Download Chapters • Lesson 16

Chapters • Lesson 16
Relationships among Organisms
Objective: 2,1,3
Key Termsy Words
• ecosystem • population • stable ecosystem • habitat • niche • symbiosis • mutualism
• commensalism • parasitism • predation • competition • pheromone • territory
Getting the Idea
Organisms do not live in isolation. They interact with the other organisms that share their
ecosystems as well as with the nonliving parts of their environments. Recall that an
ecosystem is the sum of all the living and nonliving things in an area and the interactions
among them.
Habitats and Niches
The stability of an ecosystem depends on the interactions among the individuals and
populations living there. A population is made up of all the organisms of a species that live
in an area at the same time. A single ecosystem may be the home of many diverse
populations. A stable ecosystem is one in which the population sizes and available
resources cycle regularly or change predictably and energy flows at a fairly constant rate.
A habitat is the place where an organism lives. The resources, such as food, water, and
shelter, that a habitat provides can be divided into different niches. A niche is an organism's
role in its community, including what the organism eats, how it affects the flow of energy,
and where it feeds.
Although many species of organisms can share the same habitat, different species cannot
have exactly the same niche within that habitat. For example, a moss can grow in a moist,
shaded area, such as at the base of a tall tree. The moss and the tree both use sunlight and
carbon dioxide to make food by photosynthesis. Making their own food is part of the niche
of both the moss and the tree. Acting as a food source for other organisms is also part of
both the moss's and the tree's niche. However, the niches of the moss and tree are not
identical because the tree grows well in direct sunlight, while the moss requires shade. Also,
the tree and moss are eaten by different organisms.
Symbiosis
The stability of an ecosystem is determined by the different interactions within and among
the populations living there. One type of interaction between species is symbiosis.
Symbiosis is a close, interdependent relationship between organisms of different species.
There are three types of symbiosis: mutualism, commensalism, and parasitism. All three
can be part of a stable ecosystem.
Mutualism is a symbiotic relationship in which both organisms benefit because each
species does something that helps the other species survive. Termites have a mutualistic
relationship with bacteria and microscopic protists that live in the insects' digestive systems.
Termites feed on wood and other plant material but cannot digest it on their own. The
bacteria and protists break down cellulose, providing food to both the termites and
themselves. The termites supply the bacteria and protists with a habitat, including sources
of food.
Commensalism is a symbiotic relationship in which one organism benefits and the other
does not seem to benefit or be harmed. Most often, a smaller organism benefits from its
relationship with a larger organism that provides it with shelter, transportation, defense, or
food. Barnacles, for example, have a commensal relationship with whales. Barnacles are
animals that live attached to surfaces. They often attach to the bodies of whales without
having an effect on the whales. The whales carry the barnacles to new places, helping them
find food and reproduce.
Parasitism is a symbiotic relationship in which one organism benefits and the other
organism is harmed. The organism that benefits is the parasite. A parasite lives on or in
another organism at the expense of that organism. The parasite benefits because it gets
food or shelter from the organism in which it lives. The organism in which a parasite lives is
called the host.
Tapeworms, heartworms, and some bacteria are parasites that live within the bodies of their
hosts. These parasites feed off the host's body fluids. Aphids, fleas, and mistletoe are
parasites that live outside the body of the host. Each of these parasites can harm its host by
weakening the host or spreading disease. Most parasites do not kill their host because if the
host dies, the parasite loses its food or shelter and is likely to die also.
Predation
Predation is an interaction between organisms in which one organism kills and eats
another. The organism that obtains food is the predator. The organism that is killed and
eaten is the prey. A lion is a predator that feeds on zebras, which are the prey.
Predation plays a key role in an ecosystem by keeping population sizes balanced. If the
number of predators decreases, the number of prey will increase. Prey populations can
grow too large for an ecosystem's resources to support, leading some of the prey to starve.
Then, because there are more prey animals and because some are weakened by hunger,
they are easier for predators to find and kill. Denser populations are also more vulnerable to
disease. Both these factors can reduce the size of a prey population. Similarly, if a predator
population grows too large, there will not be enough prey to support it. Some predators will
starve, and their population will get smaller. The relationship between predator and prey
keeps the numbers of both populations sustainable.
