Population Ecology

Chapter 8 Population Ecology

Chapter Overview Questions  What are the major characteristics of

populations?  How do populations respond to changes in environmental conditions?  How do species differ in their reproductive patterns?

Updates Online The latest references for topics covered in this section can be found at the book companion website. Log in to the book’s e-resources page at www.thomsonedu.com to access InfoTrac articles.  

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InfoTrac: One Hatchling at a Time. Brownsville Herald (Brownsville, TX), July 2, 2006. InfoTrac: Where the Cattle Herds Roam, Ideally in Harmony With Their Neighbors. Jim Robbins. The New York Times, July 11, 2006 pF3(L). InfoTrac: A nudge for nature. Milwaukee Journal Sentinel, July 10, 2006. Earth Island Institute Environmental Defense: Creating a Conservation Community in Oregon’s Williamette Valley Marine Bio: Habitat Conservation

Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?  They were over-

hunted to the brink of extinction by the early 1900’s and are now making a comeback. Figure 8-1

Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?  Sea otters are an

important keystone species for sea urchins and other kelp-eating organisms.

Figure 8-1

POPULATION DYNAMICS AND CARRYING CAPACITY

 Most populations live in clumps although other

patterns occur based on resource distribution. Figure 8-2

(a) Clumped (elephants)

Fig. 8-2a, p. 162

(b) Uniform (creosote bush)

Fig. 8-2b, p. 162

(c) Random (dandelions)

Fig. 8-2c, p. 162

Changes in Population Size: Entrances and Exits  Populations increase through births and

immigration

 Populations decrease through deaths and

emigration

Age Structure: Young Populations Can Grow Fast  How

fast a population grows or declines depends on its age structure. 

 

Prereproductive age: not mature enough to reproduce. Reproductive age: those capable of reproduction. Postreproductive age: those too old to reproduce.

Limits on Population Growth: Biotic Potential vs. Environmental Resistance  No population can increase its size

indefinitely. 



The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Exponential and Logistic Population Growth: J-Curves and S-Curves  Populations grow

rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.

Figure 8-4

Environmental Resistance

Population size (N)

Carrying capacity (K)

Biotic Potential Exponential Growth

Time (t)

Fig. 8-3, p. 163

Exponential and Logistic Population Growth: J-Curves and S-Curves  As a population

levels off, it often fluctuates slightly above and below the carrying capacity.

Figure 8-4

Overshoot

Number of sheep (millions)

Carrying capacity

Year

Fig. 8-4, p. 164

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size  Members of

populations which exceed their resources will die unless they adapt or move to an area with more resources.

Figure 8-6

Number of reindeer

Population overshoots carrying capacity Population Crashes

Carrying capacity

Year

Fig. 8-6, p. 165

Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size  Over time species may increase their

carrying capacity by developing adaptations.  Some species maintain their carrying capacity by migrating to other areas.  So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

How Would You Vote? To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu for Living in the Environment.

 Can we continue to expand the earth's

carrying capacity for humans? 



a. No. Unless humans voluntarily control their population and conserve resources, nature will do it for us. b. Yes. New technologies and strategies will allow us to further delay exceeding the earth's carrying capacity.

Population Density and Population Change: Effects of Crowding  Population density: the number of individuals

in a population found in a particular area or volume. 

A population’s density can affect how rapidly it can grow or decline. • e.g. biotic factors like disease



Some population control factors are not affected by population density. • e.g. abiotic factors like weather

Types of Population Change Curves in Nature  Population sizes may stay the same, increase,

decrease, vary in regular cycles, or change erratically. 





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Stable: fluctuates slightly above and below carrying capacity. Irruptive: populations explode and then crash to a more stable level. Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. Irregular: erratic changes possibly due to chaos or drastic change.

Types of Population Change Curves in Nature

 Population sizes often vary in regular cycles

when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7

Population size (thousands)

Hare Lynx

Year

Fig. 8-7, p. 166

Case Study: Exploding White-Tailed Deer Populations in the United States  Since the 1930s the white-tailed deer

population has exploded in the United States. 

Nearly extinct prior to their protection in 1920’s.

 Today 25-30 million white-tailed deer in U.S.

pose human interaction problems.  

Deer-vehicle collisions (1.5 million per year). Transmit disease (Lyme disease in deer ticks).

REPRODUCTIVE PATTERNS  Some species reproduce without having sex

(asexual). 

Offspring are exact genetic copies (clones).

 Others reproduce by having sex (sexual).  



Genetic material is mixture of two individuals. Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly. Major advantages: genetic diversity, offspring protection.

Sexual Reproduction: Courtship  Courtship rituals

consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners.

Figure 8-8

Reproductive Patterns: Opportunists and Competitors  Large number of

smaller offspring with little parental care (rselected species).  Fewer, larger offspring with higher invested parental care (Kselected species). Figure 8-9

Carrying capacity

K

Number of individuals

K species; experience K selection

r species; experience r selection

Time

Fig. 8-9, p. 168

Reproductive Patterns

 r-selected species tend to be opportunists

while K-selected species tend to be competitors.

Figure 8-10

Cockroach

r-Selected Species Dandelion

Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species

Fig. 8-10a, p. 168

K-Selected Species Elephant

Saguaro

Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species

Fig. 8-10b, p. 168

Survivorship Curves: Short to Long Lives  The way to represent the age structure of a

population is with a survivorship curve.   

Late loss population live to an old age. Constant loss population die at all ages. Most members of early loss population, die at young ages.

Survivorship Curves: Short to Long Lives  The populations

of different species vary in how long individual members typically live. Figure 8-11

Percentage surviving (log scale)

Late loss

Co n

st an t

lo ss

Early loss

Age

Fig. 8-11, p. 169