Download Introduction to Ecology Organisms don`t live in a vacuum!

4/15/11
Organisms don't live in a vacuum!
Introduction to Ecology
by Dr. W.
Without any controls, given unlimited resources, a
population would grow exponentially—as shown by
this exponential curve, or J-curve.
• Ecology is the study of how organisms interact
with each other and with their surroundings.
• Ecology can be studied at various levels of
organization:
– Population — the set of all members of a single species
in one area.
– Community — a set of interacting populations of
different species in one area.
– Ecosystem — the sum of communities and their abiotic
(non-living) surroundings in one area.
– Biome — a set of similar ecosystems.
– Biosphere—all life on Earth.
Suppose we had a
single E. coli bacterium
which was kept under
perfectly ideal
conditions. Under such
conditions a bacterium
can divide about once
every twenty minutes.
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The life cycle of common fruit flies (Drosophila melanogaster)
takes about seven days under ideal conditions. Starting with one
pair of flies, in one year you would have a ball of flies as large as
the Earth. Clearly this can’t happen in the real world—something
must act to keep populations down.
Carrying Capacity
• Many factors, both biotic (living) and abiotic
(nonliving), determine an ecosystem's carrying
capacity —the maximum number of individuals in
a population that it can stably support.
In a simple model, populations often grow to meet
the carrying capacity and then level off. This is called
logistic growth, and this type of growth curve is
called a sigmoid curve.
– Biotic factors might include presence of predators or
parasites, presence of prey or other food sources,
etc. . . .
– Abiotic factors might include amount of sunlight,
temperature, moisture levels, nutrient availability,
oxygen levels, etc. . . .
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In other cases, a population might increase, overshoot the
carrying capacity, and then fall back—or possibly oscillate
around the carrying capacity. (Note that the carrying
capacity is abbreviated K in the mathematical equations
that describe population behavior.)
In many other cases, populations may influence each other. In
northern Canada, the Hudson Bay Company buys animal furs
from trappers, and it keeps records of the furs that are bought —
its records go back over 150 years. . .
This could be caused by some factor in the environment—
but notice how the lynx population usually peaks just
slightly after the hare population peaks.
The
populations of
snowshoe
hares, and of
lynxes, follow
regular cycles
of boom and
bust.
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The reason seems to be that the hares are the lynxes'
primary food supply. As hares increase in numbers,
there's more food for the lynxes, and the lynxes raise
more kittens and become more abundant. . .
But when lynxes are abundant, they eat many hares, and
the hare population falls. Then there's less food for the
lynxes, and the lynxes become less abundant. . .
. . . but with few lynxes, the hares start increasing in
number again. Thus you have these regular cycles of
population size in both hares and lynxes.
Carrying Capacity
• The carrying capacity of an environment is not
fixed. – An organism might modify the carrying capacity of its
own environment by its own activities.
– EXAMPLE: New farming methods have increased the
world's food supply dramatically over the past few
hundred years, thus boosting the carrying capacity of
humans. – On the other hand, 10% of the world's fertile topsoil has
been lost since 1945, often due to human
mismanagement, pulling the carrying capacity down. . .
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Largely because of overgrazing by livestock and
poor irrigation, 3900 square miles of the Sahel
becomes desert each year. (That's about equal to six
medium-sized counties in Arkansas.)
Case Study:
The dry
grassland south
of the Sahara
Desert is called
the Sahel.
The carrying
capacity of the
countries in the
Sahel— such as
Mali, Burkina Faso,
Chad, Niger, etc. —
is dropping. At the
same time, the
human population
itself is rapidly
rising. . .
Life Histories
• Organisms tend to evolve towards one of two
possible life history strategies (although inbetween strategies are very common as well):
– r-selected life history: Many offspring, but low parental
investment of energy in each one. Most of the mortality
is among the younger members.
– K-selected life history: Few offspring, but high parental
investment of energy in each one. Most of the mortality
is among the older members.
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Humans are a strongly K-selected species, as are most
mammals. Reproduction is relatively low, but we invest much
energy (in pregnancy, nursing, and child care) in each
offspring.
Oysters are strongly r-selected species. The clouds of
"smoke” are countless millions of eggs and sperm being
released freely into the water. The vast majority of the larvae
will die before reaching adult size.
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