Introduction
As surely as individuals are born and die, populations grow and decrease. At any time in its history, the size of a population is the product of births and deaths, and immigration and emigration. The rate at which individuals enter the population (births and immigration) and individuals leave (deaths and emigration) will determine whether the population is growing, shrinking, or is stable, and if it is changing, how fast.

A population is define as a group of one species of organisms occupying the same general area, using the same resources, and acted upon by the same environmental factors. Populations cannot grow indefinitely, many populations will become stable over a period of time while others will show sharp increases followed by similar decreases. Population characteristics that are studied are its density and the spacing of its individuals. Population density is the number of individuals per unit area or volume. Population dispersion is the pattern of spacing among the parameters of the geographical boundaries of the population.

DENSITY AND DISPERSION

Every population has geographical barriers and a population size
Population density is the number of individuals per unit area or volume

Measuring Density:
Since it is impractical to capture and/or count each individual in a given area, ecologists use a variety of methods to determine the density of various populations of organisms. Some of the techniques used are as follows: Counting the number of nests or burrows in a given area, analyzing the number of tracks, examination of solid waste products left behind by a species, and an actual capture method used to tag and release the specimen studied.

Mark-recapture method- traps are placed within the boundaries of the population being studied and the captured animals are marked and released. After a few weeks, traps are set again. The proportions of marked to unmarked that are captured the second time give an estimate of the size of the entire population.

N= (number of marked)(total catch second time) / number marked recaptured

Patterns of Dispersion:
Dispersion is the pattern of spacing among individuals within the geographical boundaries of the population. Local densities, within a population's range, may vary considerably due to differences in the limiting factors present. There are three types of patterns of dispersion in relationship to other individuals: clumped, uniform, and random.

• Clumped - is a pattern when individuals are aggregated in patches. This style is caused by a heterogeneous environment with resources concentrated in patches. Mating or social behavior of the individuals may also contribute to this type of dispersion.
   
• Uniform - is a pattern of equally spaced individuals. Competition between individuals may set up zones or territories for feeding, nesting or breeding.
   
• Random - is a spacing pattern based on total unpredictability. This form of dispersal is highly uncommon in nature. If it does, it usually results from the absence of a strong competition among individuals.

Demography is the study of the vital statistics affecting a population size. This branch of science deals with the influence that immigration and emigration have on a given population, this does not usually include the new organisms leaving or entering the population birth and death rates are also studied.

Age structure and Sex Ratio states that each age group has a characterized birth and death rate (Juveniles and old people are more likely to die). Birth Rate is the number of individuals produced during a certain amount of time (is greatest for individuals of intermediate age).

Age and Sex Ratios - several populations have gone beyond generations where individuals of more than one generation coexist. This situation produces an age structure in most populations. Every age group has a characteristic birth and death rate. A standard rule of thumb sets a high mortality for the lower and upper age groups and a low mortality rate for the intermediate age group.

Generations over time, is the average time span between the birth of individuals and the birth of their offspring and is strongly related to the body size over a broad range of organisms a shorter generation time will result in faster population growth. Body size has a major effect on generation time.

For example, Elephant takes 2 years to produce an offspring and several more years before the offspring can successfully reproduce. On the other hand, mice can produce a litter of individuals ever 21 days. Their offspring usually are ready to reproduce in 3 to 4 weeks after birth.

Females play a central role in a population since they are the ones to produce the offspring, but males may mate with several females each breeding season.

Density-Dependent and Density-Independent Factors:

Density Dependent Factors
Population Cycles is a couple of concepts - one idea is that crowding regulates cyclic populations and another is that population cycles are caused by a lag time in the response to the density-dependent factors, creating large fluctuations of population size above and below carrying capacity. Increasing population size reduces available resources and this eventually limits population growth. In restricting population growth, a density-dependent factor strengthens as the population size increases, affecting each individual more strongly. On the other hand, the population growth declines because the death rate increases, birth rate decreases or both. If there is a reduction in the food supply or other major resources, could also restrict the reproduction cycle resulting in less offspring.

The animal species’ battle to establish territories is a behavioral trait that may restrict population growth and available food supplies. Predators concentrate in areas where there is a high concentration of organisms. As long as the natural resources are available in sufficient quantity, the population will remain constant. As the population decreases so, do the predators.

Density-Independent Factors
They are unrelated to population size and will affect the same # of individuals regardless of weather and climate Examples are weather, climate, and natural disasters such as freezes, seasonal changes such as unusually hot or cold temperatures, hurricanes, and fires. All of these factors are unrelated to population size but affect everyone in the population regardless of population size or location.

Interspecific Competition
As we stated in the last lesson populations do not live alone, just as individuals do not. When individuals of the same species, or of two different species, depend on a common important resource then competition occurs. Competition can be defined as "interactions between individuals brought about by a shared requirement for a resource in limited supply leading to a reduction in survivorship, growth, and reproduction of individuals." Interspecific competition (inter = between) is the competition between two or more different species for a resource.


MODES OF POPULATION GROWTH
http://en.wikipedia.org/wiki/Carrying_capacity

A population that begins at low levels in a favorable environment may increase rapidly for a while, but eventually the numbers must stop growing.

Exponential Population Growth

In an ideal environment, there are no restrictions on the abilities of individuals to harvest energy, grow and reproduce thus, the population will grow in size with every birth and with immigration, and the population will decrease with every death and emigration. In a population living under ideal conditions, the population grows fast, because all the members have access to abundant food and are free to reproduce.

Populations increase under these conditions and the size of the population increases rapidly, resulting in the population actually accumulating more new individuals per unit of time when it is large than when it was small, and it will pass the threshold point (the environment will not be able to sustain the population anymore)

Logistic Population Growth
In most populations, there is a limit to the number of individuals that can occupy a habitat. The carrying capacity is the maximum stable population that a particular environment can support of a relatively long period of time. Crowding and resource limitations can have a profound effect on the population growth.

The logistic model is a model of species-specific competition: the competition between two or members of the same species. As the population sizes increases, the competition become more intense.

Territoriality is the defense of a well-bounded physical space. The model is the idea that even at low populations each individual added to the population has the same negative effect on population growth rate and that some populations show an effect in which individuals may have a time that is more difficult surviving and reproducing if the population size is too small.

Experiments for Home and Classroom

You can try these desert experiments at home. You can create a mini-pond at home and grow your own algae, see how cyanobacteria grow mostly in the warm, upper layers of a lake or see how warm temperatures speed up bacterial growth. Desert plants grow better in the heat than plants imported from colder places. See how plants require the right temperature in order to grow. A little bit of air or water, pollution goes a long way. Find out how.
http://chainreaction.asu.edu/ecology/trythis/

Here are 12 ecology-related experiments for you to try:
Before starting any of the experiments listed be sure to read the instructions carefully.
http://www.galeschools.com/environment/experiment/index.htm

Build a small ecosystem and make a mold terrarium, watch tiny blue, green and white plants grow on leftover food.
http://www.exploratorium.edu/science_explorer/mold.html