Chapter 4
Ecosystems: How They Change
Introduction
Section 4.1
Dynamics of Natural Populations
Dynamics of Natural Populations
• In any population, births and deaths will cause
the population to grow or shrink
• If births and deaths are ore or less equal over
time, the population is said to be in
equilibrium
Population Growth Curves
• Exponential Increase
– Every species has the capacity to increase its
population when conditions are favorable
– Growth under absolutely ideal conditions will be
exponential
• Ex – a pair of rabbits producing 20 offspring, 10 male,
10 female, may grow by a factor of 10 each generation
– When graphed, produces a J-Shaped Curve
• Leads to population explosions
Population Growth
• Populations may increase exponentially for a
time, but then one of two things may occur
– 1. Natural mechanisms may cause the population
to level off and continue in a dynamic equilibrium
• When this happens it is graphed as an S-Shaped Curve
– 2. In the absence of natural enemies, the
population keeps growing until it exhausts
essential resources and then dies off
• When this happens it is graphed as the opposite of a JShaped Curve
Two Types of Growth
The J-Curve (blue) demonstrates population growth under optimal conditions, with
no restraints. The S-curve (green) shows a population at equilibrium. The horizontal
line (red) shows the carrying capacity of the environment for that population. Notice
how the J-curve spikes well above and then crashes below the carrying capacity,
whereas the S-curve rises up to the carrying capacity and then oscillates between
slightly above and slightly below it
Equilibrium Populations
• Natural ecosystems are made up of
populations that are usually in the dynamic
equilibrium
– Represented by an S-shaped curve
• J-curve come about when there are unusual
disturbances, such as the introduction of a
foreign species, elimination of a predator, or
the sudden alteration of a habitat
Gypsy Moth Caterpillar
An introduced species that has often caused massive defoliation of oak
trees, now seems to been brought under natural control in forests
Biotic Potential
• The ability of a population to increase is
known as biotic potential
– Number of offspring that a species may produce
under ideal conditions
– Biotic potential of different species varies greatly
• To have an effect on future generations, offspring must
survive and reproduce
– Survival through the early growth stages to become part of
the breeding population is called recruitment
Recruitment
• Replacement level recruitment
– When just enough offspring are born to replace
the adults
• Population will remain in equilibrium
– If there is less offspring, populations will decrease, and if
there are more offspring, populations will increase
Reproductive Strategies
• Two common reproductive strategies in the
natural world
– r – selected species, K-selected species
• Variable r = population growth rate
• Variable K = carrying capacity
r – Selected Species
• 1 – produce massive numbers of young, but
then leave survival to the whims of nature
– Often results in very low recruitment
– Species has a high biotic potential, but the
population will not increase because of high
mortality of the young
– Organisms with this strategy are usually small,
with rapid reproductive rates and short life spans
K-Selected Species
• Much lower reproductive rate (lower biotic
potential)
– Care for and protect young until they can compete
for resources with adult members
– Larger, longer lived, and well adapted to the
normal environmental fluctuations
Environmental Resistance
• Biotic and abiotic factors tend to cause
mortality in populations, and limit a
population’s increase
– Biotic factors that cause resistance
• Predators, parasites, competitors, lack of food
– Abiotic factors that cause resistance
• Unusual temperatures, moisture, light, salinity, pH, lack
of nutrients, fire
Carrying Capacity
• Maximum population of a species that a given
habitat can support without the habitat being
degraded over the long term
Density Dependence
• The size of a population generally remains
within a certain range hewn environmental
resistance factors are density dependent
– As the number of individuals per unit area
increases (population density) , environmental
resistance becomes more intense and causes an
increase in mortality that ceases population
growth, and vice, versa
Density Independence
• Factors in the environment that cause
mortality no matter what the population
density is
– Frequently true of abiotic factors
• Ex – deep freeze during spring germination, fires
Critical Number
• The survival and recovery of a population
depends on a certain minimum population
base, which is referred to as the population’s
critical number
Endangered Species Act
• Calls for the recovery of two categories of
species
– Species whose populations are declining rapidly
are classified as threatened
– Population that is near what scientists believe to
be its critical number, is classified as endangered
• These definitions, when officially assigned by the U.S.
