Chapter 52
Population Ecology
Distinguish between density and
dispersion of a population
 Density – the number of individuals per
unit area or volume
 Dispersion- pattern of spacing among
individuals within the population
Explain how ecologists may estimate
the density of a species
• It is almost always impractical to count all individuals in a
population.
• They might count the number of individuals in a series of
randomly located plots, calculate the average density in the
samples, and extrapolate to estimate the population size in
the entire area.
• A sampling technique that researchers commonly use to
estimate wildlife populations is the mark-recapture
method.
Describe conditions that may result in clumped
dispersion, uniform dispersion, and random
dispersion of individuals in a population.
 Dispersion is clumped when individuals aggregate in patches.
 Plants and fungi are often clumped where soil conditions favor
germination and growth.
 Animals may clump in favorable microenvironments (such as isopods
under a fallen log) or to facilitate mating interactions.
 Dispersion is uniform when individuals are evenly spaced.
 Animals often exhibit uniform dispersion as a result of
territoriality, the defense of a bounded space against encroachment
by others.
 In random dispersion, the position of each individual is
independent of the others, and spacing is unpredictable.
 For example, plants may grow where windblown seeds land.
 Random patterns are not common in nature.
Describe the characteristics of
populations that exhibit Type I, Type II,
and Type III survivorship curves.
Explain, with examples, how limited
resources and trade-offs may
affect life histories
 Limited resources mandate trade-offs between investment in
reproduction and survival.
 Plants and animals whose young are subject to high mortality
rates often produce large numbers of relatively small
offspring
 In other organisms, extra investment on the part of the
parent greatly increases the offspring’s chances of survival.
Compare the exponential model of
population growth with the logistic
model
Explain how an environment’s carrying
capacity affects the per capita rate of
increase of a population
 Ecologists define carrying
capacity (K) as the maximum stable
population size that a particular
environment can support.
 Population growth is greatest when
the population is approximately half
of the carrying capacity.
Distinguish between r-selected
populations and K-selected
populations
 r-selection tends to maximize r, the rate of increase, and
occurs in environments in which population densities
fluctuate well below K, or when individuals face little
competition.
 K-selection tends to maximize population size and operates
in populations living at a density near K.
Explain how density-dependent factors
affect population growth.
 Density-dependent factors have an increased effect on a
population as population density increases.
 This is a type of negative feedback.
Explain, with examples, how biotic and
abiotic factors may work together to
control a population’s growth.
 Resource limitation in crowded populations can reduce
population growth by reducing reproductive output
 In animal populations, territoriality may limit density.
 Population density can also influence the health and thus the
survival of organisms.
 Predation may be an important cause of density-dependent
mortality for a prey species if a predator encounters and
captures more food as the population density of the prey
increases.
Describe boom-and-bust population
cycles, explaining possible causes of
lynx/hare fluctuations.
 Three main hypotheses have been
proposed to explain the lynx/hare
cycles.
 The cycles may be caused by food
shortage during winter.
 The cycles may be due to predatorprey interactions.
 The cycles may be affected by a
combination of food resource
limitation and excessive predation.
Describe the history of human
population growth.
Describe the problems associated with
estimating Earth’s carrying capacity for
the human species.