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
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