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Chapter 6: Population Dynamics
Dynamics of Population Growth
Exponential Growth and Doubling Times
• Exponential growth: growth at a constant rate of increase per unit
time.
- Growth curve called a J-curve because of its shape.
- Also called geometric growth
• Arithmetic growth: growth that increases at constant amount per
unit time
Table 6.1. Doubling times at various compound interest rates
Annual % Increase
0.1
Doubling Time (years)
700
0.5
140
1.0
70
2.0
35
5.0
14
7.5
9
10.0
7
100.0
0.7
• At 1% per year, a population or a bank account doubles in
about 70 years
• Therefore:
70/annual % growth = approximate doubling time in years
Biotic Potential
• Biotic Potential: maximum reproductive rate of an organism
• Given unlimited resources and no limiting factors, many species can
produce enormous populations very quickly (e.g. houseflies).
Table 6.2. Biotic potential of houseflies (Musca domestica) in one year
Assuming that:
- a female lays 120 eggs per generation
- half of these eggs develop into females
- there are seven generations per year
Generation
1
2
3
4
5
6
7
Total Population:
If all females in each
If all generations survive one year
generation lay 120 eggs
and all females reproduce maximally
and then die
in each generation
120
7,200
432,000
25,920,000
1,555,200,000
93,312,000,000
5,598,720,000,000
120
7,320
446,520
27,237,720
1,661,500,920
101,351,520,120
6,182,442,727,320
Source: Data from E.J. Kormondy, Concepts of Ecology, 3rd ed., 1984 Harper and Row
Publishers, Inc.
Population Oscillations and Irruptive Growth
• In the real world, there are limits to growth.
• Dieback: when the population decreases as fast as, or faster , than it
grows
- Some limiting factor comes into effect.
• Overshoot: extent to which a population exceeds the carrying capacity
of its environment.
• Irruptive or Malthusian growth: pattern of population explosion
followed by a population crash.
• Populations may go through repeated oscillating cycles of exponential
growth and catastrophic crashes.
Growth to a Stable Population
•
Not all populations go through cycles of irruptive population
growth and catastrophic decline.
•
Growth rates of many species are regulated by internal and
external factors so that they can come into equilibrium with their
environmental resources.
•
Logistic growth: exponential growth when resources are unlimited
and slowed growth as species approach carrying capacity of
environment.
- Growth curve called an S-curve because of its shape.
•
Environmental resistance: factors that tend to reduce population
growth rates.
Chaotic and Catastrophic Population Dynamics
• Populations under natural conditions rarely follow linear dynamics and
smooth growth curves.
• Chaotic systems: systems that exhibit variability that can be of a
complexity whose pattern is not observable over a normal human
timescale.
-The "Butterfly effect" described by meteorologist Edward Lorenz
is a metaphor for describing chaotic dynamics.
“If a butterfly flapping its wings in Rio de Janeiro could, through
a long series of unpredictable multiplying effects in weather
systems, eventually result in a tornado in Texas.”
• A catastrophic system may jump abruptly from one seemly steady state
without any intermediate changes.
Strategies of Population Growth
•
Malthusian Strategies (externally controlled growth)
- Insects, rodents, marine invertebrates, parasites, and annual
plants
•
Logistic Strategies (intrinsically controlled growth)
Wolves, elephants, whales, and primates
Table 6.3. Characteristics of contrasting reproductive strategies
Externally Controlled Growth
Instrinsically Controlled Growth
1.Short life
2.Rapid growth
3.Early maturity
4.Many small offspring
5.Little parental care or protection
6.Little investment in individual offspring
7.Adapted to unstable environment
8.Pioneers, colonizers
9.Niche generalists
10.Prey
11.Regulated mainly by extrinsic factors
12.Low trophic level
1.Long life
2.Slower growth
3.Late maturity
4.Fewer large offspring
5.High parental care and protection
6.High investment in individual offspring
7.Adapted to stable environment
8.Later stages of succession
9.Niche specialists
10.Predators
11.Regulated mainly by intrinsic factors
12.High trophic level
Factors that Increase or Decrease Populations
Natality, Fecundity, and Fertility
• Natality: production of new individuals by birth, hatching,
germination, or cloning.
- Sensitive to environmental conditions.
* Nutritional levels
* Climate
* Soil
* Water conditions
• Fecundity: physical ability to reproduce.
