Population Growth

Population Dynamics What types of things affect the size of a population? What types of things affect the size of a population? Immigration: New members entering a population from outside the area Immigration: New members entering a population from outside the area Emigration: Members of an existing population leaving the area Emigration: Members of an existing population leaving the area Birth Rate Birth Rate Death Rate Death Rate

Survivorship Curves Much of a death rate for a population depends on the particular species. Survivorship curves show the likelihood that an organism will survive at a particular age during its typical lifespan. Much of a death rate for a population depends on the particular species. Survivorship curves show the likelihood that an organism will survive at a particular age during its typical lifespan. Type I: Small chance of dying until late in life Type I: Small chance of dying until late in life Type II: Equal chance of dying at any point during lifespan Type II: Equal chance of dying at any point during lifespan Type III: High chance of dying when young, but lives to old age if survives early life Type III: High chance of dying when young, but lives to old age if survives early life

A Scary Story Visualize a single bacterium. Now visualize this bacterium reproducing asexually so that now there are two bacteria. These two bacteria reproduce to bring the total to four bacteria. If this reproduction continued for only a day and a half (36 hrs) the bacteria would form a layer a foot deep over the entire Earth! Yikes! Visualize a single bacterium. Now visualize this bacterium reproducing asexually so that now there are two bacteria. These two bacteria reproduce to bring the total to four bacteria. If this reproduction continued for only a day and a half (36 hrs) the bacteria would form a layer a foot deep over the entire Earth! Yikes! So why aren’t we swimming in bacteria? So why aren’t we swimming in bacteria?

There are environmental limits on population growth The bacteria example represents what is called exponential population growth, where a population grows rapidly in a short period of time. The bacteria example represents what is called exponential population growth, where a population grows rapidly in a short period of time. On a graph, exponential growth is characterized by a J-shaped curve. On a graph, exponential growth is characterized by a J-shaped curve. Although a species may go through a period of exponential growth, this type of growth cannot continue indefinitely. Although a species may go through a period of exponential growth, this type of growth cannot continue indefinitely.

Exponential Model Birth rate far exceeds the death rate

More on environmental limits on population growth Exponential growth requires unlimited resources, which do not exist in nature. Exponential growth requires unlimited resources, which do not exist in nature. Therefore, there is a limit to the total number of individuals that can occupy a given amount of space called a carrying capacity. Therefore, there is a limit to the total number of individuals that can occupy a given amount of space called a carrying capacity.

More on Carrying Capacity The carrying capacity differs for each species, as well as with space and time. The carrying capacity differs for each species, as well as with space and time. For example, the carrying capacity for bacteria is much greater than that for humans. Also, available space and resources are not constant, and as these change, so will the population’s carrying capacity. For example, the carrying capacity for bacteria is much greater than that for humans. Also, available space and resources are not constant, and as these change, so will the population’s carrying capacity. Limiting factors affect carrying capacity and include: food, shelter, predator levels, water, nutrients, weather etc. Limiting factors affect carrying capacity and include: food, shelter, predator levels, water, nutrients, weather etc.

Types of Limiting Factors Density-independent factors: affect a population the same way, regardless of its size (ex: fires and floods) Density-independent factors: affect a population the same way, regardless of its size (ex: fires and floods) Density-dependent factors: begin to affect a population when it increases in size (ex: food and nesting sites) Density-dependent factors: begin to affect a population when it increases in size (ex: food and nesting sites)

The logistic growth model accounts for limiting factors Due to limiting factors, populations more closely fit a model of logistic population growth. Due to limiting factors, populations more closely fit a model of logistic population growth. This type of growth is characterized by an S- shaped curve. This type of growth is characterized by an S- shaped curve. The population grows almost exponentially until carrying capacity is reached, begins to slow, then stays at this capacity The population grows almost exponentially until carrying capacity is reached, begins to slow, then stays at this capacity

Logistic Model Birth rate approximately equals the death rate Carrying Capacity