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

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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

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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 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)

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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.       


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