Age structures reveal social and economic trends

Population ecology is the study of how and why populations change.

Density and dispersion patterns are important population variables. Population density is the number of individuals in a given area or volume. Environmental and social factors influence the spacing of individuals in various dispersion patterns: clumped (most common), uniform, or random.

Life tables track survivorship in populations. Life tables and survivorship curves predict an individuals statistical chance of dying or surviving during each interval in its life. The three types of survivorship curves reflect species differences in reproduction and mortality

Idealized models predict patterns of population growth. Exponential growth is the accelerating increase that occurs when growth is unlimited. The equation G = rN describes this J-shaped growth curve; G = the population growth rate, r = an organisms inherent capacity to reproduce, and N = the population size. Logistic growth is the model that represents the slowing of population growth as a result of limiting factors and the leveling off at carrying capacity, which is the number of individuals the environment can support. The equation G = rN(K N)/K describes a logistic growth curve, where K = carrying capacity and the term (K N)/K accounts for the leveling off of the curve.

Multiple factors may limit population growth. As a populations density increases, factors such as limited food supply and increased disease or predation may increase the death rate, decrease the birth rate, or both. Abiotic, density-independent factors such as severe weather may limit many natural populations. Most populations are probably regulated by a mixture of factors, and fluctuations in numbers are common.

Some populations have boom-and-bust cycles. Boom-andbust cycles alternate population growth and decline at regular intervals.

Evolution shapes life histories. Natural selection shapes a species life history, the series of events from birth through reproduction to death. Populations with so-called r-selected life history traits produce many offspring and grow rapidly in unpredictable environments. Populations with K-selected traits raise few offspring and maintain relatively stable populations. Most species fall between these extremes.

Principles of population ecology have practical applications. For example, resource managers use population ecology to determine sustainable yields.

The human population continues to increase, but the growth rate is slowing. The human population grew rapidly during the 20th century and currently stands at more than 6.8 billion. Demographic transition, the shift from high birth and death rates to low birth and death rates, has lowered the rate of growth in developed countries. In the developing nations, death rates have dropped, but birth rates are still high. The age structure of a populationthe proportion of individuals in different agegroupsaffects its future growth. Population momentum is the continued growth that occurs despite reduced fertility and is a result of girls in the 014 age-group of a previously expanding population reaching their childbearing years.

Age structures reveal social and economic trends.

An ecological footprint is a measure of resource consumption. An ecological footprint estimates the amount of land required by each person or country to produce all the resources it consumes and to absorb all its wastes. The global ecological footprint already exceeds a sustainable level. There is a huge disparity between resource consumption in more developed and less developed nations.

Use this graph of the idealized exponential and logistic growth curves to complete the following. a. Label the axes and curves on the graph. b. Give the formula that describes the blue curve. c. What does the dotted line represent? d. For each curve, indicate and explain where population growth is the most rapid. e. Which of these curves best represents global human population growth?

The graph at the top of the next page shows the demographic transition for a hypothetical country. Many developed countries that have achieved a stable population size have undergone a transition similar to this. Answer the following questions concerning this graph. a. What does the blue line represent? The red line? b. This diagram has been divided into four sections. Describe what is happening in each section. c. In which section(s) is the population size stable? d. In which section is the population growth rate the highest?

After seeds have sprouted, gardeners often pull up some of the seedlings so that only a few grow to maturity. How does this practice help produce the best yield? a. by increasing K b. by decreasing r c. by changing the populations age structure d. by reducing intraspecific competition e. by adding a density-independent factor to the environment

To figure out the human population density of your community, you would need to know the number of people living there and a. the land area in which they live. b. the birth rate of the population. c. whether population growth is logistic or exponential. d. the dispersion pattern of the population. e. the carrying capacity

The term (K N)/K a. is the carrying capacity for a population. b. is greatest when K is very large. c. is zero when population size equals carrying capacity. d. increases in value as N approaches K. e. accounts for the overshoot of carrying capacity.

With regard to its rate of growth, a population that is growing logistically a. grows fastest when density is lowest. b. has a high intrinsic rate of increase. c. grows fastest at an intermediate population density. d. grows fastest as it approaches carrying capacity. e. is always slowed by abiotic factors.

Which of the following represents a demographic transition? a. A population switches from exponential to logistic growth. b. A population reaches zero population growth when the birth rate drops to zero. c. There are equal numbers of individuals in all age-groups. d. A population exhibits boom-and-bust cycles. e. A population switches from high birth and death rates to low birth and death rates.

Skyrocketing growth of the human population appears to be mainly a result of a. migration to thinly settled regions of the globe. b. a drop in death rate due to sanitation and health care. c. better nutrition boosting the birth rate. d. the concentration of humans in cities. e. social changes that make it desirable to have more children.

According to data on ecological footprints, a. the carrying capacity of the world is 10 billion. b. the carrying capacity of the world would increase if all people ate more meat. c. the current demand on global resources by industrialized countries is less than the resources available in those countries. d. the United States has a larger ecological footprint than its own resources can provide. e. nations with the largest ecological footprints have the fastest population growth rates.

What are some factors that might have a density-dependent limiting effect on population growth?

What is survivorship? What does a survivorship curve show? Explain what the three survivorship curves tell us about humans, squirrels, and oysters.

Describe the factors that might produce the following three types of dispersion patterns in populations.

The mountain gorilla, spotted owl, giant panda, snow leopard, and grizzly bear are all endangered by human encroachment on their environments. Another thing these animals have in common is that they all have K-selected life history traits. Why might they be more easily endangered than animals with r-selected life history traits? What general type of survivorship curve would you expect these species to exhibit? Explain your answer.

How does the age structure of the U.S. population explain the current surplus in the Social Security fund? If the system is not changed, why will the surplus give way to a deficit sometime in the next few decades?

Many people regard the rapid population growth of developing countries as our most serious environmental problem. Others think that the growth of developed countries, though slower, is actually a greater threat to the environment. What kinds of environmental problems result from population growth in (a) developing countries and (b) developed countries? Which do you think is the greater threat? Why?