• Fertility: measure of actual number of offspring produced.
Immigration
• Organisms are introduced into new ecosystems by a variety of
methods.
- Seeds, spores, and small animals may float on winds or water
currents.
- Sometimes organisms are carried on the fur, feathers, or
intestines of animals.
- As adults, some animals travel by flying, swimming, or walking.
Mortality and Survivorship
Mortality: death rate determined by dividing the number of organisms
that die in a certain time period by the number alive at
the beginning of the period.
Survivorship: percentage of a cohort that survives to a certain age.
Life expectancy: probable number of years of survival for an individual
of a given age.
• Life span: longest period of life reached by a given type of organism.
• Maximum age is dictated primarily by physiological aspects of the
organism itself.
• Enormous difference exists in the life span between different species .
- Microorganisms live whole life cycles in a matter of hours or
minutes.
- Bristle cone pine trees have life spans up to 4600 years.
• Most individuals in a population do not live anywhere near the
maximum life span for their species.
- Many factors are involved in early mortality.
* Predation
* Parasitism
* Disease
* Accidents
* Fighting
* Environmental influences (e.g. climate and nutrition)
Age Structure
• Growing or declining populations will have very different proportions
of individuals in various age classes.
• Populations will often be distinguished by differences in distribution
among prereproductive, reproductive, and postreproductive age classes.
• Population momentum: potential for rapid increase in natality once
youngsters reach reproductive age in populations with a large number of
prereproductive individuals.
Emigration
• Emigration: movement of members out of a population
• Many organisms have specific mechanisms to facility migration of one
or more of each generation of their offspring.
- Genes will still be carried by descendants in other places that
may be better suited for their survival.
Factors that Regulate Population Growth
Various factors regulate population growth, primarily by affecting
natality or mortality and can be classified in different ways.
• Intrinsic: operating within individual organisms or between
organisms in the same species.
• Extrinsic: imposed from outside the population.
• Biotic: caused by living organisms.
• Abiotic: caused by non-living components of the environment.
These factors can act in two ways.
• Density-dependent: effects are stronger or a higher proportion of
the population is affected as population density increases.
• Density-independent: effect is same or a constant proportion of the
population is affected regardless of population density.
Biotic regulation of factors tend to be density-dependent, while abiotic
factors tend to be density-independent.
Density-Independent Factors
Factors that affect natality or mortality independently of population
density tend to be abiotic components of ecosystem
•
•
•
•
•
Extreme cold or even moderate cold during the wrong time of year.
High heat
Drought
Excess rain
Severe storms
Abiotic factors can have beneficial effects.
• Desert blooms can occur after rainfall.
• Fire can dominate or create many biomes.
Density-Dependent Factors
Density-dependent mechanisms tend to reduce population size by
decreasing natality or increasing mortality as population size
increases.
Reduction in population size can result from interactions between
populations or interactions within a population.
Interspecific interactions
Prey species may actually benefit from predation by helping to prevent
population overshoot.
Predator and prey populations often oscillate in synchrony with each
other (e.g. lynx
and hare)
Parasites and the host often develop some sort of balance with each other
• If the parasite kills the host, food will be unavailable for offspring.
• If parasite and host have lived together for a long time, the host is
likely to have developed some resistance.
• If parasite is introduced into a new ecosystem, the effects can be
disastrous.
In interspecific competition, two species can compete for the same
environmental resources in an ecosystem.
Not all interspecific interactions are harmful to one of the species
involved (e.g. mutualism and commensalism).
Intraspecific interactions
Individuals within a population can also compete for resources.
Territoriality is one principal way many animal species control access to
environmental resources.
• Individual, pairs, or group that holds the territories will drive off
rivals if possible either by threats, superior features (colors, size,
dancing ability), or fighting equipment.
Stress and Crowding
Stress shock or stress-related diseases
• Loose set of physical, physiological, and/or behavioral changes
that are thought to result from the stress of too much competition and
too close proximity to other members of the same species.
• Best evidence for existence of these diseases comes from
experimentation in the lab.
- Lab animals are grown in very high densities with plenty of
food and water, but little living space.
* Symptoms reported include 1) reduced fertility; 2) low
resistance to infectious diseases, 3) hypoactivity;
4) hyperactivity; 5) aggression; 6) lack of parental
instincts; 7) sexual deviance; and 8) cannibalism.