Lecture 9

Question 1

Population size can be determined by
________ and ________
factors.

Answer 1

density-dependent and density-independent

factors.

Question 2

Some factors are a function of
population density, other are not
dependent on density—

Answer 2

density-dependent factors

Question 3

Cause birth rates, death rates, and dispersal rates
to change as the density of the population changes

As densities increase, birth rates often
decrease, death rates increase, and
dispersal from the population
(emigration) increases, all of which tend
to decrease population size.

Answer 3

density-dependent factors

Question 4

occurs when
density-dependent factors cause
population to increase when density is
low and decrease when density is high.

Ultimately, food, space, or other essential
resources are in short supply and
population size decreases.

Answer 4

Population regulation

Question 5

Population increases 
rapidly at first, then stabilizes at the 
carrying capacity (maximum 
population size that can be supported 
indefinitely by the environment). 

incorporates limits to
growth and shows how a population may
stabilize at a maximum size, the carrying
capacity

Answer 5

Logistic growth

Question 6

Changes in phytoplankton abundance in 
Lake Erie could reflect changes in a 
wide range of environmental factors, 
including nutrient supplies, 
temperature, and predator abundance.

Answer 6

In some populations, fluctuations occur
as increases or decreases in
abundance from an overall mean
value.

Question 7

For some populations, fluctuations
can be large.

Populations may explode, causing
a population outbreak.

Answer 7

Biomass of the comb jelly
Mnemiopsis increased more than
a thousand-fold during a 2-year
outbreak in the Black Sea.

Question 8

Some populations have alternating
periods of high and low abundance at
regular intervals.

Populations of small rodents such as
lemmings and voles typically reach a
peak every 3–5 years.

Answer 8

Population Cycles

Question 9

can cause populations to fluctuate in size

The effects of population density often 
have a lag time or delay. 
Commonly, the number of individuals 
born in a given time period is 
influenced by population densities that 
were present several time periods ago.

Answer 9

Delayed density dependence

Question 10

The occurrence of fluctuations depends
on the values of r and .
Robert May (1976) found that when is
small (0

Answer 10

damped oscillations result.

Question 11

When is large ( > 1.57), the
population fluctuates indefinitely about
the carrying capacity.
This pattern is called a ____ ____ ____

Answer 11

stable limit cycle.

Question 12

A. J. Nicholson studied density
dependence in sheep blowflies in
laboratory experiments.

In the first experiment, adults were
provided with unlimited food, but the
larvae were restricted to 50 g liver per
day.

Answer 12

• populations tend to regulate themselves

- can have different population growth patterns

Question 13

The risk of extinction increases greatly in
small populations.

Many factors can drive populations to 
extinction: 
Predictable (\_\_\_\_\_\_\_\_) factors, as 
well as "fluctuation" in population growth 
rate, population size, and chance 
events.

Answer 13

deterministic

Question 14

Small populations are vulnerable to the effects

of genetic drift for three reasons:

Answer 14

1. Loss of genetic variability reduces the
   ability of a population to respond to future
   environmental change.
2. Genetic drift can cause harmful alleles to
   occur at high frequencies.
3. Small populations show a high frequency of
   inbreeding (mating between related
   individuals)

Question 15

chance
events related to the survival and
reproduction of individuals.

population-level birth and death rates
are constant within a given year, but
the actual fates of individuals differ.

Answer 15

Demographic stochasticity

Question 16

population growth rate 
decreases as population density 
decreases; individuals have difficulty 
finding mates at low population 
densities. 

In small populations, Allee effects can
cause the population growth rate to
drop, which causes the population size
to decrease even further.

Answer 16

Allee effects

Question 17

unpredictable changes in the
environment.

changes in the average birth or death
rates that occur from year to year
because of random changes in
environmental conditions.

Answer 17

Environmental stochasticity—

Question 18

Example:

Answer 18

Heath hen populations were 
reduced by hunting and habitat 
loss to one population of 50 on 
Martha’s Vineyard, 
Massachusetts. 
A reserve was established, and 
population size increased, but 
then a series of bad weather, 
fires, diseases, and predators 
caused the heath hen’s 
extinction.