Fish and Wildlife Service, set into motion a number of
actions aimed at the recovery of the species in question
Section 4.2
Mechanisms of Population
Equilibrium
Mechanisms of Population Control
• Top-Down Regulation
– Control of a population by predation
• Bottom-Up Regulation
– Most important control of a population occurs as
a result of the scarcity of some resource
Predator-Prey Dynamics
Parasites
• Affect the populations of their hosts
organisms in a density-dependent manner
– As population density of the host increases,
parasites and their vectors (agents that carry the
parasites from one host to another), will have
little trouble finding new hosts
Plant-Herbivore Dynamics
• Overgrazing
– If herbivores eat plants faster than the plants can
grow, the plants will eventually be depleted and
the animals will suffer.
In 1944, a population of 29 reindeer (5 males, and 24 females) was
introduced onto St. Matthew Island, where they increased exponentially to
about 6,000 and then died due to overgrazing
Predator Removal
• Eliminating predators or other natural
enemies upsets basic plant-herbivore
relationships in the same way as introducing
an animal without natural enemies.
– Ex – Sea urchins harm coastal marine ecosystems
of eastern Canada
• This is due to the over harvest of lobsters which are a
predator of sea urchins.
Keystone Species
• In the West Coast rocky intertidal zone, a sea
star species feeds on mussels (herbivores that
feed on plankton), thus keeping mussels from
blanketing the rocks
– As a result, barnacles, limpets, anemones, whelks,
and other invertebrates are able to colonize the
habitat
Keystone Species
• Ecologist Robert Paine experimentally
removed the sea star from the shoreline, and
the mussels crowded everything, decreasing
biodiversity
• Paine referred to the star as a keystone
species
– Species that has a crucial role in maintaining the
integrity of an ecosystem
Competition
• Species may compete for scare resources
• When they do, their ecological niches overlap
• Competition is a form of bottom-up regulation
because it occurs only when a resource is in
limited supply
Intraspecific Competition
• Competition from members of the same species
– Territory
• Territoriality refers to individuals or groups defending a
territory against the encroachment of others of the same
species
• In territoriality, what is being protected is an area suitable
for nesting, for establishing a harem, or for adequate food
resources
– Self – Thinning
• When crowded conditions lead to competition for resources
Effects on Species
• Competition for scare resources led Charles
Darwin to identify the survival of the fittest as
one of the forces in nature leading to
evolutionary changes in species
– “Fit” meaning the ability to have offspring
– Certain organisms have adaptations to their
environment that makes them more “fit” than
others, thus passing on that desired adaptation
Interspecific Competition
• Competition different species compete
Introduced Species
• Rabbits
– 1859 rabbits were introduced into Australia from
England to be used for sport shooting
• Rabbits had no natural enemies capable of controlling the
population, therefore, it exploded.
– Devastated native marsupials and sheep
– Was temporarily brought under control by introducing a virus to
the rabbits
» Over time rabbit became resistant to virus and population
continued Exploding
» Today rabbits are still Australia’s most destructive pest
animal, costing farmers $100 million in agriculture each year
In the first photo, the island is largely devoid of vegetation and heavily
eroded. Following the eradication of the rabbits in 1988,the island vegetation
recovered spectacturaly
Introduced Species
• American Chestnut
– Prior to 1900, dominant tree in eastern deciduous
forest of the U.S. was the American chestnut
– In 1904 a fungal disease called chestnut blight was
accidentally introduced when some Chinese
chestnut trees carrying the disease were planted
in New York
– Fungus spread and killed nearly every American
chestnut tree by 1950
Introduced Species
• Zebra Mussel
– Introduced into the Great Lakes with the discharge
of ballast water from European Ships
– Mussels are now spread throughout the
Mississippi River basin and cause ecological and
commercial damage
• Displace native mussel species and clog water0intake
pipes
Lessons from Introduced Species
• Regulation of populations is a matter of complex
interactions among the members of the biotic
community
• Relationships are specific to the organisms in
each particular ecosystem
• Therefore, when a species is transported over a
physical barrier from one ecosystem to another, it
is unlikely to fit into the framework of the
relationships in the new biotic community
Introduced Species
• In some cases, the introduced species simply
joins the native flora or fauna, or will be put
under too many environmental strains and die
out
• In other cases, the species becomes INVASIVE
– When the conditions are favorable, and there are
no natural predators, the species will thrive and
outcompete native organisms
Section 4.3
Evolution as a Force for Change
Selective Pressures
• Most young organisms in nature do not
survive; instead they fall victim to various
environmental resistance factors
– Parasites, predators, drought
– These factors are known as Selective Pressures
• Each factor can affect which individuals survive and
reproduce and which are eliminated
Natural Selection
• In nature, there is a constant selection, and
consequently, a modification of a species’
gene pool toward features that enhance
survival and reproduction within the existing
biotic community and environment
– Usually in response to selective pressures
– Because the process occurs naturally, it is referred
to as NATURAL SELECTION
Natural Selection
• Discovered independently by Charles Darwin and
Alfred Wallace
• Concept was first published by Darwin in his book
On The Origin of Species by Means of Natural
Selection (1859)
• The change in gene pool of a species by natural
selection over the course of many generations is
the main idea behind biological evolution
– This is significant because Darwin and Wallace
conducted their research, mainly on observations.
