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 (+/+
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 other species.
• 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 a