Transcript Biology\Ch.5 Populations

Populations/Resources
Objectives: I can …
• Differentiate between exponential and logistic population growth.
• Predict under which circumstances a population graph will appear
J-shaped (exponential) and when it will be S-shaped (logistic growth).
• Explain how natural selection relates to population growth.
• Compare and contrast monoculture versus crop rotation practices.
• List the pros and cons of the industrial revolution.
• Compare and contrast renewable and nonrenewable resources.
• Evaluate the pros and cons that come with use of each natural
resource or their alternatives.
• Explain some of the causes and effects of pollution.
• Explain the impact of invasive species on an ecosystem.
• Give examples of renewable and nonrenewable resources.
Vocabulary:
Population * population density * immigration * emigration *
exponential growth * logistic growth * carrying capacity * limiting
factor * green revolution * monoculture * crop rotation *
renewable and nonrenewable resources * fossil fuels * acid rain *
natural selection
Populations are groups of individuals of the same species living in
the same area. The number of individuals of a population living in the
same area is called the population density. For example: A nice thick
lawn has a greater population density of grass than a patchy, thin lawn.
The size of a population is affected by the number of births, deaths,
immigration (movement into an area), and emigration (movement out of
an area). A search for food, mates, space, etc. generally drives
immigration and emigration.
If a species is introduced to a new area abundant in space, food, etc.
Reproduction will occur at a very rapid rate. Although the population
increase will be slow at first due to the low numbers of individuals
brought into the area, as their offspring and “grandchildren” begin
reproducing too, the population will begin increasing at a greater rate.
Graphing the growth in population will show a J-shaped graph. This is
exponential growth (like graphing x2 , when x > 0)
The ideal conditions found with exponential growth don’t usually
continue forever. As the population grows, there is less space, less
food, etc. so the reproduction rate slows down considerably and may
level off. If we were to graph the population growth now, it would
create an S-shaped graph. This is called logistic growth. When an
area reaches the maximum number of individuals it can sustain, it’s
called the carrying capacity. Birth rate plus immigration equals or
barely exceeds death rate plus emigration. This is the typical scenario
for most populations.
Anything that slows down or stops further population growth is called
a limiting factor. This may be food, water, space, parasites/illness,
predation, etc. As resources become more limited, competition
increases. Competition can lead to “survival of the fittest” (natural
selection) where those individuals with the best characteristics for
survival in that environment will survive and reproduce - keeping their
genes in the gene pool. But those with less beneficial characteristics
may die, removing their genes from the gene pool. This is what drives
evolution.
Human population patterns have been influenced by improved crop
production, medicine, technology, etc. These have lead to longer survival rates
(lower death rates) in industrialized countries. Birth rates have also stabilized
in more developed countries. However, third world countries still experience
famines, disease, etc. killing large numbers of people at a young age.
While newer intensive farming practices using better fertilizers,
pesticides, plant strains, etc. (known as the green revolution) has made
it possible to produce more food in less space and feed more people, it
has also lead to a turn toward monoculture. Monocultures are when a
single crop is planted in large fields year after year. This makes it easier for
farmers to minimize equipment costs but tends to deplete the soil of certain
nutrients and can create an increase in disease or pest infestations. For
example, the smut fungus that can infest corn can live in the soil over winter
and infect even more plants next year and corn borers from this year will lay
their eggs nearby and infect next year’s crop. That’s why crop rotation is a
better idea. For example: corn depletes nitrogen in the soil but soybeans put
nitrogen back. Corn isn’t attacked by bean beetles and beans aren’t affected
By corn borers. The populations of these pests are more likely to die off if
every other year they have no food source (no corn for borers, etc.)
Therefore crop rotation requires less fertilizer and pesticide use than
monoculture but may increase the need for additional machinery.
Although the green revolution had an impact on human population
growth, so did the industrial revolution. While the industrial revolution
improved our standard of living, it also increased pollution levels and
began to deplete our natural resources.
