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BIOA02H3 Lecture Notes - Intraspecific Competition, Secondary Succession, Interspecific Competition

Biological Sciences
Course Code
Mary Olaveson

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Biology Chapter 55
Community: The species that live an interact in an area
Ecological communities are not an aggregate unit of organisms that move together when environmental
conditions change. Rather, each separate unit independently interacts with its biotic and abiotic
Trophic Level: Consists of the organisms whose energy source has passed through the same
number of steps to reach them
The organisms in a community can be divided into different trophic levels based on their source of
1. Primary Producers: Consists of plants and other photosynthetic organisms which gain their
energy from the sun (consists the tropic level: photosynthesizers)
2. Primary Consumers: Non-photosynthetic organism that consume the energy rich organic
molecules of the primary producers (constitute the trophic level:
3. Secondary Consumers: Organism that eat primary consumers
4. Tertiary Consumers: Organism that eat secondary consumers
5. Decomposers: Organisms that eat the dead bodies of organisms or their waste
products a.k.a. detritivors
6. Omnivores: Organisms that receive their food source from more than one trophic
Because so many organisms are omnivores, the boundaries between trophic levels are fuzzy
Food Chain: The process by which a primary producer is eaten by a primary consumer, which
in turn is eaten by a secondary consumer, and so on
Food Web: An interconnection of food chains because communities eat/are eaten by more
than one species
Biomass: Weight of living matter
There is a loss of energy and biomass as you rise in trophic levels

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In most cases, the distribution of energy and biomass within trophic levels in all ecosystems are similar,
where one trophic level usually dominates the others in terms of energy and biomass storage
In most terrestrial ecosystems, it is the photosynthetic plants which dominate because of their
abundance as well as their ability to hold energy for long periods of time
The next stage up from the primary producers is herbivores. The amount of energy/biomass herbivores
have vary and depend on the ecosystem
i.e. in forests, where wood dominates as the primary producer, the herbivores share of energy/biomass
is very small because wood contain very difficult-to-digest energy, therefore not many herbivores eat it
whereas in grasslands, where there is little hard-to-digest food sources, much of the grasslands will be
eaten by herbivores, therefore they will have a larger share of energy/biomass
In aquatic conditions, the energy distribution is the same as the terrestrial ecosystems however the
actual biomass distribution is inverted. Herbivores, instead of photosynthesizers, contain the majority of
the biomass
This is because in aquatic ecosystems, the primary photosynthesizers are protists and bacteria (which
have very small biomass). However, these organisms have such high rates of cell division that they can
feed a much larger biomass of herbivores, even though their own biomass is so small
Much of the energy/biomass ingested by organisms is eventually transferred to decomposers when they
die and are eaten by detritivores
They convert this energy/biomass into free-mineral nutrients that can be taken up by plants again;
completing the cycle
The way organisms interact with one another can also be divided into categories:
1) Predation/Parasitism: Interactions in which one participant is harmed, but the other
benefits (+/- interactions)
2) Competition: Interactions in which two organisms use the same resources
and those resources are insufficient to supply their combined
needs (-/- interactions)
3) Mutualism: Interactions in which both participants benefited (+/+
4) Commensalism: Interaction in which one participant benefits but the other is
unaffected (+/0 interactions)
5) Amensalism: Interactions where one participant is harmed but the other is
unaffected (-/0 interactions)
These 5 types of interactions plus the physical environment determines population densities
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