Density and Dispersion Information

1. Density and DispersionInformation Any population within a certain area can be defined using two key factors, density and dispersion. Density is the number of individuals per unit area or volume. An example of density would be the number of African elephants per square mile in Kruger National Park, South Africa. Dispersion on the other hand is defined as the pattern of spacing among individuals within the boundaries of the population. There are many different types of spacing under dispersion; the most common of them is clumped. Most of the populations throughout the entire world are dispersed as clumped, for example, many insects clump together in rotten logs and under damp and dark places such as rocks. A population’s density can be determined in several sampling techniques. One way of doing this is by actually counting all of the individuals within the boundaries of the population. For example, one may count the number of sea stars in a tide pool to find its density. This method is very tedious if populations carry great sizes, therefore there are more techniques. Another way of determining a population’s density is by finding the density of a small section in the boundaries, the scientist will then extrapolate to estimate the density within the total area. One of the most common ways of determining density is using the mark-recapture method. In this method, researchers place traps within the boundaries of the population. Captured animals are tagged and then released back into the wild. The scientists will then return to the population’s area and then place traps again. The animals caught are both tagged and untagged individuals. From this data the researchers can estimate the total number of individuals in the population. This method assumes that each tagged individual has the same probability of being caught as the untagged individual. Interactions between individuals in the population can result in the maintaining of certain patterns of spacing between these individuals, this can also contribute to changes in the population density. The clumped pattern of dispersion has the individuals aggregated in small areas. Examples of species that are clumped include plants, fungi, insects, and more. Plants are usually found clumped where soil conditions and other environmental factors are in favor for germination and growth. A uniform or evenly spaced pattern of dispersion results from direct interactions between the individuals in the population. For example, animals show a uniform dispersion when aggressive social interactions. Territoriality is an example of such aggressive social interactions. The last type of dispersion is random, random dispersion is not as common in nature as clumped or uniform.

2. Population Growth Information The formal definition for population growth starts with the fact that populations increase as individuals are born or immigrate into a specified location, and decrease as they die or emigrate. Rates of population growth, usually expressed as a percentage, vary greatly. Population growth is a localized group of individuals that belong to the same biological species and how they increase, decrease, or keep their numbers constant with time. Population growth is a devise that is used by ecologists to study different species reproduction rates, birth rates, death rates, immigration rates, emigration rates and more. Population growth can be broken down into several topics, including exponential population growth, zero population growth, and logistic population growth. Exponential population growth is also known as geometric population growth. Population increase under conditions such as populations whose members all have access to abundant food and are free to reproduce at their physiologically capacity, is usually referred to as exponential population growth. With exponential growth the birth rate alone controls how fast (or slow) the population grows. Exponential growth causes the per capita rate of increase to assume the maximum rate for the species, this is also known as the intrinsic rate of increase and noted as rmax. Exponential growth in a single species in an area can upset the entire ecosystem, for example an increase in the population of zooplankton will result in the decrease in the population of phytoplankton. Zero population growth is also known as ZPG. When the per capita birth rates and per capita death rates are both equal, zero population growth than occurs. Births and deaths obviously still happen in the population, but the births and immigration and the deaths and emigration actually perfectly balance each other out. The next branch of population growth is known as logistic population growth. Unlike the circumstances in which exponential growth takes place, in most real populations, both food and disease become important as areas become crowded. There is an upper limit to the number of individuals the environment can support. Ecologists refer to this as the "carrying capacity" of the environment. Populations in this kind of environment show what is known as logistic growth. In the logistic population growth model the per capita rate of increase decreases as the carrying capacity is reached.

3. Global Human Population Information The global human population has been increasing at alarming rates since the beginning of the industrial revolution. The human population has sustained so much growth for so long, unlike any other large mammal. The human population has increased at a slow but steady pace until 1650, where the population suddenly doubled to 1 billion within the next two centuries, then doubled again to 2 billion between 18590 and 1930, and then doubled again by 1975 to more than 4 billion. In modern times the global human population is over 6 billion individuals and is still growing, an average 73 million people are brought into the population every year. Approximately 201,000 people every single day, this is the equivalent of adding another city the size of Madison, Wisconsin to the population. Ecologists have predicted that the global human population will reach 7.3-8.4 billion individuals by 2050. Although the population is still significantly growing, the rate of growth has begun to slow since 1650. The rate of growth for the global population reached its highest mark in 1962, a rate of growth at 2.19%. This rate has declined to 1.16% in 2003. Modern scientists are estimating a population growth of just over 0.4% by 2050. Research indicates that countries like Australia, Italy, and Tunisia, among many others, have recently shown a decline in human population growth. These declines in the rate of growth show the population is being effected by fundamental changes in population dynamics, specifically caused by diseases such as AIDS.

