II. Population Dynamics

A. Population: "all the members of a given species in a given area"

B. Biotic Potential versus Environmental Resistance

1. Biotic Potential - reproductive rate

2. Environmental Resistance - biotic and abiotic factors limiting population size
a. Recruitment  the ability to survive environment resistance factors and
become part of the breeding population.
b. Replacement level: when recruitment is just enough to replace the adults.

3. Reproductive Strategies
a. High biotic potential and low recruitment  bacteria, flies, rabbits
b. Low biotic potential and high recruitment  humans, elephants, primates

C. Growth Curve: how biotic potential and environmental resistance combine to cause a
population to grow or decline.

1. J-curve: before environmental resistance factors kick in to reduce population size

2. S- curve: a balance between environmental resistance and biotic potential
a. Population equilibrium - deaths equal births
Fig. 4.2 The J-curve (blue) demonstrates population growth under optimal conditions, with no restraints. The S-curve (green) shows a population at equilibrium.
Fig. 4.4 A stable population in nature is the result of the interaction between factors tending to increase population (biotic potential) and factors tending to decrease population (environmental resistance).
D. Carrying Capacity: "The maximum population of an organism that a given habitat
will support without the habitat being degraded over the long term." This is the upper
limit on the size of a population.

E. Density Dependence and Critical Numbers

1. The density of a population influences how much impact certain environmental
factors will have on continued population growth. These factors are called
density-dependent factors.
a. Food is a density dependent factor; the more individuals competing for the
same food source, the more difficult it is to get food.

2. The impact of some environmental factors is not dependent on the density of the
population. These factors are called density-independent factors.
a. Density-independent factors include an unusual heat wave or hard freeze. If a
particular limiting factor moves outside an organism's range of tolerance, then the
organism dies irrespective of how many there might be in the population. A spill
of pesticides into a river (as happened on the upper Sacramento River in the
early 1990s because of a train accident) resulted in the sudden death of the river
ecosystem that was not related to the population size of any species.

3. Critical Number - the minimum number of organisms necessary for a species to
a. If the number of organisms drops below the critical number, extinction is
almost certain.
b. As a population nears its critical number density  independent factors become
very important. A single fire, hurricane, toxic chemical spill, housing
development, or logging operation could eliminate the species.
c. The most difficult aspect of this concept is that we do not know the critical
number for most species. To determine the critical number for a species we must
need to know its biotic potential, all its environmental resistance factors, the
range of possible values for each of the factors, the possible interactions among
the factors, etc.
III. Mechanisms of Population Equilibrium

A. Predator-Prey and Host-Parasite Dynamics

1. The interaction between predator and prey and host and parasite keeps both
populations in balance. As the population size of the prey or host increases, the
population size of the predator or parasite increases because there is more food. As
the predator or parasite population increases, the number of prey or hosts declines
because the predator or parasite has eaten them.

2. The reintroduction of wolves to Yellowstone had results that nobody anticipated.
Changes included increased riparian habitat plant diversity (elk, to protect themselves
from wolves, spend less time along rivers and more time in the trees), increased song
bird numbers and type (elk no longer trample riparian plants, and bird habitats have
returned), and increased raptor numbers (wolves have decreased coyote population,
causing an increase in the rodent population, which has provided raptors with more

B. Introduced Species - examples abound: islands are perfect for explaining how
specialized and isolated ecosystems are especially vulnerable. An introduced species is one
that has no (or few) predators and the other species in the ecosystems have not had time
to adapt to the habitats and niches it might provide. It will take many hundreds or t
thousands of years for species to adapt to the habitats and niches provided by the invader.

C. Territoriality

1. The territory size is determined by the amount of space needed to successfully
raise offspring. Territory is important when resources are limited.

2. Birds use song to announce territory. Physical conflict is unimportant in avian species.
Fig. 4.5 This figure shows wolf and moose populations on Isle Royale from 1955 to 2000.
D. Plant-Herbivore Dynamics

1. Herbivores consume plants and therefore keep the size of a plant population
in check. (This is one of the mechanisms in plant competition.) If there are too
many herbivores, the herbivores can consume the plants faster than the plants
can reproduce. This is called overgrazing.

2. "For sustainability, the size of consumer populations is controlled so that
overgrazing or overuse does not occur"  the third basic principle of ecosystem

E. Competition between Plant Species

1. Habitats and niches reduce the competition between plants. Different
combinations of soil types, moisture, temperature, light, etc., produce different
habitats and niches. Plants (and other organisms) have adapted to different
biotic and abiotic requirements.

