Chapter 55: Community Ecology
What Are Ecological Communities
• The species that live and interact in an area constitute an ecological
Communities are loose assemblages of species
• Ecological communities are not assemblages of organism that move together
as units when environmental conditions change. Rather, each species has
unique interactions with its biotic and abiotic environment.
The organisms in a community use diverse sources of energy
• A trophic level consists of the organisms whose energy source has passed
through the same number of steps to reach them.
• Plants and other photosynthetic organisms constitute a trophic level called
photosynthesizers or primary producers. The produce energy rich organic
molecules that nearly all other organisms consume.
• Heterotrophs consume the energy-rich organic molecules produced by
• Organisms that eat plants constitute a trophic level called primary
• Organisms that eat herbivores are called secondary consumers.
• Organisms that eat the dead bodies of organism or their waste products are
called detritivores or decomposers.
• Organisms that obtain their food from more than one trophic level are called
omnivores. Many species are omnivores; trophic levels are often not clearly
• Biomass: the weight of living matter.
• Distributions of energy and biomass for a particular ecosystem usually have
similar shapes. Variations in their dimensions depend on the nature of the
dominant organism at each trophic level and how they allocate their energy.
• In most terrestrial ecosystems, photosynthetic plants dominate.
• Relative to the biomass of plants, the biomass of herbivores is larger in
grasslands than in forests.
• In most aquatic ecosystems, the dominant photosynthesizers are bacteria
and protists which have such high rates of cell division that a small biomass
of photosynthesizers can feed a much larger biomass of herbivores, which
grow and reproduce more slowly (inverted distribution of biomass).
• Detritivores transform detritus into free mineral nutrients that can again be
taken up by plants. If there were no detritivores, nutrients would eventually
be tied up in dead bodies, where they would be unavailable to plants.
What Processes Influence Community Structure
• Interactions of organism with one another:
o Predation or parasitism: interactions in which one participant is
harmed, but the other benefits (+/- interactions).
o Competition: interactions in which two organisms use the same
resources and those resources are insufficient to supply their
combined needs (-/- interactions).
o Mutualism: interactions in which both participants benefit (+/+
interactions). o Commensalism: interactions in which one participant benefits but the
other is unaffected (+/0 interactio