Chapter 55: Community Ecology
55.1 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 primary
• 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.
55.2. 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 interactions).
o Amensalism: interactions in which one participant is harmed but the
other in unaffected (0/- interactions).
• These influence the population densities of species.
• May also restrict the range of environmental conditions under which species
• If there were no predators or pathogens, most species would be able to persist
under a broader array of abiotic conditions that they do in the presence of
• Presence of mutualists may increase the range of physical conditions under
which a species can persist.
Predation and parasitism are universal
• Predation and parasitism are universal processes.
• Predators are typically larger than and live outside the bodies of their prey.
• PREDATOR AND PREY POPULATIONS OFTEN OSCILLATEL growth of a
predator population nearly always lags behind growth in its prey population.
• PREDATORS MANY RESTRICT SPECIES’ RANGES: predators may also
restrict the habitat and geographic distribution of their prey.
• MIMICRY EVOLVES IN RESPONSE TO PREDATION:
o Predators do not capture prey individuals randomly. Prey individuals
vary in ways that make them more or less susceptible to being
o A palatable species may mimic an unpalatable or noxious one—
Works because a predator that captures an individual of an
unpalatable or noxious species learns to avoid other prey
individuals of similar appearance.
If a predator captures a palatable mimic; learns to associate
palatability with the appearance of that prey.
Directional selection causes unpalatable species to evolve away
from their mimics and can be maintained only if the mimic
evolves toward an unpalatable species faster than the
unpalatable species evolves away from it (happens if the mimic is
less common that the unpalatable species).
o Two or more unpalatable or noxious species may converge to resemble
one another –Mullerian mimicry.
All species of this mimicry benefit when inexperienced predators
eat individuals of any of the species because the predators learn
that all species of similar appearance are unpalatable.
• HOSTS RESIST INFECTION BY MICROPARASITES
o For a microparasite population to persist in a host population, at least
one new host individual must become infected with the microparasite
before each infect host dies.
o A microparasite can readily