Introduction
Interacting species have a tremendous influence
on the size of each other's populations.
The various mechanisms for these biotic influences
are quite different from the way in which abiotic factors
effect the size of populations. Biotic factors
also regulate the size of populations more intensely.
Finally, the influence of biotic interactions can occur
at two different levels. Interspecific effects
are direct interactions between species, and the intraspecific effects
represent interactions of individuals within a single
species.
Neutralism
Neutralism is
the most common type of interspecific interaction.
Neither population directly
affects the other. What interactions occur are slight
and indirect. The simple presence of the two species should
not directly affect the population level of either.
An example of neutralism would be the interaction between
rainbow trout and dandelions living in a mountain valley.
Competition
When two or more organisms in the same
community seek the same resource (e.g., food, water,
nesting space, ground space), which is in limiting supply
to the individuals seeking it, they compete with
one another. If the competition is
among members of the same species,
it is called intraspecific.
Competition among individuals of different species it
is referred to as interspecific
competition. Individuals in populations experience
both types of competition to a greater or lesser degree.
Competition may be the result of two different
processes: exploitation or interference. Competition by exploitation occurs
between individuals when the indirect effects of two
or more species or individuals reduce the supply of the
limiting resource or
resources needed for survival. The exclusion of one organism
by another can only occur when the dominant organism
requires less of the limiting resource to survive. Further,
the dominant species must be able to reduce the quantity
of the resource to some critical level with respect to
the other organism. Resource exploitation, however, does
not always cause the exclusion of a species from a community.
It may just cause the species involved in this interaction
to experience a reduction in their potential growth.
Competition by interference occurs
when an individual directly prevents the physical establishment
of another individual in a portion of a habitat. Established
plants can preempt the invasion and colonization of other
individuals by way of dense root mats, peat and litter
accumulation, and mechanical abrasion.
Amensalism
Amensalism is
an interaction where one species suffers
and the other interacting species experiences no effect.
One particular form of amensalism is allelopathy which
occurs with plants. Allelopathy involves the production
and release of chemical substances by one species that
inhibit the growth of another. Allelopathic substances
range from acids to bases to simple organic compounds.
All of these substances are known under the general term: secondary
substances. Secondary substances are chemicals
produced by plants that seem to have no direct use in metabolism.
A good example of a secondary substance is the antibiotic
juglone which is secreted by Black
Walnut (Juglans nigra) trees. This
substance is known to inhibit the growth of trees,
shrubs, grasses, and herbs found growing near Black
Walnut trees. In the chaparral vegetation of California,
certain species of shrubs, notably Salvia leucophylla (mint)
and Artemisia californica (sagebrush) are
known to produce allelopathic substances. Often these
chemicals accumulate in the soil during the dry season
reducing the germination and growth of grasses and herbs
in an area up to 1 to 2 meters from the secreting plants.
Mutualism
Mutualism is
the name given to associations between pairs of species that
bring mutual benefit. The individuals in the populations of
each mutualist species grow and/or survive and/or reproduce
at a higher rate when in the presence of individuals
of the other species. In most ecology or biogeography
textbooks mutualisms are generally underemphasized or
ignored. Yet this type of interaction is an extremely
widespread phenomena. For example, most rooting plants
have mutualistic associations with fungal mycorrhizae.
Mycorrhizae increase the capability of plant roots to
absorb nutrients like
nitrogen and phosphorus. In return, the roots of the
host provide support and a constant supply of carbohydrates for
consumption.
Mutualistic interactions between species
can be of two types: symbiotic or nonsymbiotic. In a symbiotic mutualism, individuals
interact physically and their relationship is biologically
essential for survival. At least one member of the pair
cannot live without close contact with the other. For
example, the fungal-algal symbiosis that occurs
in lichens.
The morphological structure of a lichen is a mass of
fungal hyphae that
forms around a small colony of algae cells. In this mutualism,
the alga produces carbohydrates and other food by products
through photosynthesis and metabolism,
while the fungus absorbs the required minerals and water
to allow for these processes to occur.
More common in nature is the nonsymbiotic
mutualism. In this interaction, the mutualists
live independent lives yet cannot survive without
each other. The most obvious example of an interaction
of this type is the relationship between flowering
plants and their insect pollinators (Figure 9f-1).
| Figure 9f-1:
Bees and many species of flowering plants interact
with each other in a mutualistic fashion. In this
interaction, the flower becomes pollinated by the
insect, while the bee receives food in the form
of pollen and nectar. |
Predation, Parasitism, and Pathogens
Pathogens, parasites, and predators obtain food at the
expense of their hosts and prey.
These processes are basic to the entire grazing food
chain above the autotroph level. Predators tend to be
larger than their prey and consume them from the outside
(Figure 9f-2). A parasite or pathogen is smaller
than its host and consumes it either from the inside
or from the outside of the organism.
| Figure 9f-2:
The tiger (Panthera tigris) hunts at
night preying on a variety of animals, including
deer, wild hog, and wild cattle. Tigers are ambush
predators that try to approach their prey as closely
as possible. They often attack their prey from behind,
biting its neck or throat in the capture process. (Source: Wikipedia) |
It is easy to believe that the predator-prey
interaction is somehow detrimental to the prey population.
This idea has led to extensive efforts to control predator
populations in the name of wildlife conservation. However,
functional relationships between predator-prey between
species, within natural ecosystems, have coevolved over
long periods time creating a dynamic balance between
their interacting populations. Thus, the population sizes
of predator and prey species are
interregulated by delicate feedback mechanisms that control
the densities of both species.
A classic example of the balance between
predator and prey involves the prickly pear cactus, Opuntia spp.
In the 19th century, prickly pear cactus was introduced
into Australia from South America. Because no Australian
predator species existed to control the population size
of this cactus, it quickly expanded throughout millions
of acres of grazing land. The presence of the prickly
pear cactus excluded cattle and sheep from grazing vegetation
and caused a substantial economic hardship to farmers.
A method of control of the prickly pear cactus was initiated
with the introduction of Cactoblastis cactorum,
a cactus eating moth from Argentina, in 1925. By 1930,
densities of the prickly pear cactus were significantly
reduced.
Some times predator species
can drive their prey into localized extinction.
In complex communities, this does no particular harm
to the predator if several other species exist as alternative
prey.
