Chapter 8
Core Case Study:
Southern Sea Otters: Are They Back from the Brink of Extinction?
 They are classified as a keystone species
o Help keep sea urchins and other kelp-eating species from depleting highly
productive kelp forests that provide habitats for a number of species in offshore
coastal waters
 They are making a comeback
o Almost went extinct in the 1900’s
 Population dynamics: is a study of how and why populations change in their
distribution, numbers, age structure, and density in response to changes in environmental
conditions
Population Dynamics and Carry Capacity
Population Distribution
 There are three general patterns of population distribution/dispersion in a habitat
o Clumping
 The location and size of these clumps vary with availability
 Why?
 The resources a species needs vary greatly in availability from
place to place
 Living in herds/flocks/schools can provide some animals with
better protection from predators and population decline
 Living in packs gives some predator species such as wolves a
better chance of getting a meal
 Some animal species form temporary groups for mating and caring
for their young
o Uniform dispersion
 Better access to scarce water resources
o Random dispersion
 Species that do this are fairly rare
 Most individuals in the populations of most species live in clumps or groups
Changes in Population Size: Entrances and Exits
 Populations increase through births and immigration and decrease through disease and
emigration
 The four variables that govern changes in population size are:
o Births
o Deaths
o Immigration—arrival of individuals from outside the populations
o Emigration—departure of individuals from the population
 Population change= (Births + Immigration) – (Deaths + Emigration)
Age Structure: Young Populations Can Grow Fast
 How fast a population grows or declines depends on its age structure
 Age Structure: the proportions of individuals at various ages
o Can have an effect on how rapidly its size increases or decreases
 Prereproductive Ages: not mature enough to reproduce
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Reproductive Ages: those capable of reproduction
Postreproductive Ages: those too old to reproduce
The size of a population made up mostly of individuals in their reproductive ages or soon
to reach these ages is likely to increase
 A population dominated by individuals past their reproductive age will tend to decrease
 The size of a population with a fairly even distribution among these three age groups
tends to remain stable
o Because reproduction by younger individuals is roughly balance by the deaths of
older individuals
Limits on Population Growth: Biotic Potential versus Environmental Resistance
 No population can increase its size indefinitely
 Biotic potential: populations vary in their capacity for growth
 Intrinsic rate of increase (r): the rate at which a population would grow if it had
unlimited resources
 A rapidly growing population reaches some sixe limit imposed by a shortage of one or
more limiting factors
 There are always limits to population growth in nature
 Environmental resistance: consists of all factors that act to limit the growth of a
population
o This is an example of negative and corrective feedback
 Biotic potential and Environmental resistance together determine:
o Carrying Capacity (K): the maximum population of a given species that a
particular habitat can sustain indefinitely without degrading the habitat
 The growth rate of a population decreases as its size nears the carrying capacity of its
environment
o Because resources such as food and water begin to dwindle
Exponential and Logistic Population Growth: J-Curves and S-Curves
 With ample resources a population can grow rapidly, but as resources become limited, its
growth rate slows and level off
 A population with few, if any, resource limitations grows exponentially at a fixed rate
such as 1% or 2% a year
 Exponential/geometric growth starts slowly but then accelerates as the population is
increasing
 Logistic Growth: involves rapid exponential population growth followed by a steady
decrease in population growth with time until the population size levels off
o This slowdown occurs as the population encounters environmental resistance and
approaches the carrying capacity of its environment
o After leveling off, a population with this type of growth typically fluctuates
slightly above and below the carrying capacity
 Changes in the population sizes of keystone species can influence the composition of
communities by causing decreases in populations of species dependent on such keystone
species and increases in the populations of species that move in to occupy part or all of
their ecological niches
Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size
 When a population exceeds its resource supplies, many of its members will die unless
they switch to new resources or move to an area with more resources
 Overshoot: when populations use up their resource supplies and temporarily or exceed
the carrying capacity of their environment
 Reproductive Time Lag: the period needed for the birth rate to fall and the death rate to
rise in response to resource overconsumption
o Populations will suffer a dieback or a crash, unless the excess individuals can
switch to new resources or move to an area with more resources
 The carrying capacity of an area or volume in not fixed
o Sometimes when a population exceeds the carrying capacity of an area, it causes
damage that reduces the area’s carrying capacity
 Over time species may increase their carrying capacity by developing adaptive traits
through natural selection that reduce environmental resistance to their population growth
 Carrying capacity can increase or decrease seasonally and from year-to-year because of
variations in weather, climate, and other factors
 Some species nearing their carrying capacity may be able to keep growing in size by
migrating to other areas with more resources
 Technological, social, and other cultural changes have extended the earth’s carrying
capacity for humans
 Can we continue to expand the earth's carrying capacity for humans?