• Modern understanding of DNA, mutations, and genetics
wasn’t published until after Darwin’s publication
Adaptations to the Environment
• All traits of any organism can be seen as
features that adapt to the organism for
survival and reproduction
– “Fitness”
Adaptations to the Environment
• Essentially all characteristics of organisms can
be grouped as follows:
– Coping with abiotic factors (climate)
– Obtaining food and water (animals), or nutrients,
energy (plants)
– Finding or attracting mates (animals) or pollinating
and setting seed (plants)
– Migrating (animals) dispersing seed (plants)
Modifications of body shape and color that allow species to blend into the background
and thus protect their populations from predation are among the most amazing
adaptations. First picture = spanworm, second picture = leaf katydid
Limits of Change
• When facing a new, powerful selective
pressure, species have only three choices
– Adaptation
– Migration
– Extinction
Adaptation
• The population of survivors may gradually
adapt to the new condition through natural
selection
– For adaptation to occur, there must be individuals
with traits (alleles – variations of genes) that
enable them to survive and reproduce under the
new conditions
– Also must be enough survivors to maintain a
viable breeding population
Migration
• Surviving populations may migrate and find an
area where conditions are suitable to them
Extinction
• Failing the first two possibilities, extinction is
inevitable
Keys to Survival
• Four key variables among specie that will
affect whether or not a viable population of
individuals is likely to survive new conditions:
– Geographical distribution
– Specialization to a give habitat or food supply
– Genetic variations within the gene pool of the
species,
– Reproductive rate relative to the rate of
environmental change
Vulnerability of different organisms to environmental changes. A summary of factors
supporting the survival and adaptation of species, as opposed to their extinction.
The Evolution of Species
• Prerequisites
– Original population must separate into smaller
populations that do not interbreed with one
another.
• Reproductive isolation is important so that genes stay
separate
– Separated subpopulations must be exposed to
different selective pressures
Darwin’s Finches
Section 4.4
Ecosystem Responses to Disturbance
Ecological Succession
• Over the course of years, a grassy field may
gradually be replaced by a woodland, and in
time the woodland may develop into a mature
forest
– This phenomenon of transition from one biotic
community to another is called ECOLOGICAL
SUCCESSION
• Occurs because the physical environment may be
gradually modified by the growth of the biotic
community itself.
Ecological Succession
• Pioneer species start the process
• As pioneer species grow, they create conditions
that are favorable to more longer-lived colonizers
– This process is called facilitation
• Succession does not occur indefinitely
– A stage of development is eventually reached in which
there is a dynamic balance between all of the species
and the physical environment. The final state is called
a climax community
Primary Succession
• If the area has not been occupied previously,
the process of initial invasion and then
progression from one biotic community to
another is called primary succession.
– Gradual invasion of bare rock or gravel surface
Secondary Succession
• When an area has been cleared by fire or by
humans and then left alone, plants and
animals from the surrounding ecosystem may
gradually reinvade the area
– This is known as secondary succession
– Example – abandoned agricultural field turning
back into a forest
Typical Succession Pattern
• Lichens  mosses  grasses  shrubs 
pine trees  hardwoods 
Reinvasion of an agricultural field by a forest ecosystem occurs in the stages shown.
Primary vs. Secondary
• Main difference between primary and
secondary succession is that secondary
succession starts with preexisting soil
Aquatic Succession
• Succession occurs because soil particles
inevitably erode from the land and settle out
in poinds or lakes, gradually filling them in
• Aquatic vegetation also produces detritus
which contributes to filling in pond/lakes
– As the buildup occurs, terrestrial species can cross
over and live there
• Lake/pond  bog  forest
In this photograph, taken in Banff National Park in the Canadian Rockies, you can
visualize the lake that used to exist in the low-level area. It is now filled it with
sediment and covered by scrub willow. Spruce and fir forest is gradually encroaching.