2. Competition between plants is limited when plants release chemicals to inhibit
the growth of other plants, when grazers reduce the population of plants, when
parasites, viruses, and other plant pathogens reduce the vigor, reproductive
capacity, or life span of plants.

3. Mutualism reduces competition. Epiphytes live on the branches of trees and
the epiphyte is thought to provide nutrients captured by rainfall to the trees.
Fig. 4.11 In 1944, a population of 29 reindeer was introduced onto St. Matthew Island
where they increased exponentially to about 6,000 and then died off due to overgrazing.
III. Mechanisms Producing Biodiversity

A. Ecological Succession: the transition from one biotic community to another

1. Because different niches and habitats form during the transition from one
biotic community to another, circumstances are favorable for the existence of
a large number of species.

2. "Ecological succession is not a matter of new species developing, or even
old species adapting, to new conditions. It is a matter of populations of
existing species taking advantage of a new area as conditions become
favorable." The beginning stage:

a. Primary succession
·If an area has not been occupied by organisms previously, the
initial invasion and progression from one biotic community to the
next is called primary succession.
·Soil and soil organisms do not exist prior to the beginning of this
successional process.
b. Secondary succession
·If an area has been previously occupied by organisms and
something has occurred to leave only the bare soil, then the
invasion and progression from one biotic community to the next
is called secondary succession.
·Soil and soil organisms exist prior to the beginning of this
successional process.
c. An artificial construct  the climax ecosystem
·Succession does not go on indefinitely. A stage is reached during
which there is not continued change. The species are in dynamic
balance with one another and with the physical environment.
·The major biomes are descriptions of climax ecosystems.
·In any climax ecosystem there are other, earlier, stages of
succession present. Without the presence of these earlier stages,
species would be lost and ecosystems could not recover from

B. Ecosystem Disturbance

1. Disturbances provide habitat for a wide array of species. In any area, all
stages of succession are likely to be represented because of large and small

2. Fire - a necessary factor for diverse number of species
a. Certain species e.g., the fire pines, are dependent on fire. Without fire their
cones do not  open, and the bare ground necessary for seed germination does
not exist.
b. Other species are adapted to fire conditions. Manzanita will sprout rapidly
after a fire as long as its extensive root system has not been damaged. The
wetter portions of the Great Plains were dependent on fire to maintain the
c. Fire helps maintain a balance among species (in a forest, fire favors pines,
lack of fire favors broadleaf trees) or may release nutrients that have not
decomposed because of arid conditions.
d. Creates pockets of secondary succession.

3. Hurricanes in southern Florida the Everglades depend on the periodic
hurricanes for the continued existence of the mangrove. Plant species that
compete with the mangrove are reduced during a hurricane. Pockets of secondary
succession are created.

4. Disturbance can be a problem when a species is close to its critical number. If
humans have caused a population to decrease close to its critical number and a
density independent disturbance event occurs, then a species can become extinct.
Fig. 4.15 A plant species may experience a population explosion as it invades an open area. If it dies back and is held down by a herbivory, space is opened up for a second invader, which may experience the same fate.
Fig. 4.17 Reinvasion of an agricultural field by a forest ecosystem occurs in the stages shown.
Fig. 4.18 Ponds and lakes are gradually filled and invaded by the surrounding land ecosystem.
IV. Equilibrium and Non-Equilibrium Systems

A. Equilibrium Systems

1. "Equilibrium theory is the view that ecosystems are maintained by 
balances between species."
2. The mechanisms of population equilibrium are thought to create this
balance between species in an ecosystem.
3. As a biotic or abiotic factor increases (or decreases) then species that
exploit that biotic or abiotic factor increase (or decrease), resulting in a
decline (or increase) in the biotic or abiotic factor and the balance
re-establishing itself.

B. Non-equilibrium Systems

1. Disturbance events are non-equilibrium events.
2. Disturbance events are important in structuring ecosystems; organisms
are removed, populations are reduced and opportunities for other species
are created.

V. Fourth Principle of Ecosystem Sustainability

A. For sustainability, biodiversity is maintained.

B. This derives from three concepts:
1. "The most stable population equilibria are achieved by a diversity of
natural enemies."
2. "Simple systems, especially monocultures, are inherently unstable."
3. "Most or all succession depends on a preservation of biodiversity, and
succession underlies the ability of an ecosystem to recover from damage."