1. Ch. 19 Population Ecology

2. Population Ecology is the Study of How and Why Populations Change
A population is a group of individuals of a single species that occupy the same general area
Individuals in a population
rely on the same resources
are influenced by the same environmental factors
are likely to interact and breed with one another

3. Population Ecology is the Study of How and Why Populations Change
Population ecology is concerned with
the changes in population size
factors that regulate populations over time
Populations
increase through birth and immigration to an area
decrease through death and emigration out of an area.

4. Density and Dispersion Patterns are Important Population Variables
Population density is the number of individuals of a species per unit area or volume
Examples of population density include
the number of oak trees per square kilometer in a forest or
the number of earthworms per cubic meter in forest soil
Ecologists use a variety of sampling techniques to estimate population densities

5. Density and Dispersion Patterns are Important Population Variables
Within a population’s geographic range, local densities may vary greatly
The dispersion pattern of a population refers to the way individuals are spaced within their area

6. Density and Dispersion Patterns are Important Population Variables
Dispersion patterns can be clumped, uniform, or random
In a clumped dispersion pattern
resources are often unequally distributed
individuals are grouped in patches.

7. Density and Dispersion Patterns are Important Population Variables
In a uniform dispersion pattern, individuals are
most likely interacting and equally spaced in the environment

8. Density and Dispersion Patterns are Important Population Variables
In a random dispersion pattern, individuals are spaced in an unpredictable way, without a pattern, perhaps resulting from random dispersal of windblown seeds

9. Life Tables Track Survivorship in Populations
Life tables track survivorship, the chance of an individual in a given population surviving to various ages
Survivorship curves plot survivorship as the proportion of individuals from an initial population that are alive at each age
There are three main types of survivorship curves:
Type I survivorship curves
High survival in early and middle life, followed by a rapid decline in survival later in life
Typical of species that produce few offspring but care for them well
Ex’s. humans, large mammals

10. Life Tables Track Survivorship in Populations
Type II curves
Constant mortality rate/survival is experienced regardless of age; so survivorship is independent of age
Ex. birds, some lizards, rodents
Type III curves
Low survivorship for the very young followed by high survivorship for those individuals that survive to a certain age
Characteristic of species that produce a large number of offspring
Ex. most marine invertebrates, fish, sea turtles

11. Percentage of maximum life span
100 I
Table 36.3 Life table for the U.S. population in 2008 10 II
Percentage of survivors (log scale) 1
III
0.1
Percentage of maximum life span

12. Few large offspring, low mortality until old age
Figure 36.UN01
Few large offspring, low mortality until old age I
Percentage of survivors
Many small offspring, high mortality II
III
Figure 36.UN01 Reviewing the concepts, 36.3
Percentage of maximum life span

13. Population Growth Predicted by the Exponential Growth Model
The rate of population increase under ideal conditions is called exponential growth
It can be calculated using G = rN where
G is the growth rate of the population
N is the population size, and
r is the per capita rate of increase (the average contribution of each individual to population growth).
Eventually, one or more limiting factors will restrict population growth

14. Table 36.4a Exponential growth of rabbits, r = 0.3

15. Exponential Population Growth
500
450
400
350
300
250
200
150
100 50
G = rN
Population size (N)
Figure 36.4a-0 Exponential growth of rabbits
Time (months)

16. Population Growth Predicted by the Logistic Growth Model
Is a description of idealized population growth that is slowed by limiting factors such as an increase in population size
Includes a new expression (K) that describes the effect of limiting factors on an increasing population size
K stands for carrying capacity, the maximum population size a particular environment can sustain.

17. Breeding male fur seals (thousands)10 8 6 4 2
Breeding male fur seals (thousands)
Figure 36.4b-0 Growth of a population of fur seals
Year
Data from K. W. Kenyon et al., A population study of the Alaska fur-seal herd, Federal Government Series: Special Scientific Report—Wildlife 12 (1954).

18. A comparison of Exponential and Logistic Growth Models
G = rN
(K − N)
Number of individuals (N) K
G = rN K
Figure 36.4c Logistic growth and exponential growth compared
Time

19. Multiple Factors May Limit Population Growth
The logistic growth model predicts that population growth will slow and eventually stop as population density increases
Density-dependent factors = limiting factors as a result of increased density
Ex. Intraspecific competition- competition between individuals of same species for limited resources (food, nutrients, nesting sites)

20. Figure 36.5a-0 6 5 4 3 2 1
Mean number of offspring per female
Figure 36.5a-0 Declining reproductive success of song sparrows (inset) with increasing population density
Density of females
Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: 805–882 (1992).

21. Proportional mortality
Figure 36.5b-0
Kelp perch
1.0
0.8
0.6
0.4
0.2
Proportional mortality
Figure 36.5b-0 Increasing mortality of kelp perch (inset) with increasing density
Kelp perch density (number/plot)
Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-Dependent Mortality of a Marine Fish, Ecology 82: 245–257 (2001).

22. Some Populations have “boom-and-bust” Cycles
Some populations fluctuate in density with regularity
Boom-and-bust cycles may be due to
food shortages
predator-prey interactions
Ex. populations of the snowshoe hare and the lynx based on the number of pelts sold by trappers in northern Canada to the Hudson Bay Company over a period of nearly 100 years

23. Hare population size (thousands) Lynx population size (thousands)
Figure
160
Snowshoe hare
120 9
Lynx
Hare population size (thousands)
Lynx population size (thousands) 80 6
Figure Population cycles of the snowshoe hare and the lynx 40 3
1850
1875
1900
1925
Year
Data from C. Elton and M. Nicholson, The ten-year cycle in numbers of the lynx in Canada, Journal of Animal Ecology 11 : 215–244 (1942).

24. Some Populations have “boom-and-bust” Cycles
But what causes the boom-and-bust cycles of snowshoe hares?
One hypothesis proposed that when hares are abundant, they overgraze their winter food supply, resulting in high mortality.
Another hypothesis attributed hare population cycles to excessive predation.

25. Evolution Shapes Life Histories
The traits that affect an organism’s schedule of reproduction and death make up its life history
Key life history traits include
age of first reproduction
frequency of reproduction
number of offspring
amount of parental care
Student Misconceptions and Concerns
 Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape.
Teaching Tips
 Compromise is a key principle of biology. No adaptation can be perfect, and no reproductive strategy can maximize all types of efforts. As the text notes, an organism cannot have a great number of offspring and invest great amounts of parental care in each one. Resources, including time, are limited. Have students imagine how different their lives would have been if they had been born as one of a set of quadruplets—or if they were faced with the task of rearing four children at once!
Active Lecture Tips
 See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

26. Evolution Shapes Life Histories
Populations with r-selected life history traits
grow rapidly in unpredictable environments, where resources are abundant
have a large number of offspring that develop and reach sexual maturity rapidly
offer little or no parental care

27. Evolution Shapes Life Histories
Populations with K-selected traits
tend to be long-lived animals (such as bears and elephants) that develop slowly and produce few, but well-cared-for, offspring
maintain relatively stable populations near carrying capacity
Most species fall between these two extremes