CHAPTER 46

ECOLOGY OF POPULATIONS

Ecology

• Etymology: eco, house or home and logy, study of (Greek)

• The study of the interactions of organisms with other organisms and with the physical environment.

• Studies life on many different levels—from individual organisms to the biosphere.

Levels of Organization

• Population—all the organisms within an area that belong to the same species.

• Community—all the various populations that interact in a particular locale, e.g. a coral reef, a forest, a pond, or even a rotten log.

• Ecosystem—a community of populations along with the abiotic environment

• Biosphere—the zones of the earth’s soil, water, and air where living organisms are found.

Population density—number of individuals per unit area or volume, e.g.

m3, m2, hectare, acre, km2

Measuring density:1. Count sample plots (quadrants)2. Use indirect indicators such as nests or

burrows3. Mark-recapture method:

N = (#marked) X (total catch 2nd time)# of recaptures

4. Actual count of individuals

Dispersion—pattern of spacing for individuals within the boundaries of the population.

• Can be due to abiotic factors such as precipitation, average temperature, type of soil, and oxygen level in water.

• Limiting factors—those factors that particularly determine whether an organism lives in an area.

• Patterns of dispersal—clumped, uniform, and random.

• Clumped—patches within the organisms’ range; e.g. social species live in groups where conditions are favorable or plants needing certain soil conditions or amounts of rainfall. Most common type.

• Uniform—individuals are evenly spaced; e.g. nesting area of birds on the ground or in trees because of territoriality.

• Random—no regular spacing; trees in a forest or moose over a suitable habitat.

Figure 52.2 Patterns of dispersion within a population’s geographic range

Figure 52.2ax2 Clumped dispersion: buffalo, swans, fish, lupine

Population Growth

• Population size and density reflect the relative rates of processes that add to or eliminate individuals from the population.

• Additions come from births and immigration.

• Subtractions come from deaths and emigration.

Population Growth• The letter “r” stands for the intrinsic rate of

natural increase.• This intrinsic rate of natural increase can be used

to calculate the growth and size of a population per any given unit of time.

• For example, in a herd of 100 elephants, 10 births and 2 deaths occur during the year.

• Birthrate is 10/100 or 0.10• Deathrate is 2/100 or 0.02• Therefore r is equal to 0.08 per year. (X 100 =

108; second year 0.08 X 108 = 116.6)

Exponential Growth• Characteristic of a small population with

access to abundant resources.

• Also called geometric growth or j-shaped curve when graphed.

• Growth curve has two phases:

a. lag phase—growth is slow because population is small

b. exponential growth phase—growth is accelerating

Biotic Potential• During exponential growth a population is

exhibiting its biotic potential, the maximum population growth under ideal conditions (no limiting factors).

• Example: female housefly lays 300 eggs

generation total #flies total #females

1 1 1

2 300 150

3 45,000 22,500

4 6,750,000

3,375,000

Figure 52.8 Population growth predicted by the exponential model

Figure 52.9 Example of exponential population growth in nature

Human Population Growth

Billion Date Reached Years Required

1 1823 1 million

2 1929 96

3 1961 32

4 1974 13

5 1987 13

6 1999 ? 12

Figure 52.20 Human population growth

Logistic Growth• Represented by a sigmoid or S-shaped curve.• Brings in the concept of carrying capacity, the

maximum number of individuals of a given species the environment can support.

• Has four phases:a. lag phase—growth is slow because the population is smallb. exponential growth phase—growth is acceleratingc. deceleration phase—growth slows downd. stable equilibrium phase—little if any growth because births and deaths are about equal

Figure 52.11 Population growth predicted by the logistic model

Figure 52.12 How well do these populations fit the logistic population growth model?

Figure 52.17 Long-term study of the moose (Alces alces) population of Isle Royale, Michigan

Figure 52.18 Extreme population fluctuations

Mortality Patterns• Cohort—all the members of a population born at

the same time.• Survivorship—the probability of newborn

individuals of a cohort surviving to particular ages.

• There are three types of survivorship curves:Type 1—death comes near end of life span; e.g. humansType 2—death rate fairly uniform throughout life span; e.g. hydrasType 3—high death rate for very young and then decreases for adults; e.g. barnacles

Figure 52.3 Idealized survivorship curves

Age Distribution

• A population contains at least three major age groups: prereproductive, reproductive, and postreproductive.

• Populations differ according to what proportion of a population falls in each age group.

