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Unit 2: Principles of Ecology
Ecology and Ecosystem Structure—Notes—KEY
(**Unit 2 does not use Managing Our Natural Resources)
Ecology Defined:
* Ecology: Interactions among organisms and their environment
* Ecology draws upon many other fields including mathematics, chemistry, physics, geology, biology, computer
programming and many more.
Aspects of Ecological Studies:
1. Ecological studies take place within the biosphere, which is the portion of the Earth that supports life from the top of
the atmosphere to the bottom of the oceans.
2. Ecology includes studying how abiotic factors (or nonliving factors) affect an organism’s life.
Examples: water, sunlight, chemicals, air
3. Ecology includes studying how biotic factors (or living factors) affect an organism’s life.
Examples: deer populations impacted by a bacterial infection
Levels of Organization in Ecology:
1. Organism is any one living thing that exhibits the characteristics of life.
Eg. 1 deer
2. Population is a group of organisms of the same species that interbreed and live in the same place at the same time.
Eg. Deer herd
3. Community is many different populations of living things interacting in a given space. Eg. Deer, trees, grass, humans,
etc.
4. Ecosystems include the interactions among populations in a community and the interactions of this community with its
abiotic or physical features.
Three Kinds of Ecosystems:
A. Terrestrial ecosystems: Ecosystems located on land masses like forests, deserts, and grasslands.
B. Freshwater ecosystems: Ecosystems located in freshwater, usually land-locked areas like rivers, lakes, and
streams.
C. Saltwater/marine ecosystems: Ecosystems located in saltwater like coral reefs or deep oceans.
5. Biomes are geographical regions containing several ecosystems that have the same climate and dominant communities.
Eg. Desert, tundra, taiga, forest, grassland
6. Biosphere includes interactions between all ecosystems on Earth.
Organisms in Ecosystems:
1. All organisms have a habitat or a place in which the organism lives and gains its resources.
2. All organisms also have a niche or a role in their environment. This role includes how the organism meets its need for
food, shelter and resources. It also explains how an organism reproduces and survives. The niche of a species includes all
its interactions with biotic and abiotic parts of its habitat.
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Eg. Darwin’s finches on the Galapagos Islands all live in the same environment but each species has a different
food source (seeds, insects, fruit, cacti) and so occupies a slightly different niche.
3. A limiting factor is any biotic or abiotic factor that restricts the existence, numbers, reproduction, or distribution of
organisms.
Eg. Temperature, wind, precipitation, elevation, nutrients, sunlight
*Factors that limit one population may have an indirect effect on another population.
Eg. No grass = no mice  no mice = no hawks
4. Tolerance is the ability of an organism to withstand fluctuations in biotic and abiotic environmental factors.
*Some species can tolerate conditions that another species cannot.
Eg. Cacti vs. ferns
5. Range of tolerance is the entire range of conditions which can support growth of a given organism. Optimal range is the
range of conditions which support maximum growth of an organism.
Relationships between Living Things:
1. Predator-Prey (+/-  one organism benefits while the other is harmed) is a relationship where a predator hunts and
kills prey for food or resources.
Eg. Lions kill gazelles for food
2. Competition (-/-  both organisms are harmed even though one might outcompete another) is a relationship when
two organisms compete for the same resource like food, habitat, water, sun etc.
Eg. 2 squirrels compete for acorns
3. Symbiotic relationships are close and permanent relationships between living things. There are many types of
symbiosis.
A. Commensalism (+/0): A symbiotic relationship in which one species benefits and the other species is neither benefited nor
harmed. Eg. Bird nests in tree
B. Mutualism (+/+): A symbiotic relationship in which both species benefit. Eg. Bees pollinate flowers & get food
C. Parasitism (+/-): A symbiotic relationship where one organism benefits (parasite) and the other organism is harmed (host).
Eg. Tick on a dog
Questions to Consider:
1. Think about a forest and list 3 abiotic and 3 biotic factors that interact inside this community.
Abiotic- air, water, weather, sunlight, soil/rock, temperature
Biotic-trees/plants, birds, grass, insects, animals, fungi, bacteria
2. Define the words niche and habitat. Give one example for each of these concepts that demonstrates the difference between them.
Niche—owl—predator, nocturnal, specific prey species
Habitat—trees/holes/cacti
3. Place the type of relationship being exhibited by the following organisms in the blank before the letter.
Commensalism (+/0)
A. Clownfish are protected by sea anemones but the sea anemones are not affected by the clownfish.
Mutualism (+/+)
B. A lichen is an algae and fungus growing together and both benefiting from the relationship.
Competition (-/-)
C. Two birds fight over nesting grounds.
Predator-Prey (+/-)
D. A hawk kills a mouse by the roadside.
Parasitism (+/-)
E. Lice living on a human head.
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