Download Maintaining Balance

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Principles of Ecology
Maintaining Ecosystem Balance—Notes, KEY
(**This note guide does not use Managing Our Natural Resources)
What maintains ecosystem balance?
* As you read in Ch.3 of Managing Our Natural Resources (Unit 1), balance of ecosystems actually
refers to slow, gradual change.
*Predators, disease, and parasites help to keep ecosystem balance. If one species begins to dominate an
ecosystem, it becomes more susceptible to disease, parasites, or predation.
Organism specialization:
1. Habitat: The place in which an organism lives—it provides food, water, cover, and space.
2. Niche: The role of an organism in the ecosystem—its “occupation”.
3. Competition: Organisms live where they are most able to compete for resources. Competition brings
about an ecological separation of closely related species—competitive exclusion principle—no two
species can occupy the same niche at the same time.
Examples: Darwin’s finches—multiple species on the Galapagos Islands, but they coexist by filling slightly different
niches (utilize different food resources).
What keeps population balance in an ecosystem?
1. Biotic potential: Factors that contribute to a species ability to increase.
 Ability to migrate
 Population growth
o Reproductive rate: number of births, eggs, seeds, or spores
o Recruitment: number of individuals that reach breeding age
 Ability to invade other habitats
 Defense mechanisms against predation
 Ability to cope with adverse conditions (adaptability and resilience)
2. Environmental resistance: Factors that limit a species from growing out of control.
 Predation
 Lack of habitat
 Disease
 Fire
 Adverse weather
 Competition
 Lack of food/nutrients
 Natural toxins
Host-Parasite Relationships : Generally, parasites don’t kill their host but weaken it so that the host is more vulnerable
to adverse weather conditions and predators.
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Unit 2: Principles of Ecology
Predator-Prey Relationships: As prey numbers increase over time, predators also increase with a slight lag behind the
prey numbers. This increase in predator numbers in turn results in a decrease of prey which leads to a decline in predator.
Summary of Population Balance
1. Population balance is a balance between biotic potential and environmental resistance.
2. Population balance is dynamic: there are year to year changes but population is stable over the long-term average.
3. Population balance is maintained because factors of environmental resistance are density dependent: as population
density increases, environmental resistance becomes more intense (i.e., predators slow the population growth of their
prey.
4. Species diversity provides ecosystem stability: Because of food webs, consumers have alternate prey species (a fox can
eat a mouse or a bird). The more complex the food web, the more stable the ecosystem.
5. Population balances take time to evolve: disaster can happen when man interferes with this balance.
Example: Rabbits were introduced to Australia in the mid-1800s and rabbits numbers increased exponentially so
that by the early 1900s the rabbits were consuming the already sparse vegetation and thus competing with the
cattle, sheep, and native wallabies for resources.
Questions to Consider:
1. Why is one biotic community eventually replaced by another during succession? Successional forces result in this replacement: A
new plant species immigrates into an area via seed dispersal. This new species competes with existing plant species for resources
(water, light, nutrients). The better competitor of the two species will dominate the site. Plants can modify their habitat as they add
organic matter to the soil. This alters the soil properties which may then allow a new species to be a better competitor.
2. How do human populations change environmental resistance? By and large, the global human population has overcome
environmental resistance to continue to increase exponentially. Modern medicine, advances in farming techniques, and technology
have contributed to the human ability to decrease environmental resistance. This may one day change if human population reaches a
certain threshold.
3. How would introducing a new competitor into an ecosystem affect the stability of that ecosystem? Provide an example. A new
competitor could potential destabilize that ecosystem by competing with an existing species. For example, cheatgrass, an invasive
grass from Eurasia, competes with native plant species in Idaho for resources. Cheatgrass is of little food value to elk and deer and of
little habitat value to birds like sage grouse. If cheatgrass outcompetes native plant species this could lead to decreased populations of
herbivores which in turn could decrease predator populations.
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