23.1. Scope of Ecology (p. 384)
A. Ecology
Chapter 23 Outline and Terms
23.1. Scope of Ecology (p. 384)
A. Ecology
1. The study of interactions of organisms with their environment.
2. Concept of ecology was first
voiced by German zoologist Ernst Haeckel.
3. Modern ecology studies how
environmental factors determine the distribution and
abundance of populations.
4. Ecology and evolution are
intertwined because natural selection has long-term effects.
5. A population is a group of the
same species occupying a certain area. (Fig. 23.1a)
6. A community consists of all
populations at one locale (e.g., a coral reef population, etc.).
7. An ecosystem contains the
community organisms and abiotic factors (e.g., energy flow,
chemical cycling).
8. Modern ecology is both descriptive
and predictive, with application to wildlife management,
agriculture, etc.
B. Density and Distribution of Populations
1. Density of organisms refers to how
many live per unit of area.
2. Population distribution of
organisms can vary from uniform to random to clumped. (Fig. 23.2)
3. Ecologists study the causes for
"patchiness" of organisms across space and through time.
4. Environment includes both biotic
(living) and abiotic (physical) factors.
5. Physical (abiotic) factors include
types of precipitation and amounts, averages, and daily and
seasonal variations in temperature; type of soil (sand, clay or loam) and few nutrients,
moisture or temperature may serve as limiting factors.
6. Biotic factors are illustrated by
red kangaroos limited to inland Australia by their food source.
23.2.
Characteristics of Populations (p. 386)
A. Population Size
1. Population size is number of
individuals contributing to gene pool of the population.
2. At any one point in time,
populations have a certain size.
3. Future population size depends on
births and deaths, immigration and emigration (often
immigration and emigration are presumed equal).
4. Birthrate and death rate are used
to calculate the net reproductive rate.
5. Net reproductive rate is used to
calculate the growth and size of a population per unit time.
B. Patterns of Population Growth (Fig. 23.3)
1. There are two general patterns:
organisms that reproduce once cease to grow as adults and
expend energy in reproduction and die, and organisms that reproduce through their
lifetime, which invests energy in future survival.
2. Most organisms do not exactly fit
these two patterns.
a. Slime
molds produce spores that swarm together into a sticky mass.
b. Many
plants can reproduce both by seeds and vegetative extensions.
c. Aphids can
switch between sexual and asexual reproduction according to weather.
a. Soil seed
banks allow the next generation to come from many previous years' seeds.
C. Exponential Growth (Fig. 23.4)
1. The J-shaped exponential growth
curve has two phases.
a. Lag phase
(growth is slow because population is small).
b.
Exponential growth phase (growth is accelerating).
2. A mathematical equation calculates
exponential growth and size for any population that has
discrete generations. (Fig. 23.4c)
3. Biotic potential (Fig. 23.6)
a. Exhibited
during esponential growth, this is the maximum population growth under ideal
circumstances.
b. Includes
plenty of room for each member, unlimited resources (e.g., food, water), and no
hindrances (e.g., predators).
4. Environmental resistance curbs
exponential growth; includes all environmental factors that limit
population size.
D. Logistic Growth (Fig. 23.5)
1. Organisms with repeated
reproductive events experience an S-shaped or logistic
growth curve.
2. Pearl (1930) estimated growth in a
yeast and arrived at a graph and formula for logistic
growth.
(Fig. 23.3b,c)
3. In addition to the lag phase and
exponential growth, there is a deceleration phase where
rate of population growth slows down and a stable equilibrium phase with little if any growth
because births equal deaths.
4. Curve is called logistic because
exponential portion of curve would plot as a straight line as log
of N.
5. A mathematical equation calculates
logistics growth (Fig. 23.5c)
6. Environmental resistance results
in the deceleration phase and the stable equilibrium phase;
population is at carrying capacity.
E. Carrying Capacity
1. Carrying capacity (K) is maximum
size population that can be supported by environment year
after year.
2. When N is small, a large portion
of the carrying capacity has not been utilized, but as N
approaches K, population growth slows down because
3. Examples: over fishing drives a
population into the lag phase; it is best to maintain populations
in a growth phase; and reducing crop pests places them in exponential phase
again.
F. Mortality Patterns
1. A life table shows how many
members of a cohort (group born at one time) are surviving at
different ages.
2. Survivorship is the percentage of
remaining survivors of a population over time; usually shown
graphically. (Fig. 23.7) [transp. 126]
a. Type I
survivorship curve: most individuals live out their life span and die of old age
(e.g., humans).
b. Type II
survivorship curve: individuals die at a constant rate (e.g., birds, rodents,
and
perennial plants).
c. Type III
survivorship curve: most individuals die early in life (e.g., fishes,
invertebrates, and
plants).
4. The grass Poa annua is
intermediate; most survive till 6-9 months and then chances of
surviving diminish.
G. Age Distribution (Fig. 23.8) [transp. 127]
1. There are three major age groups
in a population: prereproductive, reproductive and
post reproductive.
2. An age structure diagram is a representation of the number of individuals in each age group
in a population.
3. A pyramid-shape indicates the
population has high birthrates; population is undergoing
exponential growth.
