Download 3.4 Ecosystem Changes

3.4 Populations and
Ecosystem Changes
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Wild species are important for
their economic, medical,
scientific, ecological, aesthetic,
and recreational value.
Wild tropical plants once provided
90% of today’s food crop seeds;
80% of medicines came from
plants that were wild.
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The average specie lasts about 4 million years.
Background (normal) extinction rate
is about 3 species per year.
Currently, it is 1000 times that
amount.
There are a number of selection pressures acting on populations
including:
Predation
Sexual Selection
Competition
Kin Selection
Symbiosis
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There are 3 levels of declining population:
1. local decline - no longer found in one
native area but are found elsewhere in the
world.
2. ecological decline specie is present but can’t
play its role in the ecosystem
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3. biological decline -
a. threatened - declining in numbers
b. endangered - too few individuals,
extinction soon
Species need 10,000 organisms to maintain
evolutionary potential.
c. extinct - means gone forever - when
numbers drop below 1,000 for animal
species and 120 species for plants, the
species is considered extinct because of
the problems finding mates.
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Populations are doomed when the death rate is greater than the
birth rate. This is referred to as fatal subtraction.
These organisms have fallen below their minimal viable
population size. Specialization may make species vulnerable to
changing environments.
For example, if a food source is destroyed, populations may
not be able to eat different food sources.
They will become extinct unless
change occurs somewhere.
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Causes of declining species are from:
- habitat loss or fragmentation
(primary factor)
- human population growth
- pest and predator control
- harvesting wild plants
- hunting, fishing, poaching
- for sale as exotic pets
- pollution and related diseases
- exotics have altered habitat giving other species new advantages
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Genetic Drift is a concept of changing populations by chance
and is directly opposed to the notion of Natural Selection,
where specific pressures such as predator-prey relationships,
or coloration affect survival.
A natural catastrophe
such as forest fire,
earthquake, flood or
volcanic eruption might
kill off certain members of
a population by chance
alone.
NOAA
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Several terms are associated with genetic drift.
1. Bottleneck Effect is when only
certain members of a population
can live to reproduce.
This could be the result of a
catastrophe that results in a loss
of many of the original members
of the population.
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2. Founder Effect occurs when a small group of organisms breaks
off from a larger population and establishes a new population.
USGS
Random chance plays a large role in
determining which alleles are
represented in the new population.
A good example is the different kinds of tortoises that developed on
each of the Galapagos Islands who developed as they were isolated.
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3. Mass extinctions result in large scale die-offs of species.
Scientists believe these have happened many times over the
geologic history of the earth.
Llbl.gov
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One of the most important mass extinctions is thought to have
occurred 65 million years ago.
Scientists have found evidence of a giant asteroid that may have
hit in the present Yucatan Peninsula of Mexico and blanketed the
earth with a layer of iridium from space. Debris could have been
carried around the world through the atmosphere and may account
for a mass extinction of the dinosaurs of the Mesozoic Era.
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4. Climate shifts may also contribute to the changes in
populations. When temperatures fluctuate dramatically, the
normal populations may die off due to loss of food and habitat.
NASA
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