and transported in hives to the crop fields during pollina-
tion time.
When the organisms benefiting each other through
addition interferences become dependent on each other

for optimal performance and even survival, then the rela-
tionship is mutualism . A good example of mutualism is
the relationship between certain soil-dwelling fungi and
their vascular-plant associates. The fungi are made up of
mycorrhizae , special compound structures that can form
connections with plant roots. The mycorrhizae allow the
root to provide sugars for the fungus, and the fungus in
return to provide water and minerals to the plant. There
are two types of mycorrhizae: (1) ectotrophic , in which
the fungal mycelium forms a dense mantle covering the
surface of the root, with many hyphae that extent outward
into the soil, and others that extend inward and force
themselves between the cells of the epidermis and cortex
of the root (very common in the Pinaceae ); and
(2) endotrophic , the most common type, in which there is
no surface mantle but instead some of the hyphae actually
inhabit the protoplasts of parenchymatous tissues and
extend outward into the soil (common in most flowering
plant families, especially important crop species such as
corn, beans, apples, and strawberries).
Another important example of a mutualism is the rela-
tionship between legumes (plants in the Fabaceae family)
and Rhizobium bacteria (Figure 11.3). The bacteria enter