POPULATION ECOLOGY

I. Population ecology

A. Population ecology is the study of populations

B. Population = group of individuals of the same species occupying a common geographical area

1. Habitat - where a species normally lives

II. Characteristics of populations 

A. Density = number of individuals per unit of area or volume, e.g. persons/square mile

B. Distribution = the general pattern in which the population members are dispersed through habitat. 

1. Distributions may be clumped, uniformly dispersed or randomly dispersed

2. Clumped distribution is most common type because:

a. Suitable physical, chemical, and biological conditions are patchy, not uniform

b. Many animals form social groups

c. Many offspring are not highly mobile and are forced to live "where they landed"

3. Uniform distribution is rare in nature. If it does occur, it is usually the result of fierce competition for limited resources

4. Random distribution occurs if environmental conditions are rather uniform in the habitat and members are neither attracting nor repelling each other

C. Population size =  the number of individuals making up its gene pool

1. At any one point in time, populations have a certain size

2. Future population size depends on births and deaths, immigration and emigration

3. Birthrate and death rate are used to calculate the net reproductive rate

4. Net reproductive rate is used to calculate the growth and size of a population per unit time

5. Zero population growth designates a near balance of births and deaths

6. Population size may also be controlled by density dependent or density-independent factors:

a.  Density-dependent factors - growth is affected by the density of individuals

Predation, parasitism, competition are considered density-dependent; the more organisms crowd together, the more damaging are food shortages, parasites, and predators

Density-dependent factors have some effect relative to the size of the population

b. Density-independent factors.- population growth factors not associated with the number of individuals:, e.g. adverse weather, floods, droughts, cold temperatures

D. Age structure =  the relative proportions of individuals of each age

1. Not all individuals in a population are the same age

2. Age structure defines the relative proportions of individuals of each age:

a. pre-reproductive

b. reproductive

c. post-reproductive

3.  The age structure of a population will determine its future

E. Patterns of population growth

1. There are two general patterns: 

a. organisms that reproduce once, cease to grow as adults, and expend energy in reproduction and die

b. organisms that reproduce through their lifetime, which invests energy in future survival

2. Most organisms fit somewhere between these two extremes

A. If a population lives under ideal conditions it may display its biotic potential - the maximum rate of increase under ideal conditions

B. However few populations live under ideal conditions because a number of factors limit their growth

C. Limiting factor = any resource that is in short supply, e.g. food, minerals, light, living space, refuge from predators, etc.

D. Carrying capacity = maximum number of individuals of a species or population a given environment can sustain

1. Each habitat or area can only support so many individuals

IV. Population growth patterns

A. Exponential growth - indicated by a J shaped growth curve

1. If birth and death rates of a population remain constant they can be combined into one variable r = net reproduction per individual per unit time (rate of increase)

2. Population growth may be represented mathematically as: G = rN

Where G = population growth per unit time, r = rate of increase and N= the number of individuals

3. Plotted against time a graph in the shape of a J will be obtained 

4. A J shaped growth curve denotes exponential growth, i.e. one variable increases much faster than the other

5. As long as per capita birth rates remain even slightly above per capita death rates, a population will grow exponentially, with ever-increasing rates and shorted "doubling times"

6.  It took 2.5 million years for the world's human population to reach 1 billion, yet it took only 12 years to reach the sixth billion  

B. Logistic growth 

1. Because of limiting factors populations rarely exhibit J shaped growth curves

2. Early on populations will exhibit very rapid growth but as they near the carrying capacity they will level off

3. This is called logistic growth and it produces an S shaped curve

4. Logistic growth is density dependent, i.e. the growth is affected by the density of individuals

a. E.g. 26 reindeer were introduced onto an island off the coast of Alaska in 1910. Within 30 years the herd increased to 2,000. However, overgrazing reduced the food supply and the population crashed to 8 animals by 1950

b. High density and overcrowding put individuals at greater risk of being killed. E.g., predators, parasites and pathogens have greater numbers of prey and hosts in a smaller area, to interact with

Bubonic plague swept through Europe in the 14th century, killing at least 25 million. The disease spread rapidly in overcrowded cities where sanitary conditions were poor and rats were abundant

V. Mortality

A. Each species has a characteristic life span and the probability of dying increases with age

B. Population ecologists, as well as insurance companies, track cohorts and construct life tables for populations

1. Cohort = group of individuals born at the same time, e.g. baby boomers are a large group of individuals born just after World War II.

2. Life table =  an age-specific death schedule. Such a schedule is often converted to a more palatable survivorship schedule. For each age interval there is an predicted life expectancy or survivorship 

C. Ecologists divide populations into age classes and assign birth rates and mortality risks to each class. Absolute population numbers mean very little unless their age structure is known

1. E.g. population A might have many more members than population B. However, all the members of A might be post-reproductive, whereas population B might consist of mostly pre-reproductive and reproductive age individuals. Population A might be in danger of extinction.

