Community Ecology
Divya Patel, Nastassja
Kosinski, Amanda Manzione,
Renée Buttel
Concept 53.1- A community's interactions include
competition,predation, herbivory, symbiosis, and disease.
Competition (-/-)
Competition for resources between plants, between animals, or between decomposers
when resources are in short supply. An ecological niche is the sum total of a species’
use of the biotic and abiotic resources in its environment. The competitive exclusion
principle states that two species cannot coexist in the same community if their niches
are identical. Character Displacement is the tendency for characteristics to be more
divergent in sympatric (geographically overlapping) populations of two species than in
allopatric (geographically separate) populations of the same two species.
Predation (+/-)
An interaction between species in which one species, the predator, eats the other, the
prey. Because of predation, many species have developed special adaptations, such as
various forms of mimicry and coloration.
Herbivory (+/-)
An interaction in which a herbivore eats a part of a plant or an alga. Because of
herbivory many plants and algae have developed many chemical toxins and physical
defenses (spines and thorns), while many herbivores have developed the ability to
distinguish the difference between nutritious and malnutritious plants.
Parasitism (+/-)
A type of symbiotic relationship in which the symbiont (parasite) benefits at the
expense of the host by living either within the host (endoparasitism) or outside the
host (exoparasitism). Parasitoidism is a type of parasitism in which an insect lays eggs
on or in a living host; the larvae then feed on the body of the host eventually killing it.
Disease (+/-)
Pathogens (disease- causing agents) can be bacteria, viruses, or protists, but fungi and
prions can be pathogenic also. Pathogens inflict lethal harm on their hosts. Pathogens
are very much like parasites.
Mutualism (+/+)
A symbiotic relationship in which both participants benefit. Mutualistic relationships
sometimes involve the evolution of related adaptations in both species, with changes in
either species likely to affect the survival and reproduction of the other.
Commensalism (+/0)
An interaction between two species in which one of the benefits, but the neither
harms nor helps the other. There few if any cases of pure commensalism.
Interspecific Interactions and Adaptation- Evidence for coevolution, involving
reciprocal genetic change by interaction populations, is scarce. However, generalized
adaptations of organisms to other organisms in their environment is a fundamental
feature of life.
Populations are linked by interspecific interactions that affect the survival and
Concept 53.2-
Dominant and keystone species exert strong
controls on community structure
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Species Diversity- Species diversity measures the number of species in a community- its species richness-
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Trophic Structure- The structure and dynamics of a community depend to a large extent on the feeding
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Species with a Large Impact – Dominant species and keystone species exert strong controls on community
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Bottom-Up and Top-Down Controls- The bottom-up model proposed an unidirectional influence from lower
and their relative abundance.Species richness is the total number of different species in the community.
Relative abundance is the proportion each species represents of the total individuals in the community.
Measuring species diversity is easier said then done, there are very few methods used to accurately measure
species diversity. Measuring species diversity is important not only for understanding community structure but
also to conserve biodiversity.
relationships between the organisms- the trophic structure of the community. A food chain is the transfer of
food energy up the trophic levels from its source in plants (primary producers) to primary consumers to
secondary and tertiary consumers. A food web is the connection of many food chains; it is the elaborate,
interconnected feeding relationships in an ecosystem. The energetic hypothesis suggests that the length of a
food chain is limited by the inefficiency of energy transfer along the chain. The dynamic stability hypothesis
proposes that long food chains are less stable than short chains.
structure. Dominant species are the most abundant species in a community and their dominance is achieved by
having high competitive ability. Keystone species are usually less abundant species that exert a
disproportionate influence on community structure because of their ecological niche. Ecosystem “engineers,”
also called foundation species, exert influence on community structure through their effects on the physical
environment.
to higher trophic level, in which nutrients and other abiotic factors are the main determinants of community
structure, including the abundance of primary producers. The top-down model proposes that control of each
trophic level comes from the trophic level above, with the result that predators control herbivores, which in
turn, control primary producers.
Concept 53.3-
Disturbance influences species diversity and
composition
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Disturbance- Increasingly, evidence suggests that disturbance and nonequilibrium rather
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Human Disturbance- Humans are the most widespread agents of disturbance, and their
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Ecological Succession- Ecological succession is the sequence of community and ecosystem
than stability and equilibrium are the norm for most communities. According to the
intermediate disturbance hypothesis moderate level of disturbance can foster higher species
diversity than can low or high levels of disturbance.
disturbance to communities usually reduces species diversity. Humans also prevent some
naturally occuring disturbances, such as fire, which can be important to community structure.
changes after a disturbance. Primary succession occurs where no soil exists when succession
begins; secondary succession begins in a are where soil remains after a disturbance.
Mechanisms producing community change during succession include facilitation and inhibition.
Concept 53.4-
Biogeographic factors that affect community
diversity
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Research done by Charles Darwin and Alfred Wallace proved that tropical habitats support many more species
than temperate and polar regions. An example of this is that there are 711 different types of tree species in
Malaysia while there are only 50 tree species in the area north of the Alps in Western Europe.
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Species richness in equatorial-polar gradients are affected by evolutionary history and time.
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As time passes, more speciation events will occur, causing species diversity to increase.
