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Ch. 44: Population Ecology
Ecology = study of interactions among all organisms and with their physical environment
Habitat = place where an organism lives
Hierarchy of Ecological Systems:
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Population = all the organisms of the same species within an area at the same time
Community = all the populations that interact in a given area
Ecosystem = the community plus its abiotic environment
Biomes = large areas of the Earth that have similar ecosystems
Biosphere = all the areas on Earth where life exists
Demography = the statistical study of a population’s density, distribution, and growth
POPULATION DENSITY AND DISTRIBUTION
Population Density = the number of individuals per unit area
Population Distribution = the pattern of dispersal of individuals across an area of interest
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Affected by availability of resources = abiotic (nonliving) and biotic (living) components
of an environment that support living things
o Light, water, space, mates, and food
3 Distribution Patterns:
Clumped
Random
Uniform
(Draw in class tomorrow)
POPULATION GROWTH
Rate of Natural Increase (r) (a.k.a. growth rate) = the difference between the number of
individuals born each year and the number that die
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Ex. 30 births in a year and 10 deaths per 1000 individuals in the population would be
calculated as: (30-10)/1000 = 0.02 = 2.0%
Biotic Potential = highest possible rate of natural increase for a population
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Depends on:
o The number of offspring that survive to reproductive age
o Amount of competition within the population
o Age of and number of reproductive opportunities
o Presence of disease and predators
Mortality Patterns
Cohort = all the members of a population born at the same time
Survivorship Curves = graph of the number of individuals still alive at each age
Age Structure Diagrams = shows the number of individuals at each age group
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Includes major groups: Pre-reproductive, Reproductive, and Post-reproductive
POPULATION GROWTH MODELS
Two models of population growth:
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Semelparity = individuals have only one single reproductive event in their lifetime
o Ex. Many bugs or annual plants
Iteroparity = members of a population experience many reproductive events throughout
their lifetime
o Ex. Vertebrates, trees, shrubs, etc.
Growth Patterns
Exponential Growth
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Produces a J-shaped curve
Common for a species that moves into a new area or for species that reproduce in large
numbers and then die off all at once as resources are depleted
Two phases:
o Lag Phase = when growth is slow because the population is small
o Exponential Growth Phase = when growth rapidly accelerates
Calculated as: dN/dt = rmaxN
o dN = change in population size (B-D or births-deaths)
o dt = change in time
o rmax = rate of increase
o N = population size
Logistic Growth
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Produces a sigmoidal or S-shaped curve
Four Phases:
o Lag Phase
o Exponential Growth Phase
o Deceleration Phase = growth slows
o Stable Equilibrium Phase = little to no growth due because birth and death rates
become about equal
 Happens when the population reaches the carrying capacity
 Carrying Capacity (K) = the maximum number of individuals of a given
species that a community can support
Calculated as: <do in class>
o (K-N)/K = effect of carrying capacity on population growth
Don’t typically have to use real numbers in calculations, but need to apply in talking
about what part of each growth pattern a population is in (Lag, Exponential Growth, or
Deceleration Phase)
REGULATION OF POPULATION SIZE
Density-Independent Factors = when the effect does not increase or decrease with population
density
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Random events like droughts, freezes, hurricanes, etc.
Density-Dependent Factors = when the percentage of the population affected increases as the
density of the population increases
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Includes:
o Competition = when members of a species attempt to use the same resources that
are in limited supply
o Predation  when more prey is available, predation increases
o Parasitism  as parasites increase, the number of available, healthy hosts
decreases
o Disease  spreads more quickly the denser a population is
Other Factors:
o Anatomy, physiology, and behavior of an organism
o Territoriality and dominance hierarchies
o Migration and recruitment
LIFE HISTORY PATTERNS
Depend on number of births per reproduction, age of reproduction, and life span
r-Selected Populations = tend to only get to exponential growth
o In fluctuating or unpredictable environments
o Opportunistic Species that usually are:
o Small individuals with short lifespans
o Fast to mature and produce many offspring
o Provide little to no care
o Many offspring die before they reach maturity
K-Selected Populations = tend to show logistic growth
o In stable environments
o Equilibrium Species that are usually:
o Large individuals with long lifespans
o Slow to mature and produce few offspring
o Provide lots of care to children
o Most young survive to reproductive age
HUMAN POPULATION GROWTH
(Currently, the world population is about 6.7 billion.
1800  1 billion
1930  2 billion
1960  3 billion
Not sure when the population will reach the deceleration phase, which concerns us.
Biggest reason for change is the decrease in death and infant mortality rates)
More-developed countries (MDCs) = low population growth and people enjoy high living
standards
o Ex. USA, Canada, Europe, Japan, Australia
o Age distribution diagrams show relatively constant cohort sizes = zero population
growth or replacement reproduction
Less-developed countries (LDCs) = fast population growth and the majority of people live in
poverty
o Ex. Latin America, Africa, and Asia
o Asia has 59% of the world’s population living on 31% of the world’s farmable (arable)
land
o Major water shortages, loss of biodiversity, and increased urban pollution
expected
o Age distribution diagrams are bottom-heavy  exponential growth to continue
o As LDCs become more industrialized, their consumption of resources and production of
pollution will increase
o Ex. China competes with us for oil and metals and has a huge smog problem
Environmental Impact (EI)
MDCs use a much larger portion of the Earth’s resources, despite having less people
Two types of overpopulation possible:
o Population Growth
o Increased consumption of resources