• Because of the postwar “baby boomers” the postreproductive group will soon be the largest group in the United States.

Regulation of Population Size

• Density-independent factors (abiotic factors) such as:– Weather conditions such as a rare freeze in a

tropical region– Natural disasters such as hurricanes and

volcanic eruptions, forest fires– Do not necessarily kill a larger percentage of

individuals in a dense population than in a less dense population

Regulation of Population Size

• Density-dependent factors (biotic factors) include:

--resources—food, shelter, nesting sites

--predation---predators aggregate where prey is dense and it is easier for predators to find the prey

--parasites and pathogens

Regulation of Population Size

• Density-independent factors may amplify the effects of density-dependent factors on population growth.– Heavy snowfall may have low temperatures

below freezing, but may also reduce food sources and increase intraspecific competition.

– A severe cold spell may hit those animals without secure shelters harder than those with shelters. Shelters depend on density of the population.

Regulation of Population Size

• Other regulatory factors may be intrinsic to the organism:

--territoriality and dominance hierarchies affect population size and growth rates; e.g. study of Great Tit in England showed that nesting boxes that were too close remained unoccupied

--some populations may have an innate stability; e.g. Dungeness crab populations have wild fluctuations in population size (termed chaos)

Population Cycles

• Regular fluctuations of populations of birds, mammals and insects; e.g. artic voles and lemmings cycle every 3-4 years and snowshoe hares every 10 years.

• Crowding may regulate cyclical populations, perhaps by affecting endocrine system.

• Hares may cycle because of changes in food source brought about by overfeeding.

Figure 52.19 Population cycles in the snowshoe hare and lynx

Life History Patterns

• Populations vary in number of births per reproduction, age of reproduction, life span, and probability of living the entire life span.

• The logistic population growth model has been used to suggest that some members of some populations are subject to r-selection and some are subject to K-selection.

• r = intrinsic rate of natural increase• K = carrying capacity

r-Selection

• Favors r-strategists

• Found in fluctuating or unpredictable environments

• Density-independent factors will keep populations in the lag or exponential phase of population growth

• Population size is low relative to K

K-selection

• Found in relatively stable environments• Populations tend to be near carrying

capacity• Minimal fluctuations in population size• Resources such as food and shelter can be

relatively scarce, and those best able to compete will have the largest number of offspring

K-strategists vs. r-strategists R-selected1. Many offspring (more

mean more will survive a population crash)

2. Small young3. Rapid maturation with

short life span4. Little or no parental care5. Reproduce once (“big

bang”)6. Often very good

dispersers and colonizers of new habitats

7. Examples: annual plants such as dandelions, insects

K-selected1. Few offspsring2. Larger young3. Slow maturation with a

fairly long life span4. Much parental care5. Reproduce many times (tend to become extinct

when their normal way of life is destroyed.

6. Are specialists rather than colonizers

7. Examples: bears, Florida panther

Figure 52.4 An example of big-bang reproduction: Agave (century plant)

Human Population Growth

• The world’s countries can be divided into two groups:--more-developed countries (MDCs)—countries like the U.S. and European countries that have a low population growth and a good standard of living.--populations doubled from 1850 to 1950 because modern medicine and better socioeconomic conditions caused a decline in death rate--experienced only modest growth from 1950-1975 because of decline in birthrate—called demographic transition

Figure 52.21 Demographic transition in Sweden and Mexico, 1750-1997

--MDCs now have a yearly growth rate of about 0.1% (r = .001)

--some countries such as Germany, Sweden and Italy are not growing or are decreasing in population

--yearly growth of U.S. population = 0.6%

Human Age Structure Diagrams

• LDCs still have a high rate of population growth because they have more women entering reproductive years than older women leaving them.

• Even with each couple having only two children, zero population growth will not occur for some time because of the age structure of populations in LDCs.

Figure 52.22 Age-structure pyramids for the human population of Kenya (growing at 2.1% per year), the United States (growing at 0.6% per year), and Italy (zero growth)

for 1995

Population Growth and Environmental Impact

• MDCs account for only about 25% of the world’s population, but are responsible for 90% of hazardous waste production (72% is from U.S. alone).

• MDCs consume 60% of the world’s fossil fuels.

• An average American family comsumes and produces wastes equivalent of 30 people in India. (Study pie charts on p. 851.)

Figure 52.23 Ecological footprint in relation to available ecological capacity