Home
Back |
Ecology:
The study of a how organisms relate to their environment.
That
which follows was inspired by a lecture series presented by:
Dr.
Alan D. Thornhill, Biologist
May
17-19, 2002 at the Quaker Retreat Center
Richmond,
Indiana
Table
of Contents:
ABC's
of Life
Biological
Controls of Growth
Resource
Use (K)
(My
sincere apologies to all biologists/ecologists for the over-simplifications
contained below)
|
|
ABC's of Life
Return to Table of
Contents
Over time, assuming that
resources (food) are available, the intrinsic nature of any
population -- from alligators to humans to zebras -- is to grow! Surpluses
and increases in food availability allow populations to grow at
exponential rates.
|
|
|
Simply stated: Every living
organism has to eat. When organisms eat - they reproduce.
Assuming generation overlap, the population will grow. As food
availability increases, a population will increase -- tracking its food
availability. |
|
|
Biological
Controls of Growth
Return to Table of
Contents
In naturally occurring systems (not
human-made), food availability does not continually increase over
time. Biological events (environmental resistance) occur that
allow a population to stabilize and be in balance with food
availability.
|
|
|
|
Biological
Controls of Growth fall into three basic categories of Environmental
Resistance:
|
|
1)
Abiotic (non-biological) factors that contribute to limiting a
population size include examples such as: wind (gentle breezes vs.
tornados), rain (drought vs. flood), rain pH (acid?), amount and quality
of sunlight, climate, lightning, fire. |
|
The impact of a abiotic events are measured in
terms of both frequency and duration.
Event
|
Event Frequency
|
Duration
of System Recovery (Environmental Impact)
|
Branch
Fall in Forest
|
High
Frequency
|
Short
Recovery
|
Volcano
Eruption
|
Low
Frequency
|
Long
Recovery
|
|
2) Biotic (biological) factors that contribute to limiting a population size include
examples such as: Disease, Resource Availability, Predators,
Competitors.
|
|
3) Social factors contribute to
limiting the size of a population within species that have social
systems. The social system of flocking birds is
hierarchical. The relationship among birds is catergorized as:
dominant, sub-dominant and juvenile. A birds place in the
"pecking order" will determine whether it has a high, medium
or low quality environment to exploit. The quality of the
environment will determine whether the bird thrives or merely
survives.
|
|
Environmental resistance creates a
continuum or gradient for a population within the environmental
system.
|
|
|
|
Resource Use (K)
Return to Table of
Contents
A population in balance with its
environment can be sustained for an indefinate period of time. The
size of the population for a particular species is expressed in terms of
K. K is the carrying capacity of an environment for a specific
species.
|
|
|
The size of a population that can be
supported by an environment falls along a continuum. At the
extremes: 1) An environment will sustainably support a large population
if resources consumed per individual is small or 2) An environment will
sustainably support a small population if resources consumed per
individual is large. |
Resources are not evenly distributed across
the surface of the earth. Certain temperate geographical regions
can produce food predictably year round. Other regions have
volatile environments and abundant food supplies are not predictable.
K in the United States of America is very large
relative to other regions of the earth due to large swaths of arable
land. For thousands of years, these areas were home to tall praries
which produced an extremely thick horizon of organic material compared
to the rest of the world.
By way of comparison KUSA is very large
when contrasted with KChile -- where nearly 1/3 of the
country is desert.
K values of all regions (land and seas) added together
equal KGlobal |
|
|
|
A "textbook" example that
illustrates how a population tracks resource availability is the well
documented relationship between the snow shoe hare and the lynx.
In a given year, weather conditions may be such that
there is an abundance of ground cover. This ground cover equates
to a surplus of hare food eventually resulting in a gradual increase in the hare population. It happens
that one of the lynx preferred food is the hare. With the
increase in hare population it can be expected that after a brief
time-lag, the lynx population will begin to increase in response to the
availability of surplus lynx food. The hare population will not
become excessive as the lynx will pick off the largest, fattest, slowest
-- pregnant -- hare. Over time, the hare population will fluctuate
via factors other than predation -- such as weather. What of the lynx
population that is dependant upon the availability of hare for its
survival? What happens to the lynx when the hare population
decreases? There will be a slight time-lag and the Lynx population
will begin decreasing. Lynx will be spending more time running and
chancing prey and not as much time producing off-spring. Females
will abort pregnancies under stress (not even knowing they were
pregnant). This is how biology works. There will not be
massive starvation among the lynx population. There will be Lynx
getting skinny and not reproducing...not by choice...but by
biology. There are an abundance of biological mechanisms
that slow and arrest population growth before starvation among a
population occurs.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|