2. A pyramid-shape indicates the population has high birthrates; population is undergoing exponential growth

3. A block-shape indicates that prereproductive and reproductive age groups are more nearly equal, with the postreproductive group being smallest due to mortality; this is characteristic of stable populations

4. An inverted pyramid-shaped diagram indicates the postreproductive group is largest and the prereproductive group is smallest, a result of the birthrate falling below the death rate; this is characteristic of declining populations

D. Survivorship curve = plots of survivorship vs. age. There are three types:

1. Type I - high survivorship until fairly late in life. Characteristics of long lived organisms that take care of their young for extended periods. E.g. humans and most large mammals

2. Type II - fairly constant death rates at all ages. Such organisms are just as likely to be killed or die at any age due to predators or disease. E.g. lizards, some birds, small mammals such as rodents

3. Type III - very high death rates early in life. Characteristic of species that produce large numbers of offspring but do not care for them. E.g. sea turtles, sea stars, most insects, many fish and most plants

VI. Life History Patterns

VII. Human population growth

A. Human populations have experienced staggering growth within the last few hundred years. It took from the beginning of human existence until 1850 to get 1 billion people on earth. But by 1987 the number of humans had reached 5 billion

B. The human population is now in an exponential part of a J-shaped growth curve

C. World population increases the equivalent of one medium-sized city (370,000) per day and 135 million per year, despite the fact that nearly 2 billion people suffer from starvation or malnutrition

D. Doubling time is the length of time for population size to double, now 47 years 

E. Human population growth has been made possible by three things:

1. Human intelligence, memory, learning, tool making and languages helped us to adapt and survive in new habitats

2. The development of agriculture increased carrying capacity

3. Controlling diseases and harnessing energy sources sidestepped limiting factors

F. Human populations have passed through three phases:

1. Hunter-gathering phase: until about 11,000 years ago populations grew slowly because people hunted animals and gathered naturally growing plant foods. The estimated population of earth 11,000 years ago was about 10 million

2. Agricultural phase: domestication of plants and animals began about 11,000 years ago. This vastly increased the carrying capacity of our habitats.  The estimated population of earth by 1750 AD 800 million

3. Industrial phase: began with industrial revolution in the mid 18th century. Machinery, particularly the steam engine, transformed agriculture and provided a means to rapidly distribute food and other necessities. This caused rapid migration into large cities because fewer farmers were needed

G. The current world population is 6 billion. If growth continues in 100 years there will be at least 12 billion

H. Larger populations require more food supplies which require more agricultural land. However, increasing population pressures are resulting in the conversion of valuable farm lands into residential land

I. Increasing agricultural production requires more water and energy input, but it causes more pollution, deforestation and the depletion of soil banks

J. Overcrowding has already contributed to epidemics of cholera and other pathogens where sewage treatment is poor or lacking

K. Recall the logistic (S-shaped) growth pattern. Eventually human population levels will approach the carrying capacity of the earth. Recall the consequences predicted by Malthus 200 years ago

VII. Controlling population growth

A. Family planning is only way to stem the tide of human growth but it is a politically, socially and theologically sensitive issue

B. To achieve zero population growth couples should only replace themselves. In underdeveloped countries this amounts to about 2.5 children per couple. In developed countries about 2.1 children per couple would maintain zero population growth

C. Total fertility rate = the average number of children born to women during their reproductive years

1. In 1995 the total fertility rate was 3.1 children, well above the replacement rate. But this is much lower than the 6.5 in 1950.

2. The introduction of the birth control pill and the movement of women into the workforce probably account for much of the reduction

D. About 1/3 of the world population is currently in the pre-reproductive phase. This does not bode well for the future

E. Delaying child bearing into the 30s rather than teens and 20s also reduces population growth

F. Some countries, e.g. China have incentives/coercive measures to encourage only one child per family. These include free contraception, abortions and sterilizations, lower taxes, better schooling and housing, etc. People pay more in taxes if they have more children. Is this cruel? Between 1958-1962 30 million died from famine in China. Since 1972 China's total fertility rate has declined from 5.7 to 1.9. Even so, its population should reach 1.5 billion by 2025

VIII. Demographic transition

A. Population growth rate and economic development are correlated. In general, more developed countries have lower fertility rates

B. The demographic transition model predicts changes in human population growth patterns correlated with changing economic development

1. During the pre-industrial stage living conditions are difficult and both birth rates and death rates are high. Therefore population growth rates are low

2. During the transitional stage industrialization begins. Food production increases and health care improves. Death rates drop but birth rates remain high. Therefore population growth is rapid

3. During the industrial stage people move from the farms to cities and they no longer need nor desire large families. Therefore population growth is relatively slow. The U.S., Canada, western Europe, Australia and Japan are in this phase

4. During the post industrial stage zero population growth is achieved

C. Many less developed countries are in the transitional stage yet they don't have enough skilled workers to complete the transition to the full industrial stage. Increased fuel prices, driven up by industrialized countries, further inhibit their transition

IX. Population or resources?

A. Compare the nearly 1 billion population of India with 250 million in US:

1. India has 16% of the world's population while the US has 4.7%
2. People in the US consume 30 times as much as people in India
3. The US generates 25% of global pollution and trash while India generates 3%
4. Therefore, on a per capita basis, India would need a population over 10 billion to equal US consumption and pollution!

X. Impact of no growth

A. There would be serious social, political and economic ramifications to no growth. For example, it takes a larger young population to support older generations, e.g. social security and Medicare and our economy and "standard of living" is based on continuous growth, i.e. increased numbers of consumers

XI. Humans are living organisms and we are subjected to the same ecological principles as all other organisms. Eventually we will reach our carrying capacity. We can do it by limiting population growth or by letting nature do it for us