4. Density and DispersionAnd DDT Density and dispersion are both affected by DDT. Density is the number of individuals per unit of area or volume. DDT, being toxic to living organisms, can lower the density of an organism. For example, birds can have a high density in an area. After DDT is applied to the area where the high density is, DDT enters the organism. It is stored in the fatty tissues and remains there. It also travels up food chains, as one organism will eat an organism affected by DDT. Since DDT is toxic, these animals start to die. As more and more organisms die, the population density decreases. Dispersion is the pattern of spacing among individuals within the boundaries of the population. DDT also has an effect on dispersion. As more and more organisms die from DDT, regular dispersion is interfered with. For example, uniform dispersion would be hard to maintain, especially with the loss of so many organisms. Clumped dispersion would also become harder, as the diminished numbers of organisms would lead to smaller, if any, groups.

5. Population Growth And DDT Population growth can slow and come to a halt with the introduction of DDT. DDT, being toxic, poisons organisms and kills them. This causes a rise in death rates. If new organisms immigrate to an area, they too will be killed by this toxin. DDT has also been shown to have an effect on the reproductive system, often resulting in sterility. If this happens, no new organisms can be born. Food and water supplies of an organisms population have a good chance of being affected by DDT. If this is so, more and more DDT will accumulate in the organism until the limit is reached, and the organism dies. If the death rate exceeds the birth rate, the population growth declines. All these factors contribute to how the population growth is affected.

6. Global Human Population And DDT DDT is not only limited to plants and animals. DDT can also have an affect on humans. When DDT is introduced to the human body it stores itself in organs rich in fatty substances such as the liver, kidneys, adrenals, thyroid, and testes. DDT can be absorbed by humans in several way. One is direct exposure to the chemical itself. Another is the consumption of food that has been exposed to DDT. DDT can also be passed down to offspring in the womb and through breast milk. Only a certain amount of DDT can be held by the human body before its damaging effects are realized. DDT has been found to be a chemical carcinogen, meaning that it can cause cancer. It can cause a range of illnesses, from fevers and nausea to fatigue and anxiety. DDT has the power to kill a human being. If DDT is applied to an area in large amounts, or if DDT has poisoned the food supply, the human population will begin to drop. Since DDT embeds itself in fatty organs, the human testes will become one of its targets. Sterility and other birth defects may occur because of the harmful effects of DDT. This could greatly diminish the number of births. Infant mortality would be high, as they will have been exposed to DDT in the womb and would now be exposed to it through the outside world. They would be extremely susceptible to the harms caused by DDT. This would mean the life expectancy would be low. Eventually, if DDT is still in use, the human population will decrease dramatically.

7. Density and DispersionAnd Modern Environmental Stress At this point in time, global warming has become the newest and possibly most destructive environmental stress. Global warming affects density and dispersion very much. As the Earth’s temperatures rise, the polar ice caps up north begin to melt. This threatens many organisms, such as polar bears. The polar bears’ habitat is ruined every day because of global warming. As the polar bears loose more and more of their home, they move to other places where the habitat they are used to is still intact. The population densities of the certain areas, which have been surviving global warming, grow more and more. Now there are many more polar bears in certain areas. Polar bears are predators, and now the population of the animals they prey on becomes lower, causing the population of whatever the prey feeds on to go higher, and on and on. Unfortunately, this isn’t only happening with the polar bears, but in many other areas.

8. Population Growth And Modern Environmental Stress Population growth is also affected by global warming. When the habitats of certain organisms are changing, everything else changes. Global warming could cause a lack of food, making the death rates higher and the birth rates lower. When an animal at the top of the food chain begins to die off, its prey multiplies, and the population of whatever the prey feeds on decreases.

9. Global Human Population And Modern Environmental Stress The global human population harmed by the contamination of water supplies, because humans rely on water as a main resource. Humans drink water, use it to bath themselves, clean their clothes and dishes, among many others uses. This allows many ways for harmful chemicals or substances to come in contact with the global human population. These pollutants can cause people to fall ill and sometimes dies. Another stress on the global human population is the increasing population. There is only so much space on Earth that is inhabitable, and it is possible the human race will meet their carrying capacity in the near future. Ideally this would mean the human population would begin to decrease, but this is uncertain. Overpopulation, is another issue, and allows for diseases to spread easily, and more demand for resources such as water, and food.

10. Credits Timothy Flynn, Sean Coffey, Caroline Kaleda, and Pierce Krauland.