4. A bell-shape indicates that
prereproductive and reproductive age groups are more nearly
equal, with the post reproductive group being smallest due to mortality; this is characteristic
of stable populations.
5. An urn-shaped diagram indicates
the post reproductive group is largest and the
prereproductive group is smallest, a result of the birthrate falling below the death rate; this is
characteristic of declining populations.
23.3. Regulation of Population Size (p. 392)
A. The J-shaped and S-shaped growth
curve models do not always predict real populations.
1. In the
winter moth (pages 394-395), many eggs did not survive winter and exponential
growth did not occur.
2. Growth
curve of reindeer herd introduced to St. Paul Island, Alaska, overshoots
carrying
capacity. (Fig. 23.9)
B. Populations do not increase in
size year after year because environmental resistance,
including both density-independent and density-dependent factors, regulates the number of
organisms.
1. Some
populations were considered to be regulated primarily by density-independent
factors.
a. The number of organisms present does not affect the influence of the factor.
b. The damage to a population from an accidental fire does not change with the
number of
organisms present.
c. Density-independent factors show no correlation with the size of the
population.
2.
Populations regulated by density-dependent factors are affected by the number of
organisms present.
a. Predation, parasitism, competition are considered density-dependent; the more
organisms crowd together, the more damaging are food shortages, parasites, and
predators.
b. Density-dependent factors have some effect relative to the size of the
population.
3. Ecologists
state that most important regulators seem to be weather, food, other animals,
pathogens, habitat.
4. While the
above factors are all external, it is also likely that internal factors
influence
population size.
5.
Recruitment, immigration, and emigration are other means by which complex
organisms
limit densities.
6. New
theories on chaos help us understand severe fluctuations over time.
23.4.
Life History Patterns (p. 396)
A. The logistic population model
predicts two main life history patterns.
1.
r-Selection
a. Species that underwent selection to maximize their rate of natural increase
are
categorized r-selected.
b. These populations are often opportunistic species, which tend to be
colonizers.
c. Strategy for continued existence is based on individuals having the following
traits: 1)
small size, 2) short life span, 3) mature fast, 4) produce many offspring, and 5) engage
in little caring of offspring. (Fig. 23.13)
d. Such populations usually exhibit a survivorship curve similar to type III.
(Fig. 23.7c)
[transp. 125]
e. Thus, they rely on rapid dispersal to new unoccupied environments.
2.
K-Selection
a. Species that hold their populations fairly constant near the carrying
capacity are K
selected.
b. Such populations are equilibrium species, tend to be specialists rather than
colonizers,
and may become extinct when their evolved way of life is disrupted (e.g., the grizzly
bear, Florida panther, etc.).
c. Overall strategy for continued existence is based on having the following
traits: 1) large
size, 2) long life span, 3) slow to mature, 4) produce few offspring, and 5) expend
considerable energy in care. (Fig. 23.13)
d. Such populations usually have a survivorship curve similar to type I. (Fig.
23.7a)
e. Thus, they rely on competitive superiority to secure limited resources.
B. Most populations cannot be
characterized as either r- or K-strategists; they have
intermediate characteristics.
23.5. Human Population Growth (p. 399)
A. The Human Population Is Growing
(Fig. 23.14) [transp. 128]
1. The human
population is now in an exponential part of a J-shaped growth curve.
2. World
population increases equivalent of one medium-sized city (200,000) per day and
88
million per year.
3. Growth
rate is the difference between birthrate and death rate per 1,000 persons.
4. Doubling
time is the length of time for population size to double, now 47 years.
5. Zero
population growth is when birthrate equals death rate and population size
remains
steady.
B. More-Developed versus
Less-Developed Countries (Fig. 23.16) [transp. 129]
1. More
developed countries underwent demographic transition 1950-1975; their growth
rate
is now 0.6%.
a. More developed countries (MDCs) were first industrialized (e.g., Europe,
North America,
Japan, etc.).
b. Demographic transition is decline in death rate followed by declining
birthrate; results in
slower growth.
2. Less
developed countries (LDCs) are now undergoing demographic transition.
a. Less developed countries (LDCs) are fully industrialized (e.g., countries in
Africa, Asia,
Latin America).
b. LDC growth rate peaked at 2.5% between 1960-1965; it is declining slowly to
about
1.8% at year 2000.
C. Comparing Age Distributions (Fig.
23.15) [transp. 130]
1.
Replacement reproduction will cause population growth to continue due to the age
structure
of the population.
2. Mere
replacement does not produce zero population growth because more women enter
reproductive years than leave them.
3. The MDCs
have a low growth rate because of a stabilized age structure.
4. The LDCs
have a higher growth rate because of a youthful age structure.
D. A Sustainable World
1. The
decision is not between preservation of ecosystems or human survival.
2. Both the
growing populations of the LDCs and the high consumption of the MDCs stress
the environment.
3. An average
American family, in terms of consumption and waste production, is equal to
thirty people in India.
4. Borrowed
carrying capacity is used to describe how cities borrow resources from the rural
areas.
5.
Overpopulation and over consumption contribute to pollution and extinction of
species.
6.
Sustainable practices include logging by draft horses, etc.