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COMMUNITY ECOLOGY
I. Community = two or more populations of different
species occupying the same geographical area
A. Community Ecology = the study of how different species
interact within communities
B. Habitat = the physical place where an organism
lives, e.g. a pine forest or fresh water lake
1. Some organisms, particularly migratory birds require more than one
habitat
C. Niche = the functional role of an organism in a
community, its job or position
1. For example, grasses on the savanna of Africa are primary producers,
wildebeests are herbivores, lions are carnivores
2. Each species has a potential niche - what they could
do with no competitors or resource limitations
3. However, due to competition and/or resource limitations, most
organisms occupy a realized niche, the part of the
fundamental niche that a species actually occupies in nature
II. Species interactions - since communities consist of many
species they all have various interactions going on at the same time
A. Neutral - two species that don't interact at all
B. Commensalism - beneficial to one species but neutral to
another, e.g. birds that nest in trees, epiphytes (plants that grow on other
plants) such as tropical orchids
C. Mutualism - an interaction that is beneficial to both
species, e.g. plants and their pollinators, plants and animals that disperse
their seeds, certain fungi and plant roots
1. Some Yucca species have an obligate mutual relationship with a moth.
The plant relies upon the moth to transfer the pollen and the developing
moth larvae feed off some of the seeds that result from the pollination.
D. Parasitism - an interaction that benefits one species
and is detrimental to another
1. Note that the host is generally not killed
E. Predation - an interaction beneficial to one species
and detrimental to another. In this case the prey is killed
1. Predators are those that kill and eat other animals. Although many
organisms eat plants they usually don’t kill them because they are a
constant supply of food. Prey are those that are killed and eaten
III. Competitive interactions
A. Competition has negative effect on both organisms competing for a
resource
B. Because resources are limited in nature there will always be competition
for them
C. Competition is the driving force of evolution, those that win leave more
offspring
D. Types of competition:
1. Intraspecific - competition among individuals of the
same species, e.g. humans compete against other humans
2. Interspecific - competition between different
species, e.g. humans compete against a wide variety of species seeking to
utilize our food resources
E. The theory of competitive exclusion maintains that
species who utilize the same resources cannot coexist indefinitely
1. Stated another way this could be called the "one niche, one
species" concept
F. Competition often leads to resource partitioning - the
resources are divided, permitting species with similar requirements to use the
same resources in different areas, ways and/or times
1. In New Guinea nine species of pigeons eat the same fruits but
each specializes in only one size fruit
2. Different warblers New England have divided up spruce trees into
specific feeding zones
IV. Community stability
A. Communities are assemblages of many different species occupying the same
geographical area
B. Communities are not static, they gradually change over time because the
environment changes and species themselves tend to also change their habitats
C. Ecological succession = a directional, cumulative
change in the species that occupy a given area, through time
1. Primary succession - starts from barren ground, e.g.
new islands or de-glaciated areas
2. Secondary succession - starts from disturbed areas,
e.g. abandoned farm land or storm ravaged land
D. Succession starts with a pioneer community, the first
organisms to occupy an area
1. Over a period of time the pioneers gradually modify the area, often
making it unsuitable for themselves and providing optimal conditions for
others to move in
a. For example, open ground might be colonized by sun tolerant plants.
However, as they grow they produce more shade, which makes the habitat
less suitable for them and more suitable for shade tolerant species
E. As members of the pioneer community gradually change the environment
several transitional communities may come and go
1. For example, the soil pH may undergo considerable change over a period
of time, alternately excluding and including different species
F. Ultimately a climax community, a stable,
self-perpetuating array of species in equilibrium with one another and their
habitat, will form
G. Glacier Bay, Alaska is one of the best
documented examples of primary succession
1. Retreating glaciers leave barren land, on which succession has been
documented for the past 200 years
2. The bare rocky soil starts with a pH of 8.0 and no ground cover
3. Mosses and mat forming plants with nitrogen fixing bacteria quickly
colonize the area
4. Willow and alder shrubs, also containing nitrogen fixing bacteria
eventually form dense thickets
5. Over a period of time plant litter enriches the soil with nitrogen,
allowing Sitka spruce trees to come in
6. Within 200 years the soil pH changes to 4.8 and the climax community,
a hemlock-spruce forest forms
V. Biodiversity
A. Biodiversity, the number of different species within an area, is
greatest in tropical areas near the equator and it decreases towards the poles
B. Tropical areas have more sunlight and of greater intensity, more
rainfall and longer growing seasons for plants
C. This environment is quite stable and contains many vertical
"layers" which provide more microhabitats
D. Therefore these areas can support more species, e.g. the number of bird
species is directly correlated with latitude
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