© 2015 Pearson Education, Inc. P OPULATION S TRUCTURE AND D YNAMICS

© 2015 Pearson Education, Inc.

POPULATION STRUCTUREPOPULATION STRUCTUREAND DYNAMICSAND DYNAMICS


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,and• are likely to interact and breed with one another.


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.


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.



• 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.



• Dispersion patterns can be clumped, uniform, or random.

• In a clumped dispersion pattern

• resources are often unequally distributed and

• individuals are grouped in patches.



• In a uniform dispersion pattern, individuals are

• most likely interacting and equally spaced in the environment.



• In a random dispersion pattern, individuals are spaced in an unpredictable way, without a pattern, perhaps resulting from random dispersal of windblown seeds.


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


• 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

Life tables track survivorship in populations


Percentage of maximum life span

Per

cen

tag

e o

f su

rviv

ors

(lo

g s

cale

)

III

II

I100

10

1

0.1


Figure 36.UN01

Percentage of maximum life span

Per

cen

tag

e o

f su

rviv

ors

I

II

III


Idealized models predict patterns of population growth

• 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.



Time (months)0 1 2 3 4 5 6 7 8 9 10 1112

Po

pu

lati

on

siz

e (N

)

500

450

400

350

300

250

200

150

100

50

0




• Logistic growth model • Is a description of idealized population growth that is slowed

by limiting factors as population size increases.• Includes a new expression 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.


Year1915 1925 1935 1945

10

8

6

4

2

0

Bre

edin

g m

ale

fur

seal

s(t

ho

usa

nd

s)

Data from K. W. Kenyon et al., A population study of theAlaska fur-seal herd, Federal Government Series:Special Scientific Report—Wildlife 12 (1954).


Figure 36.4c

Time

Nu

mb

er o

f in

div

idu

als

(N) G = rN

G = rN(K − N)K

K

0


Multiple factors may limit population growth

• The logistic growth model predicts that population growth will slow (increased deaths, decreased births) 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)


Figure 36.5a-0

Density of females 0 10 20 30 40 50 60 70 80

Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundancein an Insular Song Sparrow Population. Ecology 73: 805–882 (1992).

6

5

4

3

2

1

0Mea

n n

um

be

r o

f o

ffs

pri

ng

per

fe

mal

e


Figure 36.5b-0

Kelp perch density (number/plot)0 10 20 30 40 50 60

Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-DependentMortality of a Marine Fish, Ecology 82: 245–257 (2001).

1.0

0.8

0.6

0.4

0.2

0

Pro

po

rtio

na

l mo

rtal

ity

Kelp perch


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



• 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, and• offer little or no parental care.



• 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 and

• maintain relatively stable populations near carrying capacity.

• Most species fall between these two extremes.

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© 2015 Pearson Education, Inc. P OPULATION S TRUCTURE AND D YNAMICS