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Ecosystems and Living Organisms
Chapter 4
Communities
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Different populations of organisms that live
and interact together in the same place at
the same time
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Vary in size
No true boundaries
Example Rotting Log
Organisms in Communities and
Populations compete for resources
Ecosystem
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The sum of a community and the abiotic
environment.
i.e many populations + soil, water, weather,
terrain, etc.
Succession: How Communities
Change
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The stages of community development
Colonization and replacement
Change in species diversity and abundance
Leads to climax vegetation and stability??
Primary Succession
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Pioneer Community
New soil/uninhabited environment
Lichens = break down rocks into soil
Then mosses, ferns, grasses, shrubs, trees
Secondary Succession
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After major disturbance where plants once
were
Light seeded plants; shade intolerant;
small, fast growing, highly reproductive
plants and animals
Animals undergo succession with plant
communities
Keystone Species
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A species in an ecosystem that affects
many other species so that it cannot be
readily lost or replaced.
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Usually affect some type of resource
Do not have to be high in numbers
Example: gopher tortoise/wolves/panthers
Interactions among organisms
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Symbiosis
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Result of coevolution
Types of symbiotic relationships
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Mutualism
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Commensalism
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Lichen (algae and fungus)
Gray whale and barnacle
Parasitism
Mosquito and human
 Pathogen = causes death and/or disease
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Interactions among organisms
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Predation
Predator (larger, more complex)
 Prey (might be plants)
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Result of Coevolution
Selective forces
 Results of random mutations
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Methods of predation
Pursuit
 Ambush
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Methods of protection for prey
Plant defenses
 Spines, thorns, etc.
 Excretions
 Toxins
 Allelopathy (black walnut & cherrybark oak)
 Animal defenses
 Fight or flight
 Warning coloration
 Camoflague
 Poison glands
 Shells, herds, etc.
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Competition
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Use of common essential & limited resource
Intraspecific competition –within a
population
Interspecific competition – between
populations
Sometimes subtle and complex
Organisms that compete may also benefit
each other
The Ecological Niche
1.
2.
3.
Producer, Consumer or Decomposer?
Predator and/or prey?
Symbiotic relationships?
Ecological niche – Role in the ecosystem and
in habitat
Niches
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Fundamental niche –
Potential idealized
niche
Realized niche – Niche
that an organism
actually uses
This difference is due
to competition
Limiting Factors
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Any resource that restricts the ecological
niche of an organism
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Usually simple
Minerals
 Salt concentration in water
 Amount of precipitation
 Temperature
 Graph p. 77
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Competitive Exclusion
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One species is excluded from part of a
niche because of competition with another
species (interspecific competition).
No two organisms can occupy the same
niche
Coexistence is possible if niches are
reduced
Gause study
Resource partitioning
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Reduction in competition for resources that
occurs as a result of each species’ niche
differing from the others in one or more
ways
Species Diversity
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The number of species present in a community
High diversity: tropical rain forests
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Low diversity: isolated islands, mountaintops
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1. Abundance of potential
ecological niches
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The greater the complexity of the
community, the more potential niches
available
More niches available, the more organisms
The more organisms, the more diversity
2. Location at the margins of
adjacent communities
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Species diversity is greater at the margins
of adjacent communities
Ecotone – A transitional zone where two
or more communities meet
Edge effect - The change in species
composition at ecotones
3. Geographical isolation
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The more isolated the community, the less
diverse it will be
Cannot be readily replaced
Contain fewer potential niches
Island biogeography
4. Dominance of one species over
another
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When one species uses more than its share
of resources
It out competes other species
Kangaroo rats
5. Environmental Stress
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The more environmental stress, the less the
species diversity
Polluted stream vs. clear stream
Urban vs. rural areas
Temperature
6. Geological history
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The older the community, the higher the
species diversity
Glaciers
Volcanoes
Islands
Continental Drift
Ecosystem Services
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Provided by the ecosystem
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Clean air, water
Fertile soil
The more diverse an area, the better the
ecosystem services
Community Stability
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The ability of a community to withstand
environmental stresses and disturbances
The greater the species diversity, the more
stable the community and the less
important the species
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American Chestnut