o a. No. Unless humans voluntarily control their population and conserve resources,
nature will do it for us.
o 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
 A population’s density can affect how rapidly it can grow or decline, but some population
control factors are not affected by population density
 Population Density: the number of individuals in a population found in a particular area
or volume
 Higher population densities may help sexually reproducing individuals find mates but can
also lead to increased competition for mates, food, living space, water, sunlight and other
resources
 High densities can help shield come members of predators but can also make large
groups vulnerable to predators such as humans
 Close contact among individuals in dense populations can increase the transmission of
infectious disease
 Density-dependent factors tend to regulate a population at a fairly constant size, often
near the carrying capacity of its environment
o Includes diseases
o Mostly abiotic, can kill members of a population are density independent
 Meaning, their effect is not dependent on the density of the population
 Can include floods, hurricanes, fire, pollution, and habitat
destruction
Types of Population Change Curves in Nature
 Population sizes may stay about the same, suddenly increase and then decrease, vary in
regular cycles, or change erratically
 Four general patterns of variation in population size
o Stable
 Fluctuates slightly above and below carrying capacity
o Irruptive
 Populations explode and then crash to a more stable level or in some cases
to a very low level
o Cyclic
 Populations fluctuate and regular cyclic or boom-and-bust cycles
 Sine curve (going high up and very low from the K-line)
o Irregular
 Erratic changes possibly due to chaos or drastic change
 Ecologists distinguish between top-down population regulation by predation and bottomup regulation by the scarcity of one or more resources
 Population sizes often vary in regular cycles when the predator and prey populations are
controlled by the scarcity of resources
Case Study: Exploding White-Tailed Deer Populations in the United States
 Since the 1930s the white-tailed deer population in the U.S. has exploded
 Today 25-30 million white-tailed deer in U.S. pose human interaction problems.
o Deer-vehicle collisions (1.5 million per year).
o Transmit disease (Lyme disease in deer ticks).
Reproductive Patterns
Ways to Reproduce: Sexual Partners Not Always Needed
 Some species reproduce without having sex and others reproduce by having sex
 Two types of sexual reproduction
o Asexual Reproduction: in which offspring are generally exact genetic copies
(clones) of a single parent
 Common is single-celled bacteria
 Each cell can divide to produce two identical cells that are replicas of the
original
 Many plants and animals such as coral reefs reproduce this way
o Sexual Reproduction: which mixes the genetic material of two individuals and
produce offspring with combinations of genetic traits from each parent
 Three disadvantages
 Males do not give birth: females have to produce twice as many
offspring as an asexually reproducing organism does to maintain
the same number of young in the next generation
 There is an increased chance of genetic errors and defects during
the splitting and recombination of chromosome
 Courtship and mating rituals consume time and energy: can
transmit diseases, and can inflict injury on males of some species
as they compete for sexual patterns
 Two important advantages
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Provides greater genetic diversity in offspring
o A population with many different genetic possibilities has a
greater chance of reproducing when environmental
conditions change than does a population of genetically
identical clones
 Males of some species can gather food for the females and the
young and protect and help train the young
Reproductive Patterns: Opportunities and Competitors
 While some species have a large number of small offspring and give them little parental
care, other species have fewer offspring and take care of them until they can reproduce
 r-selected species: species with a capacity for a high rate of population increase
o The species have many, usually small, offspring and give them little or no
parental care or protection
o Overcome the loss of so many offspring by producing so many of them that a few
will survive
o Such species tend to be opportunists
 The reproduce and disperse rapidly when conditions are favorable or when
a disturbance opens up a new habitat or niche for invasion
 However, once established, their populations may crash because of
unfavorable changes in environmental conditions or invasion by more
competitive species
 K-selected species: tend to reproduce later in life and have small number of offspring
with fairly long life spans
o Typically the offspring if such species develop inside their mother (where they are
safe), are born fairly large, mature slowly, and are cared for and protected by one
or both parents until they reach reproductive age
o This reproductive pattern results in a few big and strong individuals that can
compete for resources and reproduce a few young to begin their life cycle again
o They tend to do well in competitive conditions when their population size is near
the carrying capacity (K) of their environment
o Their populations typically follow logistic growth
o They are prone to extinction
 The availability of a suitable habitat for individuals of a population in a particular area
determines its ultimate population size
 r-selected species tend to be opportunists while K-selected species tend to be competitors
Survivorship Curves: Short to Log Lives
 The population of different species vary in how long individual members typically live
 Individuals of species with different reproductive strategies tend to have different life
expectancies
 Survivorship curve: the way to represent the age structure of a population
o Three types
 Late loss population: live to an old age
 Constant loss population: die at all ages
 Early loss population: most members die at young ages
 Life table: show the projected life expectancy and probability of death for individuals at
each age in a survivorship curve
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