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COMMUNITY ECOLOGY
Introduction
In this section, we study the structure of a community. We also study the four main
types of interaction within communities.
We will investigate and discuss the human influence on the community structure of
humans
Community structure
As we already know a group a organisms of the same species living in the same area is
called a population. A number of different populations together are called a community.
Example, grassland community near Johannesburg. It may consist grass, different insect
and bird populations. A number of communities together make up an ecosystem.
Within a community organisms can die classified based on their method of obtaining
nutrients or feeding. Autotrophs e.g. plants have the ability to produce their own food by
a process called photosynthesis. These autotrophs are also called producers.
All plants and animals that obtain food from another source because they cannot make
their own food is called heterotrophs e.g. humans. These heterotrophs are also called
consumers and directly or directly are dependent on the autotrophs or producers for their
food. Consumers can be categorised by what they eat. Herbivores are consumers that
only eat plants, e.g. cows and sheep. Carnivores are consumers that only eat meat, e.g.
lions and leopards. Omnivores are consumers that eat both plant and meat, e.g. humans
and apes. These consumers can be placed into three categories:
 Primary consumers – they eat producers or plants.
 Secondary consumers – they eat the primary consumers.
 Tertiary consumers – they eat other consumers.
This can be displayed in a pyramid of numbers, see Fig. 1 below. This indicates the
number of organisms at each level to ensure healthy community structure or simply the
numbers of organisms required to maintain a food chain.
Fig.1 Pyramid of numbers
Finally, there are decomposers that include fungi and bacteria. These organisms
obtained food or energy from dead organic materials.
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Interactions within a Community
All communities are finely balanced with every individual organism involved in a battle
for survival. The four types of interactions to be discussed in this lesson are predation,
competition, parasitism, mutualism and commensalism.
Predation
Predation is the interaction between predator and prey. Predators kill others animals for
food, e.g. cheetahs catching springbok.
Fig. 2 Illustration of predation
Predation also has an added advantage because it controls population size as illustrated
below. An example is the lynx (predator) and dassie (prey) in Table Mountain. When
the number prey increases there is more food available. That means that all the cubs of a
lynx litter will receive food and all will reach maturity. That increases the predator
population. As the predator population increases the predation pressure would increase
which means that the predators will be killing more prey causing a decrease in prey
population. As the prey population decreases the predators will have less food and their
numbers will be decreasing. The decrease in predators allows the number of prey to
increase. And the cycle continues.
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Fig. 3 Graph of predator-prey interaction
Competition
Competition is the interaction between individual organisms for a limited resource, e.g.
light, water, space, shelter and food. This interaction can lead to specialization for
particular modes of life.
Intraspecific competition is the competition between individuals the SAME species.
Best known example, lions competing against other lions for the carcass (dead body) of
zebra. Because the individuals are from the same species (Panthera leo) it is
intraspecific competition. The principle is the same in the case of plants. If you plant a
large number of mielies (Zea mays), the individual plants competing for water, nutrients
and light will be of the SAME species thus making it intraspecific completion.
Interspecific competition is the competition between individuals from DIFFERENT
species. Best known example, while lions (Panthera leo) are eating the carcass of the
zebra, hyenas (Crocuta crocuta) join to get some food and fighting breaks out between
lions and hyenas. These are different species competing for the same resource and that
is why it is interspecific competition. Again, the principle is the same in the case of
plants. If you plant a large number of mielies (Zea mays) in a field with many weeds,
the individual plants, mielies and weeds, competing for water, nutrients and light will be
of DIFFERENT species thus making it interspecific completion.
Gause's law refers to the principle that when two species compete for the same resources
within an environment, one of them will eventually outcompete and displace the other.
The displaced species may become locally extinct due to death of all representative
individuals or migration to another suitable environment. This is called the competitive
exclusion principle as illustrated in the example below.
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When Paramecium aurelia and Paramecium caudatum species live in separate
environments their population thrive as illustrated below. But when present in the same
environment , they start competing with each other. Paramecium aurelia which is a
smaller, faster growing outcompetes Paramecium caudatum. So Paramecium caudatum
become extinct.
Fig. 4 Competition in Paramecium
Resource partitioning
When resources are limited, natural selection will result one of the species utilising
different resources. This allows the species to co-exist within the same community.
This phenomenon is called resource partitioning.
Example is in a forest where different species of plants survive in different environments
of light intensity. In the African savannah, lion lived in the wide open grasslands.
Leopards hunt in grassland but live in trees.
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Parasitism
Parasitism is a symbiotic relationship in which one species (parasite) benefits while the
other species (host) is damaged. Most parasites will limit the damage to prevent death of
its food source. There are two types of parasites, namely ectoparasites and
endoparasites. Ectoparasites live outside the body of its host and examples include
mosquitoes, lice, flea and ticks. Endoparasites live inside the body of its host and
examples include tape-worms, round worms and bilharzia. Parasitism is regarded by
some scientists as a form of predation.
Fig.5 Common ectoparasite (mosquito)
Most common South African example of an ectoparasite is the common mosquito shown
above. It lives outside human’s bodies and suck blood through the human skin. Other
examples include ticks that live on dogs and cows. These ticks suck blood from their
hosts and in extreme cases can actually kill the host.
Another South African animal parasite is the bilharzia parasite, Schistosoma
haematobium , a parasitc flatworm. It is a very successful parasite and completes its life
cycle in two hosts, a snail, Bulinus africanus and humans.
Most common South African plant parasite is the Dodder (Cuscuta sp .) which lives on
wheat. The dodder has small root-like structures, called haustoria which penetrate to the
wheat's vascular bundles. The parasite obtains organic nutrients, water and mineral salts
directly from the host. With all the nutrients, the parasites then spread to adjacent plants.
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Fig. 6 Dodder
Mutualism
Mutualism is a symbiotic relationship in which both species benefit. There are two
types facultative and obligate mutualism.
In facultative mutualism both organisms benefit by living in close association, but is not
essential. Example, the relationship between ants and aphids (plant lice) on rose plants.
The ants feed on the sugary fluid released by the aphids, and the aphids are protected by
the ants.
Fig. 7 Mutualism between ants and aphids
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In obligate mutualism is a relationship in which both species are forced to live in
mutualism. Example, is a lichen which is a “plant” made up of a fungus and an alga
living in close association. Lichens are visible everywhere including roof tops, rocks and
tree trunks. The fungus absorbs inorganic substances which are then used by the alga to
make food during photosynthesis. The fungus then gets food which was made by the
alga. Another every common example is the relationship between bees and flowers.
Fig. 8 Mutualism in lichens
Commensalism
Commensalism is a symbiotic relationship between two species in which one species
benefits and the other is not benefitted or harmed. The host species provides a home
and/or transportation for the other species. Most well-known example is the clown
fishes live within the waving mass of tentacles of sea anemones as shown in the movie,
“Finding Nemo”. Other, examples include birds nesting in trees. The bird receives
benefit because its nest is far from predators. The trees are neither benefitted or harmed.
Fig. 9 Commensalism