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Objectives
Identify causes and possible results of interspecific competition.
Identify some adaptations of predators and prey.
Compare and contrast symbiotic relationships.
Key Terms
interspecific competition
competitive exclusion
niche
predation
symbiotic relationship
parasitism
mutualism
commensalism
On your next walk through a patch of woods or park, or even down a
city street, look for examples of different species interacting with each
other. You might observe birds nesting in trees, bees collecting pollen
from flowers, or ferns growing in the shade of larger plants.
Populations do not live in isolation instead, they interact with other
populations living in the same area. In some cases, populations
compete for resources such as food, water, or shelter.
Competition Between Species
An organism, such as an elephant, cannot survive without other
organisms. The elephant is part of a herd of elephants that form a
population. The herd is part of a larger community of organisms. Recall
from Chapter 34 that a community is a group of species living in the
same geographic area. The elephants' community includes gazelles,
giraffes, and birds; ants, beetles, fungi, and bacteria in the soil; and
grasses and trees.
In Concept 35.2 you read that members of a population may compete
for limited resources in the environment. This competition within a
single species limits the growth of the population. Within a community,
interspecific competition (competition between species) takes place
when two or more species rely on the same limited resource. For
example, in the African savanna community, many species feed on
grasses. In times of drought, the grasses may be in short supply, and
competition may become especially intense.
Competitive Exclusion If two species are so similar in their
requirements that the same resource limits both species' growth, one
species may succeed over the other. This process is referred to as
competitive exclusion. A Russian biologist named G.F. Gause
demonstrated this principle in the laboratory. He cultured two different
species of Paramecium, P. aurelia and P. caudatum, feeding them a
constant amount of food every day. When he grew them in separate
containers, both thrived. But when he grew them together, the two
species competed for the limited food available. Because P. aurelia
could gather food more quickly, P. aurelia survived and reproduced,
while P. caudatum starved. Eventually only P. aurelia remained in the
culture.
Figure 35-14
Two similar species may each thrive in separate locations, but
one may exclude the other when they are placed together. The
results of an experiment with two Paramecium species
demonstrate this principle of competitive exclusion.
In nature, the degree to which two species can require the same
resources and still coexist depends on other factors in the ecosystem.
For example, predators may keep competing populations below the
levels at which resources such as food would become limiting factors.
Gause's laboratory experiment demonstrated the process of competitive
exclusion because he was able to isolate the two species and their
common limiting resource (food) in the laboratory.
Niches Within a community, each species has a unique living
arrangement called its niche. A niche includes an organism's living
place (habitat), its food sources, the time of day it is most active, and
many other factors specific to that organism's way of life. The local loss
of a species due to competitive exclusion is most likely to occur if two
species have niches that are very similar. But even when species in a
community compete for some of the same resources, their niches are
rarely identical. For example, one lizard species in a tropical forest may
feed on insects in low shrubs, while a similar lizard species may eat
insects high in the trees.
Predation
Within the same savanna community where grazing animals may
compete for grass, other interactions between species are taking place.
For instance, a lion chases down an injured zebra, while nearby an egret
targets a fish for its meal. These two interactions are examples of
predation, an interaction in which one organism eats another. The lion
and the egret are examples of predators, the organisms doing the eating.
The food species being eaten are the prey. Because eating and avoiding
being eaten are so important to survival, it is not surprising that many
effective adaptations have evolved in both predators and prey.
Predator Adaptations Predators that pursue their prey are
generally fast and agile. Many predators have coloring that hides, or
camouflages, them in their surroundings. Some predators, such as
wolves and killer whales, may team up in packs to capture their prey.
Most predators have acute senses that enable them to find prey.
Rattlesnakes, for example, locate their prey with heat-sensing organs
located between each eye and nostril. Adaptations such as claws, teeth,
fangs, and stingers help many predators catch prey.
Prey Adaptations Several adaptations help prey avoid being eaten.
Some organisms retreat to safe locations; others flee from predators.
And, while predators use camouflage to ambush prey, prey may use
camouflage to hide from predators. Another type of defensive coloring
has the opposite effect—it makes the organism stand out. Such
"warning coloration" serves as a caution to predators.
Some organisms aren't poisonous or dangerous themselves, but they
look like organisms that are. This type of defense is called mimicry. For
instance, the hawk moth larva puffs up its head and thorax when
disturbed, looking like the head of a small poisonous snake. It even
weaves its head back and forth and hisses like a snake.
Animals aren't the only prey organisms that have elaborate defense
adaptations. Plants cannot run away from predators, but they have other
defenses that include poisonous chemicals and structures such as
spines and thorns.
Symbiotic Relationships
A symbiotic relationship (sim bee aht ik) is a close interaction between
species in which one of the species lives in or on the other. There are
three main types of symbiotic relationships: parasitism, mutualism, and
commensalism.
Parasitism is a relationship in which one organism, the parasite, obtains
its food at the expense of another organism, the host. Usually the
parasite is smaller than the host. Both blood-sucking mosquitoes and
tapeworms that live and feed in the intestines of larger animals are
examples of parasites.
The process of natural selection, described in Chapter 14, works on
both the parasite and the host. Parasites that can locate and feed on
their hosts efficiently are most successful. For example, some aquatic
leeches locate their hosts first by detecting movement in the water.
Then they confirm their selection by using temperature and chemical
cues on the host's skin. Usually the effect of the parasite does not kill
the host quickly, which would result in the death of the parasite as well.
Natural selection has also produced defensive adaptations that help
hosts resist parasites. The immune system of humans and other
vertebrates is an example.
In mutualism, both organisms benefit from the symbiotic relationship.
One example of mutualism occurs inside your own body. Your large
intestine is inhabited by millions of bacteria. The bacteria benefit by
having a warm, moist home with a constant stream of nourishment, your
food. In turn, some intestinal bacteria produce vitamin K. As discussed
in Chapter 29, vitamin K is essential for blood clotting. Both you and
the bacteria benefit from this relationship.
Commensalism is a relationship in which one organism benefits, while
the other organism is neither harmed nor helped significantly. For
example, a spider crab may place seaweed on its back. The crab
benefits by being camouflaged from its predators. The seaweed does
not seem to be significantly affected. True commensalism in nature is
rare, since most interactions harm one species (parasitism) or help both
species (mutualism) to some degree.
Concept Check 35.4
1. How did Gause's experiment with Paramecium demonstrate
competitive exclusion?
2. Describe two methods predators use to help them capture their prey
and two methods prey use to help them avoid being eaten.
3. Define and give an example of each of the three types of symbiotic
relationships.
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