Lecture 32 (Population ecology)

1. A population is a group of individuals of a single species that simultaneously occupy the same general area.

• The characteristics of populations are shaped by the interactions between individuals and their environment.

2. Two important characteristics of any population are density and the spacing of individuals

• Populations have size and geographical boundaries.

• The density of a population is measured as the number of individuals per unit area.

• The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries.

• Measuring density of populations is a difficult task.

• We can count individuals; we can estimate population numbers.

• One sampling technique that researchers use is known as the mark-recapture method.

• Individuals are trapped in an area and captured, marked with a tag, recorded, and then released.

• After a period of time has elapsed, traps are set again, and individuals are captured and identified.

• This information allows estimates of population changes to be made.

• Patterns of dispersion.

• Within a population’s geographic range, local densities may vary considerably.

• Different dispersion patterns result within the range.

• Overall, dispersion depends on resource distribution.

• Clumped dispersion is when individuals aggregate in patches.

• By contrast, uniform dispersion is when individuals are evenly spaced.

• In random dispersion, the position of each individual is independent of the others.

3. Demography is the study of factors that affect the growth and decline
of populations

• Additions occur through birth, and subtractions occur through death.

• Demography studies the vital statistics that affect population size.

• Life tables and survivorship curves.

• A life table is an age-specific summary of the survival pattern of a population.

• A graphic way of representing the data from a life table is with a survivorship curve.

• This is a plot of the number of individuals in a cohort still alive at each age.

• A Type I curve shows a low death rate early in life (humans).

• The Type II curve shows constant mortality (squirrels).

• Type III curve shows a high death rate early in life (oysters).

4. The traits that affect an organism’s schedule of reproduction and survival make up its life history.

• Life-histories represent an evolutionary resolution of several conflicting demands.

• Sometimes we see trade-offs between survival and reproduction when resources are limited.

5. Population growth models:

a) The exponential model of population growth describes an idealized population in an unlimited environment

• Using mathematical notation we can express this relationship as follows:

• If N represents population size, and t represents time, then DN is the change is population size and Dt represents the change in time, then:

• DN/Dt = B-D

• Where B is the number of births and D is the number of deaths

• We can simplify the equation and use r to represent the difference in per capita birth and death rates.

• DN/Dt = rN OR dN/dt = rN

• If B = D then there is zero population growth (ZPG).

• Under ideal conditions, a population grows rapidly.

• Exponential population growth is said to be happening

• Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth

• dN/dt = rmaxN

b) The logistic model of population growth incorporates the concept of carrying capacity

• Typically, unlimited resources are rare.

• Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support.

• dN/dt = rmaxN((K-N)/K)

• The logistic population growth model and life histories.

• This model predicts different growth rates for different populations, relative to carrying capacity.

• Resource availability depends on the situation.

• The life history traits that natural selection favors may vary with population density and environmental conditions.

• In K-selection, organisms live and reproduce around K, and are sensitive to population density.

• In r-selection, organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K.

6. Negative feedback sometimes prevents unlimited population growth

• Density-dependent factors increase their affect on a population as population
density increases.

• This is a type of negative feedback.

• Density-independent factors are unrelated to population density, and there is no
feedback to slow population growth.

7. Population dynamics reflect a complex interaction of biotic and abiotic influences

8. Some populations have regular boom-and-bust cycles

9. The human population has been growing almost exponentially for three centuries but cannot do so indefinitely

• The Demographic Transition.

• A regional human population can exist in one of 2 configurations.

• Zero population growth = high birth rates – high death rates.

• Zero population growth = low birth rates – low death rates.

• Age structure.

• Age structure is the relative number of individuals of each age.

• Age structure diagrams can reveal a population’s growth trends, and can point to future social conditions.

10. Estimating Earth’s carrying capacity for humans is a complex problem

• Wide range of estimates for carrying capacity.

• What is the carrying capacity of Earth for humans?

• This question is difficult to answer.

• Estimates are usually based on food, but human agriculture limits assumptions on available amounts.

• Ecological footprint.

• Humans have multiple constraints besides food.

• The concept an of ecological footprint uses the idea of multiple constraints.

• For each nation, we can calculate the aggregate land and water area in various ecosystem categories.

• Six types of ecologically productive areas are distinguished in calculating the ecological footprint:

• Land suitable for crops.

• Pasture.

• Forest.

• Ocean.

• Built-up land.

• Fossil energy land.