Download Population Ecology

CHAPTER 51
Population Ecology
ECOLOGY BASICS

Terms to know…
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Ecology
Branch of biology
 Relatively new science
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Biotic factors
 Abiotic factors
 Environmental science
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Population
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ecology + human interactions
same species, same area, same time
Population ecology

numbers + changes
FEATURES OF POPULATIONS
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Properties that individuals lack:
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Population density
Population dispersion
Birth/death rates
Growth rates
Survivorship rates
Age structure
Properties that communities lack:
Common gene pool
 Reproductive success
 Evolution
 Economic importance (crops, forests, game
animals…)
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POPULATION DYNAMICS
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Density – the number of individuals per unit of
area or volume at a given period of time
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Affected by:
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Habitat
Season/Weather
Other populations
Limiting factors: the number of individuals in a
population is controlled by the ability of the environment
to support it
 Density-dependent factors – the effect increases as
population density increases
 Examples:
 Density-independent factors – affects the size of a
population but is not influenced by changes in
population density; typically abiotic
 Examples:
POPULATION DYNAMICS…
Dispersion – spacing in relation to other
members of the population
 Three basic varieties:
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Clumped (aggregated distribution,
patchiness)
1.
Individuals are concentrated in specific areas
Reasons: distribution of resources, social behavior of
animals (herds, family groups), reproduction
Advantages: reduced chance of predation
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Uniform
2.
Individuals are fairly evenly spaced
Reasons: social behavior of animals (territories), high
levels of competition between individuals
Advantages: reduced competition
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3.
Random
Individual spacing is unrelated to others in the population
 Does not occur often in nature
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CHANGES IN POPULATION
SIZE
Per capita – per individual
 Natality – average per capita birth rate (b)
 Mortality – average per capita death rate (d)
 Immigration – individuals entering a local
population (i)
 Emigration – individuals leaving a local
population (e)
 Population growth rate (r):
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r = (b + i) – (d + e)
If r = positive number  population is increasing
If r = negative number  population is decreasing
If r = zero  population is staying the same
Examples:
INTRINSIC RATE OF INCREASE
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Maximum rate of increase when:
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Conditions are ideal
Resources are abundant
Population density is low
rmax
 Factors which influence this:
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Age at which reproduction begins
 The fraction of the life span devoted to reproduction
 The number of reproductive cycles
 The number of offspring produced each cycle
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Different species have different intrinsic rates…
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Small organisms have high rates (bacteria); large
species have low rates (elephants)
EXPONENTIAL POPULATION
GROWTH
Optimal conditions allow a constant per capita
population growth (rmax)
 The larger the population gets, the faster it grows
 J shape curve:
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Organisms cannot reproduce this way
indefinitely because of increased:
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competition, predation, disease, wastes
LOGISTIC POPULATION GROWTH
Population growth rate nears zero
 Occurs near the environment’s limits to support
the population
 Carrying capacity (K) – the largest population
that an area can maintain indefinitely, assuming
no changes in the environment
 S shape curve:
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DENSITY-DEPENDENT REGULATORY
FACTORS
Cause an increase in death rate as the population
increases
 Affect a larger proportion of the population, not
just a larger number
 Can also cause a decrease in death rate as the
population decreases
 Tend to regulate a population at a relatively
constant size near the carrying capacity
 An example of a negative feedback system
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CYCLICAL POPULATION FLUCTUATIONS
INTRASPECIFIC COMPETITION
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Interference competition
Also called contest competition
 Dominant individuals obtain resources at the
expense of others (they interfere)
 Can cause small drops in population as the
individuals unable to compete die
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Exploitation competition
Also called scramble competition
 All individuals share the limited resource
 Can cause wide variation in population size
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LIFE HISTORY TRAITS
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Reproductive strategies differ:
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Semelparous – a single, large reproductive effort
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Iteroparous – repeated reproductive cycles over the
life span
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Examples:
Examples:
Two extremes:
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r-selected – rapid population growth rate
Small size, early maturity, short life span, large broods,
little or no parental care
 Found in variable, temporary, or unstable environments
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K-selected – keeps population at or near carrying
capacity
Large size, late reproduction, long life span, small broods,
parental care of young
 Found in constant or fairly stable environments
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SURVIVORSHIP CURVES
Measures the probability of survival to a
particular age
 Three basic types:
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Type I – young have a high chance of survival,
probability of survival decreases with age
1.
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humans
Type II – the probability of survival does not
change with age
2.

squirrels
Type III – the probability of death is highest early
in life
3.
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oysters
METAPOPULATIONS
Different populations within an ecosystem
 Due to variations of habitats:
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Source habitats – high quality habitats, plenty of
resources, lots of reproductive success, greater
population density
 Sink habitats – lower-quality habitats, fewer
resources, less reproductive success, lower population
density
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Humans are increasing the number of
metapopulations as they fragment ecosystems
Human Populations
Our population continues to increase, but this is
due to a decrease in the death rate rather than
an increase in the birth rate.
 This decrease is due to:
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Greater food production
 Better medical care
 Improved sanitation practices
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Zero population growth (r = 0)
Human Demographics
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Highly developed countries:
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Low population growth rates
Highly industrialized
Low infant mortality rates
Example:
Developing countries:
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Moderately developed
Birth rates and infant mortality rates high but declining
 Medium industrialization
 Examples:
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Less developed
Highest birth and infant mortality rates
 Lowest amounts of industrialization
 Examples:
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Age Structure Diagram
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The number and proportion of people at each age
of a population
Age Structure Diagrams…
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Rapid growth:
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Pyramid shaped – as children mature, they become
parents of the next generation and there are more of
them than the previous group
Slower growth or decline in population:
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More tapered bases – the number of prereproductive
and reproductive ages are close to the same 
slower, but still positive growth
Small bases – a higher percentages of
postreproductive ages than reproductive and
prereproductive ages  negative growth (decline)
Environmental degradation

People overpopulation….
Developing countries
 Examples:
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Consumption overpopulation…
Developed countries
 Examples:
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