• What are the factors that determine the size of population? How and why do they change?
• These are the type of questions that a population ecologist tries to answer.
• To study a population it is necessary to first define the study population. Does it consist of the individuals in a particular geographical area such a valley or all the individuals in an island chain?
• In humans, a researcher might study the effect of drought on populations in sub-Saharan Africa, or the impact of a regional conflict on a particular ethnic populaion.

• In small populations, such as the endangered Everglades Kite, it may be possible to count every individual. Most organisms are too numerous to do this.
• To estimate population size, researchers may count individuals within sample plots and extrapolate populations numbers from this data. In the U.S. there is a census every 10 years to try to learn about our population.
• Below are examples of uniform and clumped dispersal pattern.
The distribution and available of resources may determine the patterns seen.

• Survivorship curves are used to learn about the pattern of lifespan within a population.
• In population I most individuals reach adulthood, and there is a large die-off of individuals as the population nears the end of it normal lifespan.
• Population II has a fairly constant mortality throughout the lifespan of the population.
• Population III has very high mortality of the young, but those individuals that do reach maturity live long life with a low death rate.

• In the exponential model of population growth, you do not see the effect of limiting factors.
• Population undergo rapid and explosive growth of population.
• In the illustration below, you can see the growth of a bacterial population. It should be clear why a dangerous pathogen must be recognized and treatment quickly started.

• Logistic growth curves show an S shaped pattern, rather than the J shape of the exponential growth model.
• The slowly of rapid growth and the leveling of population numbers is the result of limiting factors in the environment.
• The term carrying capacity refers to the maximum size of the population imposed by environmental factors.

• There are many limits on the size of populations. One of the most obvious is food. Some butterfly larva are very host specific and may feed on only one or two species of plant. If those plants are killed or reduced in numbers due to disease or a change in environmental conditions, the number of butterflies decrease.
• Other limiting factors include space to set up a breeding/nesting territory; competition from other species; habitat disruption; and or climatic conditions.
• An absence of predators, warm climate and a good food supply can lead to expanding populations. In South Florida this is seen as iguanas, cuban anoles, tokay geckos and many other exotics plants and animals extend their ranges throughout Miami-Dade
County and the surrounding region. In these cases temperature may be a limiting factor. A strong winter freeze may reduce population numbers of these tropical species.

• Boom-and-Bust cycles occur when the population growth of one species is closely tied to a limiting factor that may be expended.
• Lynx and snowshoe hare population cycles are often used as an example of this phenomenon. It involves a period of rapid population increase followed by a sudden precipitous decline in numbers. The predator populations numbers increase and decrease as the prey numbers change.
• Another example of boom-and-bust cycles can be seen in the phytoploankton and zooplankton populations of freshwater lakes.
• Often the limiting factor is a nutrient, such as phosphorus. When the phosphorus is depleted, the algal populations crash, and the zooplankton, which feeds of algae, also crash. This can be observed every spring in some of the lakes on Miami-Dade
College’s Kendall Campus.

• The principles of population ecology are important to biologists in the areas of wildlife management, forestry, and fisheries.
• In all of these areas professional try to practice the principles of sustainable resource management. In the past resources were often exploited with the feeling that they were limitless. Today we know that a resource has to be carefully managed to prevent it loss.
• This is the idea of maximum sustained yield , or how much can you harvest without depleting the resource.
• Unfortunately we haven’t been good at managing many ocean fish stocks. Some fisheries, such as the northern cod fishery, collapsed due to over-fishing. A few other species threatened by over exploitation include sturgeon (caviar), Chilean Sea Bass, and
Orange Roughy.
• Wildlife managers set bag limits on game based on an estimated sustained yield. These principles are used to set the numbers of permits issued for Florida’s annual alligator hunt.

• Human population is following an exponential growth pattern.
With the advent of modern medicine, better sanitation, and increased food supplies, the biggest constraints on population growth were removed.
• At some point in time the world’s population will have to stabilize. The current increases have had many profound effects on the biosphere and its inhabitants.

• Man’s footprint upon the land is great, not just in sheer numbers of individuals, but in the resources they consume.
How many items have you used and then disposed of today?
• There will be limits on the growth of human populations. The question is: Will we manage it ourselves or will we wait for natural forces to do it for us?.

• The age structure of three countries are shown below. What does this meant for the future of each of these countries? The percent of individuals in the reproductive and pre-reproductive age groups will define future growth.
Unfortunately the greatest growth is in those countries which have the fewest resources to sustain an exploding population. War, famine, disease, and natural disaster may be among the limiting factors of the future.