Natural resources that can be replaced over a reasonable time are
considered renewable resources. Ex: Trees used for lumber can be
replanted and water is replaced through the water cycle. However,
even these have problems. Trees, especially hardwoods like Oaks,
take a long time to re-grow and the nutrients in the soil may become
depleted after removal of a mature stand of trees so young trees won’t
grow well. Water is replaced by rain but once a water source is
polluted or infested by parasites, it may not be safe to use. And, the
rain might not fall when and where it is needed. Soil can erode away
but takes 100 years to create an inch of new soil.
Nonrenewable resources are those that cannot be replaced by nature.
For example: the metal ores of the Earth were formed when the Earth
was created. So, as we use up copper, aluminum, etc. we will run out.
Even coal, oil, and natural gas are considered nonrenewable because
they take millions of years to form even under the right conditions.
These need to be conserved, recycled (if possible), and replaced by
other materials so that we don’t run out. Plastics have replaced many
metals in products but plastic manufacturing requires a lot of oil. In
addition, when plastics aren’t recycled they add a lot of waste to
landfills. Plus, plastics used for food/water packaging leach chemicals
that mimic human estrogens but can be even more powerful. This
may lead to health problems and can aggravate conditions like breast
cancer, obesity, etc.
Solar power, wind energy, and nuclear power have reduced our need
for coal, oil, and natural gas (fossil fuels). But, these are more
expensive per kilowatt hour of electricity and have other problems like
nuclear wastes from nuclear power plants, amount of wind or sun
available in various regions, etc.
Whether a resource is renewable or non-renewable, pollution can be a
problem. Air pollution can damage our lungs and cause asthmatic
reactions, lung cancer, etc. Ozone alert days are common in summer
in cities. This makes breathing very difficult. Moving to the country
isn’t always the answer. Walworth county is listed as one of the top 10
percent WORST air polluters in the US due to emissions of
carcinogenic (cancer causing) substances from a company here in East
Troy.
Some pollutants like chlorofluorocarbons (CFC’s) from aerosol
propellants are destroying the ozone layer up in the atmosphere. Here
we need ozone to block the sun’s radiation. Tears in the ozone near
the poles are getting larger.
Carbon dioxide build-up is creating a greenhouse effect. Who knows
what will be the next pollutant or its affect on the environment?!
Pollutants in the air can combine with rain. Acid rain forms when
carbon dioxide and water join forming carbonic acid or when nitrogen or
sulfur join with water to form nitric acid or sulfuric acid. These gases
are part of industrial pollution from smokestacks. Other toxins are also
released, like mercury released by coal power plants.
Not only can acid rain destroy buildings, statues, etc but it can also
lower the pH of lakes to the point that no fish can survive and it can
acidify the soil so much that forests die.
Water pollution can make water unsafe to drink. Even water in
Wisconsin’s lakes is unsafe. PCB’s are dumped LEGALLY into lakes
from the paper industry. Mercury is high in our lakes too. Fish from our
lakes are unsafe to eat, especially by women who are not past child
bearing age. Many of the toxins now found in water cannot be filtered
out.
Some water sources are infested with parasites such as Bilharzia
(schistosomiasis). And, water as a whole, is limited. Many areas have
little water available. Saudi Arabia has to desalinize salt water from the
sea. India has such bad arsenic contamination that the water is slowly
killing the residents.
Phosphorus from fertilizers and detergents has caused huge algal
blooms. First this shades the pond/lake bottom killing the plants below.
Then, when the algae begins to die, the rotting process robs oxygen
from the water causing fish to die. Phosphorus in detergents sold in
Wisconsin is now regulated but fertilizers are still a problem.
And, soil contamination often leads to water contamination as
chemicals run-off into streams and lakes or soak into groundwater. In
addition, animal droppings can wash into water supplies increasing risk
of exposure to E. coli, cryptosporidium, etc.
Other things can also alter an ecosystem. The introduction of foreign
species can have unexpected consequences. Invasive species like
zebra mussels often have no natural predators in their new homes.
They can proliferate at extremely high rates and out compete native
species. Other invasive species are purple loosestrife (competes with
cattails, etc), Japanese beetles, Asian lady bug beetles, etc. Ironically,
many invasive species were introduced on purpose to combat some
other problem but these ended up hurting more than helping the
ecosystem.
Ironically, introducing new species into an area may DECREASE
biodiversity. If the foreign species out competes the native species, the
natives may become extinct - reducing the diversity of species in the
world.