Chapter 8

Population Ecology

1 million before settlers They were over-hunted to

the brink of extinction by the early 1900’s for fur

Put on endangered species list in 1977 300 increased to 3000

Southern Sea Otters: Are They Back from the Brink of Extinction?

Figure 8-1

Core Case Study: Southern Sea Otters: Are They Back from

the Brink of Extinction? Sea otters are an

important keystone species

control sea urchins and other kelp-eating organisms.

Kelp forests provide habitat & prevent shore erosion

Figure 8-1

POPULATION DYNAMICS AND CARRYING CAPACITY

Populations change Distribution Numbers Age structure density

changes occur based on resource distribution & environmental conditions

Figure 8-2

POPULATION DYNAMICS AND DISTRIBUTION

Patterns occur based on resource distribution.

Figure 8-2

Fig. 8-2a, p. 162

(a)Clumped: Most common distributionResources are clumpedHerds/packs: provide protection, help hunting,

raising young

Fig. 8-2b, p. 162

(b) Uniform (creosote bush): even spread out to make best useOf scarce resources like rain in dessert

Fig. 8-2c, p. 162

(c) Random (dandelions): randomly scattered - rare

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. (majority here = growing pop) Reproductive age: those capable of

reproduction. Postreproductive age: those too old to

reproduce. ( majority here = declining pop)

Even distribution in age structure = stable pop

Limits on Population Growth: Biotic Potential vs. Environmental

Resistance

Populations vary in capacity for growth Reproduce early & often = high potential 1 fly = 5.6 trillion in 13 months

No pop can grow indefinitely Limiting factors:

• Sunlight, water, nutrients, living space• Predators, competition, disease

Limits on Population Growth: Biotic Potential vs. Environmental

Resistance The intrinsic rate of increase (r) is the rate at

which a population would grow if it had unlimited resources = BIOTIC POTENTIAL

Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

Exponential & Logistic Curves

J – curve

S - curve

Biotic potential

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

Fig. 8-3, p. 163

EnvironmentalResistance

Time (t)

Po

pu

lat i

on

si z

e (N

)

Carrying capacity (K)

ExponentialGrowth

BioticPotential

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

Fig. 8-4, p. 164

Carrying capacity

Year

Nu

mb

er o

f sh

eep

(m

illi

on

s)Overshoot

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.

Fig. 8-6, p. 165

Nu

mb

er o

f re

ind

eer

Populationovershootscarryingcapacity

Carryingcapacity

Year

PopulationCrashes

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.

‣ The number of individuals per unit area (for terrestrial organisms) or volume (for aquatic organisms)

At low population densities, individuals are spaced well apart. Examples: territorial, solitary mammalian species such as tigers and plant species in marginal environments.

At high population densities, individuals are crowded together. Examples: colonial animals, such as rabbits, corals, and termites.

Population Density

High density populations

Low density populations

Population Density and Population Change: Effects of Crowding

Environmental resistance = all the factors that act to limit the growth of a population. Some population control factors have a greater

effect as the population’s density increases.• e.g. biotic factors like disease

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

Density Dependent Factors

The effect increases as population density increases Competition for resources Predation Parasitism Infectious disease

These factors tend to regulate pop at fairly consistent size, often near carrying capacity

‣ The effect doesn’t depend on population’s density – doesn’t matter if crowded together or spaced far apart:

Physical (or abiotic) factorstemperatureprecipitationhumidityaciditysalinity etc.

Catastrophic eventsfloods and tsunamisfiredroughtearthquake and eruption

Density Independent Factors

Types of Population Change Curves in Nature

Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. 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

Fig. 8-7, p. 166

Po

pu

lati

on

siz

e (t

ho

usa

nd

s)

Year

LynxHare

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 (r-selected species).

Fewer, larger offspring with higher invested parental care (K-selected species).

Figure 8-9

Fig. 8-9, p. 168

r species;experiencer selection

Time

Nu

mb

er o

f in

div

idu

als

KCarrying capacity

K species;experienceK selection

Reproductive Patterns

r-selected species tend to be opportunists while K-selected species tend to be competitors.

Figure 8-10

Fig. 8-10a, p. 168

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

r-Selected SpeciesCockroach

Dandelion

Fig. 8-10b, p. 168

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

K-Selected Species

SaguaroElephant

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

Fig. 8-11, p. 169

Per

cen

tag

e su

rviv

ing

(lo

g s

cale

)

Age

Early loss

Late loss

Constant loss