Population Principles

A population can be defined as a group of individuals of the same species inhabiting the same area. Populations differ in their stability and some of them are stable for thousands of years. Populations tend to increase as far as their environment will allow. As a result, the population is influenced by limiting factors of the physical environment and interactions with other populations in the community. Sometimes a population will grow suddenly; this is called a population explosion. When this occurs, resources quickly diminish leaving species to compete with other species in the community. A good example of this is locusts. They periodically appear throughout the land devouring vegetation. They die in great numbers but destroy food and disrupt food chains, which other organisms depend on for survival. On the other hand, a sudden population decline could occur called a population crash. This can cause serious problems for dependent populations. Organisms that depend on other organisms for food could near extinction if a population crash occurs. A population crash can occur at any time when a species specific food source is diminished and have no alternative supply. There are many underlying factors that contribute to the increase or decrease of populations.

Natality, or birthrate, refers to the number of individuals added to the population through reproduction. Natality is usually sensitive to environmental conditions and to social interactions between members of the species. For reproduction to be successful, species look for ideal habitat conditions. In plant populations, they produce many seeds that must land in the right soil conditions before a plant will be produced. The natality for most organisms is typically high. Most organisms produce more offspring than are needed to replace the parents. The growth of a population is not determined by the birthrate alone. Mortality, or death rate, refers to the number of deaths in a population (6). Mortality rates are high for many organisms. For all the seeds that a plant produces, few are able to mature to a plant and reproduce. In animals, most organisms die before they can mature and reproduce (7). Let me simplify:

~ If more organisms die than are replaced in a given time, the population will decrease.

There are three types of survivorship curves that demonstrate the increase or decrease in the population:

Sex ratio is the comparison between the number of females and the number of males. Most animals have an equal amount of males and females. At times, males are often killed and therefore, the females over populate them. In result, reproduction rates slow considerably. It is common for bees, wasps, and ants to be female dominated. Population can differ in age distribution as well. Age distribution is the different age levels of the individuals in the population (6). Most animals are born during the spring, but many of them die before they mature or able to reproduce. In species that have a higher survival rate, it is possible for a stable population to have a balanced age distribution (7).

Population density is the number of individuals of a species per unit area or volume at any given time. When the population density is too high, organisms must compete with one another for necessary resources (6). Animal populations may compete for water, food, shelter, or sunlight. For example, the fruit fly is most commonly found near rotting fruit and is not typically found elsewhere. As the density increases, competition becomes fierce. Spatial distribution relieves these overcrowded conditions and makes way for new homeland. Emigration is the pressure for out-migration. Typically, needed resources are scarce and that forces animals to relocate. This is common for birds. On the other hand, immigration occurs when animals that leave one population become members of a different population (7).

Biotic potential is the maximum rate at which a population could increase under ideal conditions. A species biotic potential is influenced by their age that reproduction can begin and the number of offspring produced at one time. Larger animals have a lower biotic potential, such as man and elephants. However, microorganisms have the greatest biotic potentials. The biotic potential is greater than the number of offspring needed to take the place of the parents when they die. Not all the young survive to become reproductive adults; many of them die when they are young. The high reproductive potential each offspring has leads to a population increase. For example, if two cats were to have five offspring, which, if they survive, will produce offspring while their parents are still producing offspring. This will lead the population to grow exponentially. Eventually, the death rate will equal the birthrate and the population will become relatively stable (7).

Factors that regulate population size fall into two categories, density-dependent and density-independent (6).  The two density factors that vary in different ways from each animal are also most likely to help determine the size of the population.

 ~ The density-dependent factor is when a change in population density alters how an environmental factor affects a population. When population density increases, it causes the density-dependent factors to slow the population growth by causing an increase in the death rate. On the other hand, when the population density increases, it causes an increase the density-dependent factor. Which not only affects a large number, but a large proportion, of the population. An example of the density-dependant factor is the Bubonic Plague.

Another example, of when the population is being regulated, is the occurrence of rabbits in Australia. There was an abundance of rabbits in Australia due to the fact that there was no predator to feast on them. These rabbits devoured the vegetation, which left no resources for the other animals. As a result a deadly vaccination was introduced for these rabbits. The vaccination is better known as Myxomatosis. There aren't many rabbits left in Australia due to the Myxomatosis. Myxomatosis is a terminal disease of rabbits. After a rabbit is affected, they become blind and around their eyes become swelled. As a result, it is very difficult for the rabbit to feed, or protect its self from predators such as foxes. Not all rabbits die from the disease. Although survival is rare, probably only 5-10% of the rabbits will eventually recover from the disease. The rabbits that are injected, usually only live for months, some only weeks after the infection. It is most common for the rabbits to die within 12 days of injection. For this example, there were no predators to control the rabbit population and the population exploded. In this case, a terminal disease was introduced. Are there other options?

~ Which bunny would you prefer to see?

Predator vs. Prey

• All living things must eat to survive. Therefore, with predators always looking for a meal, prey must try to avoid being eaten. Whatever defense mechanism the prey uses to protect himself could be an advantage against his enemy. There are three ways that prey could protect themselves against their predators:

~ The first defense mechanism a prey could use would be to run. Animals could use their speed to escape from their predators. This method has been a very effective way for animals to survive.
 

~ Secondly, another very effective way of survival is camouflage or protective coloration. This allows the animal to blend in with it's surroundings, therefore, tricking the predator into thinking that the prey isn't within reach. One form of protective coloration is cryptic coloration. Which also allows the prey to blend in with its environment without being detected.

