Population Growth

Explain the concept of logistic population growth. When do populations exhibit such growth?

As resources are depleted, population growth rate slows and eventually stops: logistic population growth.

Describe the concept of carrying capacity (K)

Carrying capacity (K) is the number of individuals of a population the environment can support. Finite amount of resources can only support a finite number of individuals.

Discuss how the carrying capacity (K) may be permanently altered by a population.

Carrying capacity can be permanently altered by a population. • Overpopulation may damage the habitat (resource base) and permanently reduce productivity, so that only a lower population can be sustained. • e.g., reindeer on an Alaskan island

Provide some examples discussed in class that illustrate some factors that limit population growth.

Density Dependent Limits Food Water Shelter Disease • Density Independent Limits Weather Climate examples: population of thrips, songs of sparrow, soybean survival decreases at higher densities

Which factors (density-independent or density-dependent) are considered more common? What characteristics may mask this factor?

Density-dependence seems common, but is often masked by high r, short T.

List and discuss some density-independent and density-dependent factors limiting population growth.

Environment limits population growth by altering birth and death rates. Density-dependent factors David Lack 1954 # present & reproduction depend on existing population size (density) Disease, resource competition Density-independent factors Andrewartha & Birch 1954 Population controlled by weather, catastophes, etc. that do not depend on population size Natural disasters

Describe the terms in the equation for exponential growth used to calculate population size at any time (Nt = N0ermaxt).

For populations with overlapping generations and unlimited resources, growth can follow an exponential model: • Nt = Number individuals at time t. • N0 = Initial number of individuals. • e = Base of natural logarithms. • rmax = Per capita rate of increase. • t = Number of time intervals

What is the greatest uncertainty when discussing limits to human population growth?

Human population growth does not currently show density effects that typically characterize natural populations. • In natural populations, per capita population growth rate decreases with population size, whereas global human population growth rate has a positive relationship

Define immigration and emigration. Describe why these are difficult to measure and what assumptions are generally made in population modeling regarding these terms.

Individuals are born and die; they may also be added or lost from a population by the processes of immigration and emigration. • Immigration--addition of new individuals to a population via movement into the area. • Emigration--loss of individuals from a population by movement out of the area. • Both immigration and emigration are typically difficult to measure. As a starting point, one often assumes they balance, and thus focus on birth and death in the study of population growth, but this is a tenuous assumption.

Describe the concept of ecological carrying capacity and discuss how this estimate varies worldwide.

Limited resources eventually will cause human population growth to slow, but global human carrying capacity is not known.

Discuss the pattern in the size of an organism and the per capita rate of increase (rmax).

On average, small organisms have higher rates of per capita increase and more variable populations than large organisms.

Describe the terms in the equation for exponential growth (dN/dt = rmaxN). Does this equation apply to populations with overlapping, or non-overlapping generations?

The change in population number per unit time is: dN / dt = rmax N • As population size (N) increases, the rate of population increase (dN/dt, the number added per unit time) gets larger.

Describe what Thomas Malthus predicted about human population growth versus agricultural production and the eventual outcome.

Thomas Malthus(in 1798) predicted that populations would grow geometrically (exponentially) while agricultural production would grow only arithmetically (linearly), such that eventually population (humans) would outgrow its food supply and famine would result

Describe the terms in the equation for geometric growth (Nt = Noλt). Does this equation apply to populations with overlapping, or non-overlapping generations?

When generations do not overlap, growth can be modeled geometrically. Nt = Noλt Nt = Number of individuals at time t. No = Initial number of individuals. λ = Geometric rate of increase. t = Number of time intervals or generations.

Describe at what point (N) you could maximize the amount of organisms harvested from a population. Is this the population usually targeted by managers? If not, what is the targeted population and why?

You can maximize the amount of organisms harvested from a population if you keep the population at the maximal dN/dt, which occurs at N=K/2. • Because of the uncertainties, manage for SY

Describe the trend in births and death of a population as it transitions from a preindustrial to a postindustrial society. What is the resulting trend in population growth rate?

Zero population growth -when birth rates equal death rates • Two ways to reach ZPG. High birth and death rates or low birth and death rates. • Demographic transition is moving from the first to the second. Most developed countries have made the transition

Explain the terms in the general equation for population change over time (dN/dt = B - M +I -E).

dN/dt = B - M + I - E • B = births • M = mortality • I = immigration • E = emigration

Characterize the distribution of the current human population.

most of the human population is concentrated in Asia

Provide an example of a population that may exhibit exponential growth.

pollen accumulation rate in lake sediments can be used as an index of population size . Pollen in lake sediments indicates that Scots pine colonized the Norfolk region

Provide an example of a population that may exhibit logistic growth

population growth of willows after rabbits grazing stopped increases at first then stabilizes

Describe the terms in the equation for logistic growth [dN/dt = rmaxN(1-N/K)].

rmax = Maximum per capita rate of increase under ideal conditions. • When N nears K, the right side of the equation nears zero. • As population size increases, logistic growth rate becomes a small fraction of growth rate. Highest when N=K/2.