Population Ecology

A population is a group of individuals of the same species living in the same geographic area. The study of factors that affect growth, stability, and decline of populations is population dynamics. All populations undergo three distinct phases of their life cycle:
1. growth
2. stability
3. decline

Population growth occurs when available resources exceed the number of individuals able to exploit them. Reproduction is rapid, and death rates are low, producing a net increase in the population size.

Population stability is often proceeded by a "crash" since the growing population eventually outstrips its available resources. Stability is usually the longest phase of a population's life cycle.
Decline is the decrease in the number of individuals in a population, and eventually leads to population extinction.

Characteristics of Population:

1. SIZE - pertains to the number of individuals in a population.

Factors affecting size:

• NATALITY - refers to the number of ndividuals added to the population thru REPRODUCTION. Birth rate - refers to the number of individuals born per year 1,000 individuals per year.
• MORTALITY- refers to the number of individuals that die in a given time. Death rate - is the number of individuals die per 1,000 individuals per year.
• C. IMMIGRATION- pertains to individuals of the population moving into an area in a particular time.
• D. EMIGRATION - pertains to individuals of the population moving out from an area in a particular time.

2. Population density

(a) Given that a population is defined in terms of some natural or arbitrarily defined geographical range, then population density may be defined as simply the number of individual organisms per unit area
(b) Different species, of course, exist at different densities in their environments, and the same species may be able to achieve one density in one environment and another in a different environment
(c) Population densities may additionally be determined in terms of some measure other than population size per unit area such as population mass per unit area.

3. DISPERSION PATTERN

The dispersion pattern of a population refers to the way individuals are spaced withthin their area. These patterns are important characteristics for an ecologist to study, since they provide insights into the environmental effects and social interactions in the population.

The Three Main Types of Population Dispersion
Population dispersion is the observation of where individuals are found in a habitat. How individuals "disperse" themselves.

There are three main types of dispersion: clumped, uniform and random.

Clumped Dispersion
Is the tendency for populations to be found in tight clusters, dispersed across a large landscape. In between these population hubs, very few to no individuals are usually found. This sort of a dispersion can be caused by a number of factors. Some species cluster together for protection, while others group around natural resources necessary to their survival. For instance, fish are often clumped in schools which may reduce predation risks and increase feeding efficiency. Mosquitoes often swarm in great numbers, increasing their chances for mating.

Uniform Dispersion
Is the tendency for populations to be found evenly distributed about their habitat. This is generally caused by a species ability to survive anywhere in their habitat - they use the resources found immediately around them, and spread out as to use all of the available resources. Examples are uniform disperson of nesting king penguins and human habitations. Plant plantations are another examples.

Random Dispersion
Is the tendency for populations to be found randomly about their habitat. In immobile species, this is usually caused by their ability to live anywhere in a given habitat, except, they are limited to growing wherever they are first set root (which is usually caused randomly, from spores drifting in the wind to seeds falling and tumbling on the ground). In motile populations, individuals are able to move about their habitat, so that at any given instance, they can be found anywhere about their environment. Desert cacti are examples.

4. DOMINANCE - describes the most abundant individual of the population.

5. SEX RATIO - refer to the relative number of males and females.

6. AGE DISTRIBUTION -pertains to the number of individuals belonging in each age group of a population.

Different age groups:

a. REPRODUCTIVE JUVENILES - individuals of the population who are not yet capable of reproducing.

b. REPRODUCING ADULTS - individuals of the population who are capable of reproducing.

c. POSTREPRODUCTIVE ADULTS - individuals of the population who are no longer capable of reproducing.