Transcript Populations: Extinctions and Explosions

Populations: Extinctions and
Explosions
Chapter 28
What happens when people build
on natural habitats?
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Habitats shrink in size
Small habitats support fewer individuals
Fewer individuals means more inbreeding
Inbreeding decreases genetic variation
and increases genetic defects
• Population spirals downward toward
extinction
The Extinction Vortex
Factors that cause extinctions
• Deterministic events – predictable
events such as habitat destruction and
hunting
• Stochastic events – unpredictable and
infrequent events such as unusual storms
Deterministic Factors that lead to
extinction
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Destroying ALL the local populations
Over hunting
Habitat destruction
Habitat fragmentation
Edge effects
Stochastic factors
• A freak storm could kill most of the healthy
females in a small population.
• The next year most or all of the offspring
could be males.
• Alleles could be lost through genetic drift.
• These effects would not be likely in a
larger population
How can we preserve species?
• Some studies have predicted that about
half of all the world’s species will go
extinct in the next decade or two unless
something is done to prevent it.
• We must set aside a large percentage of
our land.
• The preserves need to be large enough to
minimize edge effects
• Corridors that allow individuals to travel
from one population to another
How do populations grow?
• Which would you rather have
– $10 a day for two weeks
– Or starting with one penny, twice the amount
you got the day before for two weeks?
• This is an example of exponential growth –
the bigger the population, the faster it
grows
• The growth rate of a population is the difference
between the average birth rate and the average
death rate = r
• Rmax – the intrinsic rate of natural increase
• Exponential growth is always temporary
because the environment cannot support that
many individuals.
• The number of individuals that the environment
can support indefinitely is called the carrying
capacity or K
• When K is exceeded, more individuals die off.
• When the birth rate is equal to the death
rate there is zero population growth.
Immigration and emigration can also
affect population growth.
• K is determined by some limiting factor.
Density-dependent factors
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Competition
Predation
Parasitism
Disease
All limit growth in proportion to the density
of the population
Density- independent factors
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Hurricanes
Tornadoes
Drought
Fires
Volcanic eruptions
The goal of every species is to have as
many descendents as possible survive
to adulthood.
1. r-selected species have as many
offspring as possible
1. Often pioneer species
2. K-selected species reproduce in
smaller numbers, and spend more time
and energy raising their young.
Age structure of a population
• Cohort – individuals that enter the
population at the same time
• Depends on surviorship and birth rate
• Growing populations have a graph like a
pyramid
• Stable populations have a rectangular
shape
Do we need to worry about the
growth of the human population?
• For many years people have been
concerned about increasing population
• Growth has slowed because people and
countries have done something about it.
Questions remain:
• Will epidemics like AIDS or wars reduce
the world’s population growth?
• What is the carrying capacity of the world
for humans?
• For humans and other creatures?
How has the human population
sidestepped limiting factors?
• People spread into more habitats because
they could build shelters, use fire, and
communicate knowledge.
• When we were hunter-gatherers the
population was probably stable.
• Farming increased the availability of food,
allowing the human population to grow
• Closed sewers, antibiotics, chlorinated water,
refrigeration and vaccines caused the death
rate to drop dramatically in the 19th and 20th
centuries.
• We subsidize our cities and agriculture –
bringing in additional energy from other
places.
• Ecological footprint – the area of land
needed to produce all of the resources a
single person uses.
• Average American – 5.1 hectares
• (hectare = 100 acres)
• Person from India – 0.4 hectares
• U.S. = 290 million people X 5.1 hectares =
1.48 billion hectares
Area of U.S. = 0.91 billion hectares
India = 1.05 billion people X 0.4 =
0.42 billion hectares
How can we feed the world?
• We supply developing countries with high yield
crops
– These crops need chemical fertilizers and irrigation
– Decreased genetic diversity
– Monoculture – growing only one crop on a farm or
in an area
• Easier to pick
• Natural disasters can wipe out entire economies
• More inviting to pests and parasites
• Crop yields have leveled off, but population is
still growing.
Grain production has leveled off, but the
human population continues to grow.
We are using up limited resources
and polluting renewable resources.
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Power plants produce thermal pollution
Increasing erosion by logging and agriculture
We dump raw sewage and wastes into water
Poisons, heavy metals, radioactive substances,
pesticides and chemicals are hard to get rid of.
• Biomagnification – toxins increase in animals
higher up the food chain
• Burning hydrocarbons increases carbon
dioxide in the air – green house gas
• Sulfur and nitrogen in fuels cause acid rain
and smog
• Chlorofluorocarbons – spray cans, air
conditioners, refrigerators and cleaners –
destroy the ozone layer
– Increase cancer
– Extinction of species
– Birth defects
What can we do?
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Reduce
Reuse
Recycle
Use environmentally friendly products
Vote!!