The diagram below shows how the size of a population of predators, such as wolves, varies
with the size of a population of its prey, such as rabbits. Whenever the rabbit population
increases, the wolf population increases. Increased predation then reduces the rabbit
population. Next, the decrease in prey leads to a smaller number of wolves. Because these
changes within and between the populations follow a pattern, predator and prey interactions
can help keep an ecosystem stable.
Competition
Organisms obtain everything they need for survival from their ecosystems. Resources that
all organisms need include food, water, air, and living space. Competition is an interaction
in which two or more organisms try to use the same resources at the same time.
Organisms compete in every ecosystem. Often organisms of the same species compete.
For example, the hyenas on a savannah compete with one another for food, water, living
space, and mates. Organisms also compete for resources with members of other
populations. For example, lions and hyenas both hunt zebras for food. A given zebra
population can support only so many predators. If the hyena population grows, the lion
population may decrease. At the same time, the lions, hyenas, and zebras, along with other
organisms that share their ecosystem, all compete for water.
All organisms compete to some extent for resources. The most successful competitors are
the organisms that are best adapted to their ecosystem.
Communication and Defense of Territory
Most animals have ways of communicating with other animals of their own or other species.
Communication helps animals survive by allowing them to identify themselves, attract
mates, warn one another of predators, and protect their living space.
Sound is a common method of communication among animals of the same and different
species. For example, some species of whale use songs to attract mates. Wolves howl to
identify their location and draw members of their packs together. Other animals use sounds
such as hissing and growling to announce their presence to animals of other species. Such
sounds may be used either to scare off potential predators or to protect living space.
Visual communication is often used to attract mates. For example, many animals perform
courtship dances. Courtship dances are usually instinctive, elaborate, and ritualistic movements
that an animal performs to attract the attention of a potential mate. Such rituals have been
observed in scorpions, spiders, and various species of birds.
Some animals communicate with chemicals, including pheromones. A pheromone is a
chemical substance that an animal produces to bring about a specific response in other animals
of the same species. Many animals use pheromones to attract mates. Such pheromones can
often be detected by other animals of the same species over great distances. Many social
insects, such as bees, ants, and termites, use a variety of pheromones. Each pheromone
communicates a different type of information to other members of the colony. For example, ants
and termites may emit one type of pheromone to mark a path that leads other members of their
colony to a food source. Many bees and ants use other pheromones to signal a threat to the
hive or colony. The pheromone may trigger other members of the group to swarm together and
attack the invader.
Animals may also use chemicals to help identify and defend their territory. A territory is an area
that an animal claims and defends to ensure it has access to resources it needs. A territory can
be a relatively small area, such as a nest or den. It may also be a large area in which an
organism lives and hunts for food or other resources. Many animals use chemical markers such
as urine or musk to identify the boundaries of their territories to others. Musk is a scented
chemical that is released from the glands of some animals. Animals often use sounds to warn
others to stay out of their territory. Animals of some species defend their territory through
aggressive behaviors such as fighting.
Focus on Inquiry
Predator-prey interactions lead to a cyclical relationship between the population sizes of the two
species. The data table below gives estimates for populations of polar bears, which are
predators, and arctic seals, which are their prey, in a tundra ecosystem.
Recall from Lesson 14 that only a small part of the energy in any organism is passed to the
organism that feeds on it. As a result, a prey population is almost always much larger than the
predator population. Predator-prey graphs are often shown without scales, so that one graph
can show changes in both populations. The same interval on thejy-axis may indicate 100 deer
and 10 wolves. Another way to draw these graphs is to label the y-axis in the usual way for one
organism and put a different scale for the other organism on the right-hand edge of the graph.
Use the data table above to construct a line graph of the seal and bear populations over ten
months. Make your graph on a separate piece of graph paper. Label your axes, and include a
key to distinguish the populations shown.
Based on your graph, what are the approximate population size ranges of each species?
If environmental conditions remained the same, what would you expect the graph to look like for
months 11-20?
Suppose that in month 11, the population of a kind of fish that was an important part of the seal
diet decreased dramatically. How would you expect this change to affect the graph over the
coming months if the fish population did not recover?
Now suppose that in month 12, the fish population recovered completely. However, in month 13,
a disease killed half the populations of both seals and bears. If all other factors in the
environment remained the same, how would you expect the graph to change in the short term
and the long term?