CHAPTER 2

Populations

Populations are defined in several ways

• 1) Ecologists define a population as a single-species group of individuals that use common resources and are regulated by the same environmental factors– a) Individuals in a population have a high

likelihood of interacting and breeding with one another

– b) Researchers must define a population by geographic boundaries appropriate to the questions being asked

Density and dispersion patterns

• 1) Population density is the number of individuals in a given area or volume– a) It is sometimes possible to count all the

individuals in a population• 1. More often, density is estimated by

sampling

Density and dispersion patterns

– b) One useful sampling technique for estimating population density is the mark-recapture method

Density and dispersion patterns

– c) The dispersion pattern of a population refers to the way individuals are spaced within their area• 1. Clumped• 2. Uniform• 3. Random

• 2) Clumped dispersion is a pattern in which individuals are aggregated in patches – a) This is the most common dispersion

pattern in nature– b) It often results

from an unequal distribution of resources in the environment

• 3) A uniform pattern of dispersion often results from interactions among individuals of a population– a) Territorial behavior and competition for

water are examples of such interactions

• 4) Random dispersion is characterized by individuals in a population spaced in a patternless, unpredictable way– a) Example: clams living in a mudflat– b) Environmental conditions and social

interactions make random dispersion rare

Density and dispersion patterns

Idealized models

• 1) Idealized models describe two kinds of population growth– a) exponential growth – b) logistic growth

Idealized models

• 2) Exponential growth is the accelerating increase that occurs during a time when growth is unregulated– a) A J-shaped growth curve, described by

the equation G = rN, is typical of exponential growth• 1. G = the population growth rate• 2. r = the intrinsic rate of increase, or an

organism's maximum capacity to reproduce

• 3. N = the population size

G = population growth rater = maximum capacity to reproduceN = population size

• 3) Logistic growth is slowed by population-limiting factors– a) It tends to level

off at carrying capacity

• 1. Carrying capacity is the maximum population size that an environment can support at a particular time with no degradation to the habitat

• Homeostasis-the tendency ofA population to remain stable

• 3) 2. What Barriers can prevent dispersal?– Climate– Physical- mountains– Behavioral- area selection like birds– Passive-wind carrying seeds– So….why no polar bears

in the South Pole.

Idealized models

• 4) The logistic growth model predicts that– a) a population's growth rate will be low

when the population size is either small or large

– b) a population’s growth rate will be highest when the population is at an intermediate level relative to the carrying capacity

Multiple factors may limit population growth

• 1) Increasing population density directly influences density-dependent rates

• a) such as declining birth rate or increasing death rate

• 2) The regulation of growth in a natural population is determined by several factors

– a) limited food supply (resources)

– b) the buildup of toxic wastes

– c) increased disease

– d) predation

Multiple factors may limit population growth

• 3) Field studies of the song sparrow have demonstrated that birth rates may decline as a limited food supply is divided among more and more individuals

4) Abiotic factors may limit many natural populations

– a) Aphids show exponential growth in the spring and then rapidly die off when the climate becomes hot and dry in the summer

“Boom-and-bust" cycles

• 1) Some populations go through boom-and-bust cycles of growth and decline– a) Example: the population cycles of the

lynx and the snowshoe hare • 1. The lynx is one of the main predators

of the snowshoe hare in the far northern forests of Canada and Alaska

2) About every 10 years, both hare and lynx populations have a rapid increase (a "boom") followed by a sharp decline (a "bust")

– a) Recent studies suggest that the 10-year cycles of the snowshoe hare are largely driven by excessive predation• 1. But they are also influenced by

fluctuations in the hare's food supply– b) Population cycles may also result from a

time lag in the response of predators to rising prey numbers

“Boom-and-bust" cycles

Evolution shapes life histories

• 1) An organism's life history is the series of events from birth through reproduction to death

– a) Life history traits include

• 1. the age at which reproduction first occurs

• 2. the frequency of reproduction

• 3. the number of offspring

• 4. the amount of parental care given

• 5. the energy cost of reproduction

2) In nature, every population has a particular life history adapted to its environment

– a) The agave illustrates what ecologists call "big-bang reproduction" • 1. It is able to

store nutrients until environmental conditions favor reproductive success

The human population

• 1) The human population as a whole has doubled three times in the last three centuries– a) Now stands at about 6.87 billion and may

reach 9.3 billion by the year 2050• 1. Most of the increase is due to

improved health and technology• 2. These have affected death rates

2) The history of human population growth

The Black Death

Birth, death rates, and age structure

• 1) Population stability is achieved when there is zero population growth – a) Zero population growth is when birth

rates equal death rates– b) There are two possible ways to reach

zero population growth (ZPG)• 1. ZPG = High birth rates - high death

rates• 2. ZPG = Low birth rates - low death rates

2) The demographic transition is the shift from high birth and death rates to low birth and death rates

• a) During this transition, populations may grow rapidly until birth rates decline

Birth, death rates, and age structure

• 3) The age structure of a population is the proportion of individuals in different age-groups– a) Age structure affects population growth

RAPID GROWTH

Kenya

Male Female

Percent of population Percent of population Percent of population

SLOW GROWTH

United States

Male Female

ZERO GROWTH/DECREASE

Italy

Male Female

Ages 45+

Ages 15–44

Under15

Under15

Ages 45+

Ages 15–44

The human population

Questions to think about? In your group discuss the following:1. Is Earth’s carrying capacity limited?2. What are some limiting factors that we

face?3. How much food do you need?4. What is a famine?5. How have human activities changed the

environment?6. Is resource management a necessity?

Practical applications

• 1) Principles of population ecology may be used to – a) manage wildlife, fisheries, and forests for

sustainable yield– b) reverse the decline of threatened or

endangered species– c) reduce pest populations

Practical applications

• 2) Renewable resource management is the harvesting of crops without damaging the resource– a) However, human economic and political

pressures often outweigh ecological concerns

– b) There is frequently insufficient scientific information

3) Integrated pest management (IPM) uses a combination of biological, chemical, and cultural methods to control agricultural pests

– a) IPM relies on knowledge of • 1. the population ecology of the pest• 2. its associated predators and parasites• 3. crop growth dynamics

– b) One objective of IPM is to minimize environmental and health risks by relying on natural biological control when possible