~ The final defense mechanism is physical or chemical combat. An example of physical combat would be a porcupine. If a predator tries to eat a porcupine they would have a very difficult time with the sharp quills. Also a predator would have a tough time trying to eat a turtle through its protective shell. Chemical combat can also be very successful. An example of a prey that could easily use chemical combat would be a skunk. As a predator approached a skunk he could spray his chemicals which would result in a terrible odor that an attacker would never forget. The dart frog also uses chemicals to try and help him survive. The frog contains poisons secreted from its skin to warn the attacker. Any animal that feasts on a poor defenseless chemical containing, frog is likely to get sick or die.

A good example of a natural relationship is the wolves and moose on Isle Royale (7). The wolves had crossed the ice from Canada to reach the Isle Royale. In 1958, the wolf population had only reached about 20. It had a great increase in the 1980's reaching a high of about 50 wolves. Shortly after, in 1993, there was a decline to 13 wolves due to the spread of a viral disease. By 1996, good reproduction brought the wolves back up to twenty-two wolves. The moose population is affected my many different factors: the availability of food, weather, accidents, disease, and wolf predation. There was a major reduction in the wolf population in the 1980's (6). However, since then the moose population has had a major increase. Since then, the wolf population has been killing many moose and has also fed on the moose that died of other causes. The high moose death has been one of the reasons why there are still so many wolves on Isle Royale. The dead moose have been the food resource for the wolves.

The Endangered Species Protection Act was formed for the sole purpose of protecting our precious wildlife. Because of human interference of species habitats, animals were losing vital resources needed for their existence. An abundance of trees and forests were being destroyed because of man-made roads, highways, and government funded projects. Even though these projects provided employment opportunities, they neglected to consider the species homeland.

Rhinos have been living for over 50 years. The black and white rhinoceros are found in central and southern Africa. These animals are endangered because poachers hunt them for their horns and then sell them on the black market. Some people believe that the horns are a powerful medicine. They are also used to make dagger handles for tribes in the Yemen. Today, five species of rhino survive but all are on the verge of extinction. Four species of rhinos could be extinct in the next 10 years.

http://www.rhinos-irf.org/rhinos/black.html

 

The African Elephant is yet another endangered specie in the world today. African elephants can be found throughout African's entire mainland. They are the largest living land animals. There are an enormous amount of elephants being killed by poachers, for ridiculous reasons. Poachers, after killing the elephants, are taking their tusks. The elephant's tusks are their enormous front teeth, made of ivory, which is often carved into ornaments and jewelry.

~ Do you think that elephants should be killed solely for the purpose of making jewelry for people like us? What do you think?

Like all other large animals, reproduction for elephants takes a long time. Female elephants can't breed until they are at least fourteen years old. Even then, it still takes two years for a pregnant elephant to give birth. Because of the fact that elephants can't breed quickly enough, it is becoming more likely that they are going to become extinct very soon (7).

 

The Pygmy Hippopotamuses can be found in the swampy forests of West Africa Sierra Leone, Guinea, Liberia, and the Ivory Coast. These Hippopotamuses are also considered to be endangered species. They are slowly becoming sick and dying because of the destruction of their habitat and the intensive hunting for the meat of the Hippos. The hippos are surviving due to the roots, plants, and the fruits in the forests. As the forests are being cut down all the necessities needed for them to survive are becoming more and more scarce as the days go on.

 

Another good example is the Northern Spotted Owl. These owls have been considered endangered since 1986. About seventy-five percent of the population of spotted owls is centered on federal lands. Spotted owls begin breeding at a young age. Many of the owls begin at two or three years of age, and continue on for life. They usually only contain 2 or 3 eggs. Many of the owls today are dying because their habitat is being destroyed. The dense old-growth forests where they live are being cut down and destroyed, therefore leaving the owl homeless and more likely to be killed or die.

~ What is the purpose of destroying the owls homes?

 

Bibliography:

1. Academy for the Advancement of Science and Technology. 1996. Animal Populations. Bergen County, New Jersey. http://www.bergen.org/AAST/projects/ES/AP/index.html (1999, September 19).

2. Arms, Karen. 1990. Environmental Science. Saunders College Publishing, Philadelphia, PA.

3. Bailey, R. (1997, June 5). Predator vs. Prey. Biology. http://biology.about.com/library/weekly/aa060597.htm?COB=home&terms=natural+population&PM=113_300_T (1999, September 15).

4. Berg, L.R., G.B, Johnson, and P.H. Raven. 1998. Environment. 2^nd ed. Saunders College Publishing, Fort Worth, TX.

5. Brewer, G. (1999, Jan 2). Population Dynamics. http://www.ndsu.nodak.edu/instruct/brewer/populatn.htm (1999, September 15).

6. Cunningham, W.P., B.W. Saigo. 1995 Environmental Science: A Global Concern. 3^rd ed. WM.C. Brown Publishers, Dubuque, IA.

7. Enger, E.D., B.F. Smith. 1998. Environmental Science: A Study of Interrelationships. 6^th ed. The McGraw-Hill Companies Inc., Boston, MA.

8. Goldfarb, T.D. 1997. Sources. Notable Selections in Environmental Studies. 1^st ed. Dushkin PublishingGroup/ Brown and Benchmark Publishers, Guilford, CT.

9. Sharov, A. (1997, Dec 8). Population System. http://www.ento.vt.edu/~sharov/PopEcol/lec1/struct.html (1999, September 12).

10. World Resources Institute. (1992). Biological Diversity: What it is and Why is it Important. Biodiversity. http://www.wri.org/biodiv/biodiv.html (1999, September 12)

 

Ó By Cara Rogers (Senior Psychology Major)

crogers1@falcon.lhup.edu