25 April 2005

Lecture 42

Reading, Chapter 29, 30, and 31

A. Population Ecology

2. Human population growth

Human population growth has been exponential for most of recorded history but the acceleration in growth rate has been dramatic over the last two hundred years. Even with uncertainty about what the human carrying capacity of the Earth is, it is certain that the human population will exceed it during your lifetime if exponential growth continues. Fortunately, the pattern of population growth in Europe and other developed regions suggests a natural slowing of human growth rate is occurring. The pattern observed has been termed the "demographic transition model".

The demographic transition model has four stages:

1) Pre-industrial

This stage is characterized by high birth rates but also high death rates, especially among children. The result is a relatively stable population or slow growth. This stage represents most of human history.

2) Industrializing

In this stage, improvements in agriculture and public health cause a decrease in the death rate, especially among children. Birth rates do not drop, however, so the initial effect is a dramatic acceleration of the rate of population growth. Awareness of this dramatic acceleration during the 20th century prompted grave concern about global over-population.

3) Industrial

In this stage, birth rates begin to fall as standards of living rise and women gain greater education and political power. Many developed countries, e.g. in Western Europe are at this stage.

4) Post-industrial

This stage is still somewhat hypothetical but some countries appear to have achieved it. Both birth and death rates are low and the population is stable, with little or no growth, well below the carrying capacity of the region.

The demographic transition model is being applied to questions of how we can keep the human population of the Earth at a level low enough to avoid a population crash and assure a high quality of life for all. The world-wide promotion of economic development for all and equal rights for women may prove to be crucial to the survival of our species.

A website describing the demographic transition model may be found at

thegeographyportal

B. Ecosystems ecology

Biological communities are groups of populations and the environment that they share. Ecosystems are groups of communities in a region of earth. One aspect of ecosystems is the flow of energy through them.

1. Biological energy flow

There are three basic components of biological energy flow:

Producers

Producers (plants and algae) absorb sunlight and use it to make carbohydrates and other biological molecules, i.e. they are photosynthetic. Nearly all the energy used by living things ultimately comes from the sun by means of photosynthesis. We are all solar-powered.

 

Consumers

Consumers (animals and some heterotrophic protists) eat producers or other consumers.

 

Decomposers

Decomposers are mostly bacteria and fungi. They break down dead bodies and waste from both producers and consumers, converting it into nutrients that are used by producers for their photosynthesis and growth.

 

There are several important points that you should know about biological energy flow:

1) Photosynthesis provides nearly all the energy used by living things. All the coal, oil, and natural gas in the earth is also the result of ancient photosynthesis. Coal, oil, and gas are the remains of ancient plants and algae.

 

2) The conversion of food into mass is about 10% efficient. In other words, a cow must eat 10 lbs of grass or grain to gain one pound of weight. The other nine pounds of food is "burned" for energy to run the cow.

 

3) The biomass (total weight) of producers is almost always much greater than that of consumers. This is because of 2) above.

 

4) Also because of 2) above, stable toxins like lead, mercury, or the pesticide DDT can exhibit "biological magnification". About 90% of the food taken in by consumers gets "burned" for energy and only about 10% is used for growth. The stable toxins above cannot be "burned" or excreted and accumulate in the bodies of consumers to 10 times the concentration present in their food. Thus, certain stable toxins are magnified ten-fold at every step in the food chain. Top consumers can have body concentrations much higher than their surroundings.