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Community Ecology
Chapter 15
Community ecology
• A community is
• Interspecific interactions
– classified according to effect on population concerned
Interspecific interactions• Competition: -/-
• Mutualism: +/+
• Predation/herbivory: +/-
• Parasite/pathogen: +/-
Each organism has a niche
• Ecological niche: a multidimensional description of a species; its way of living
• The more similar 2 niches are, the more intense the competition for resources
When niches overlap• Competitive exclusion
• OR• Resource partitioning
• Both of these result in a reduction or removal of competition
• Competition seems to be indirect, not head-to-head; both species trying for same resource, one is just a bit better at getting it
When niches overlap
Key properties to a community
• Prevalent vegetation• Species diversity• Response to disturbance
Vegetation & species diversity
• The more diverse the vegetation, the more diverse the population of animal species
• The more diverse the community, the more resistant it is to the negative effect of pathogens
Diversity defined by 2 components
• Species richness
• Relative abundance
Species richness & relative abundance together
determine community diversity
Keystone species impact diversity• A keystone species greatly
influences which other species are present
• If a keystone species is removed from community, the composition changes drastically
Community response to disturbance
• What is a disturbance?
• Not always negative; small scale disturbances can contribute to species diversity
• Succession is the change in species composition over time following a disturbance
Primary succession• Starts in lifeless area without even soil
– 1st life forms are autotrophic bacteria– photosynthetic mosses & lichen– grasses & shrubs– small trees– larger, longer-living species that eventually
become prevalent vegetation• Primary succession can take 100s or 1000s
of years
Primary succession
Primary succession
Secondary succession
• Occurs when disturbance destroyed existing community but left soil intact
• 1st grasses & shrubs followed by larger vegetation. . . – because animals depend on plants, animal
community goes through succession too; ex. diversity of birds increases as trees replace shrubs
• Secondary succession can happen in decades or even a few years
Secondary succession
More about succession• Succession can lead to a climax community
– the specific species in a climax community is dependent on the temperature & rainfall
• The most diverse communities are at some intermittent step of succession
Summary• Community ecology is the study of
interspecies interactions• Interactions can be classified as beneficial or
harmful• Interactions shape the community structure• The presence or absence of some species
play a larger role in the community structure than others (keystone species)
• How communities respond to disturbance help define the community
Predation• An interaction where one species eats
another, including herbivory– the predator benefits; the prey or plant don’t
• One of the most important forces shaping the composition & abundance of species in a community
• Defenses against predation– physical– behavioral
Physical prey defenses• Mechanical defenses
– spines, quills, armor, claws, etc.
• Chemical toxins– make prey poisonous or
unpalatable– used by many plants;
can be harmful to one species and harmless to another
– some animals also use this defense
Physical prey defenses, cont.
• Warning coloration– danger! poisonous!
& mimics
• Camouflage– patterns of
coloration that allow an organism to blend into its environment
Behavioral prey defenses• Hiding or escaping
– requires vigilance to avoid predators– safety in numbers
• Alarm calling– many species, especially birds &
mammals, warn others of danger• Fighting back
– some species mob the predator– spitting, vomiting, eliminating waste,
dropping tails, eviscerating, etc.
Parasitism• Parasitism: a symbiotic relationship where one
species benefits (the parasite) & one is harmed (the host)
• Two types of parasites– ectoparasites live outside the host’s body
• lice, leeches, ticks, fleas– endoparasites live inside the host’s body
• tapeworms, roundworms, Plasmodium
• Different from other forms of predation – the parasite is much smaller then the host – parasites stay in contact with the host for long periods
of time, weakening the host as it uses the host’s resources
Positive interspecies interactions
• Mutualism: everyone benefits– both species gain & neither is
harmed– common in virtually every
community• honeybees & flowers; cleaner fish &
the “dirty” fish
• Commensalism: one species benefits & the other neither benefits or is harmed– cattle egrets & livestock