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Tropical habitats are older than both polar and temperate regions, proving that species richness is more
abundant in the tropics than at an equatorial-pole gradient.
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Biodiversity is affected by climate. Solar energy and water are two components of climate that effect
biodiversity.
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The species area curve was recognized by Alexander von Humboldt in 1807. This curve states that the larger
area, the greater the number of species.
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The amount of species living on an island is based on the rate of immigration and the rate of extinction. These
rates are affected by the size of the Island, the amount of species living on the island, and the distance from
the island to the mainland. Larger islands have lower extinction rates because they contain more resources and
more diverse habitats.
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The island equilibrium model was creates by MacArthur and Wilson. This model shows that when the rate of
immigration is equal to the rate of extinction, an equilibrium will be reached. The island’s size and distance
from it’s mainland effect species richness( at equilibrium). Many scientist have questioned the accuracy of this
model.
Concept 53.5-
Contrasting views of community structure are the
subject of continuing debate
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The integrated hypothesis was created by F.E. Clemens, of the Carnegie Institute of Washington, in the early
1900’s. This hypothesis states that a community acts as a superorganism, as a result of required biotic
interactions .
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Then, H.A. Gleason, of the University of Chicago, created the individualistic hypothesis. This hypothesis states
that species with the same abiotic needs live in the same area.
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These hypotheses are similar because they state that communities interact.
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These hypotheses differ because the integrated hypothesis states that species depend upon their
interactions with other species and he individualistic hypothesis states that each species lives in an
environment where the abiotic factors that it needs are present.
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American scientists Paul and Anne Ehrlich proposed the idea that all species in a community depend upon and
affect one another.
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The rivet model proposed the idea that species in a community interact with species in the web of life.
Increasing or decreasing the population of one species will greatly affect the population of another species.
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The Redundancy model was created in 1992 by Brian Walker, an Australian Ecologist. This model proposed the
idea that most species are not associated with one another and that decreasing or increasing the population of
one species will not greatly affect the population of another species.
What is DDT?
• DDT is a pesticide that was released
to the public after WWII.
• Unfortunately, during that time and
even today, many people are unaware
of the side effects of DDT in the
environment especially on factors such
as herbivory, predation, and
interspecific competition.
• Thanks to the work of many
researchers and publicists, such as
Rachel Carson, people today are
becoming aware of the effects of
DDT and now DDT is banned for use.
Herbivory
• Herbivory: an interaction in which an herbivore eats
parts of plants or alga
• DDT is sprayed all over plants for the purpose of
killing unwanted insects. Because DDT is sprayed on
plants, herbivores that eat those plants will take in
the DDT. If the herbivores take in the DDT in large
consumptions, then they will be unable to carry out
necessary life functions, like reproduction, and
eventually become extinct. Hence there wouldn’t be
anymore interaction between the involved plant
species and their herbivore predators.
• Global warming also affects herbivory. Global
warming is when the global temperature increases.
Many organisms, especially plants that cannot
disperse rapidly over long distances, would probably
not be able to survive the high rates of climate
change projected to result from global warming. If
plants, don’t survive, then neither will the
herbivores, which are dependent on them.
Predation
•
•
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Predation: An interaction between species in which one species, the
predator, eats the other, the prey.
Predation is also affected severely by DDT. After DDT was released
to the public, one of the first signs that DDT was a serious threat to
the environment was when the population of various bird species, such
as pelicans, ospreys, eagles, all birds that feed on herbivores that
feed on plants. The accumulation of DDT in the tissues of these birds
interfered with the deposition of calcium in their eggshells, a trend
that may have already occurred because of environmental conditions.
When these birds tried to incubate their shells, the weight of the
parents broke the shells of the affected eggs, resulting a decline of
reproduction rates. Basically, this shows how the interaction between
predators (birds) and their preys (herbivores) leads to the decline in
the population of the predator (and in some cases even the prey)
species.
Predation is affected by acid precipitation. In a study in Norway, it
was found that fish were dying in ponds and lakes because of acid rain
from pollutants. This would cause the whole lake and pond ecosystem
to suffer not only due to the acid precipitation, but the loss of one
species would indicate the loss of the predator species and eventually
the elimination of certain food chains.
Interspecific Competition
•
•
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Interspecific Competition: competition for resources between
plants, between animals, or between decomposers when resources
are in short supply.
DDT affects interspecific competition. Competition can limit
herbivore numbers because competition may maintain herbivore
numbers below what the vegetation could feed. But as mentioned
earlier herbivore numbers can be easily reduced through DDT’s
affect on herbivory. This will immediately affect competition. The
worse part is that competition includes more than one species, so
if an herbivore species, which was controlled by the competing
species is eliminated, then the competing species will have nothing
to compete for and one of two things will happen: (1)competition
will end and the competing will find different, new preys or (2)
the competing species will die of starvation. Basically, DDT has an
indirect relationship with competition, but it still affects the
outcome of interspecific competition.
Competition is affected by the overloaded global carrying capacity
because as we approach or exceed the carrying capacity the
amount of available resources will not suffice for the amount of
organisms. Hence competition will be brought to a new level and
many species may face competitive exclusion.