ph ch 14 population ecology.ppt [read-only] - tplagge.net ch 14 population ecology.pdfa population...
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Population Ecology
Chapter 14
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What is ecology?
• The study of the interactions between organisms & their environments– can be studied at
many levels• individual/organismal• population• community• ecosystem
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A population is . . .• A group of organisms of the same
species living in a specific area • Population ecologists are interested
in how quickly or slowly populations grow– for resource management– for conservation of rare & endangered
species• Population growth influenced by
– birth & death rates– immigration & emigration
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Populations can grow quickly . . . for a while
• Exponential growth occurs when each individual produces more than a single offspring to replace itself– the bigger the population, the faster it grows
Exponential growth of rabbits
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Why is the planet not overrun with rabbits? (or ants or palm trees or. . .)
• Population growth is limited by the environment; life gets harder as it gets more crowded
• Population density: the # of individuals living in a given area (how crowded it is)
• As population size increases,– competition for resources increases– spread of disease & parasites increases– risk of predation increases
• predator population grows in response to increase in # of prey items
• the more densely packed prey, the more visible
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Limitations to population growth
• Density dependent factors– conditions whose growth-limiting effects increase
as population grows – the maximum population size an environment
can sustain, or “carry” is called the carrying capacity
– as population size reaches carrying capacity:• death rates increase• emigration rates increase• birth rates decrease (less food >> poor nutrition >>
reduced fertility)
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Logistic growth• When a population
reaches the carrying capacity of the environment (there is now a lack of resources), growth is called logistic– a much better
approximation than exponential growth of how populations grow
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Limitations to population growth• Density independent factors
– conditions whose growth-limiting effects are unrelated to population size
– causes increase in death rate or decrease in birth rate because of “bad luck”
• natural disasters, severe weather conditions, oil spills …
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Unusual population growth patterns• If an environment is
repeatedly subjected to density independent factors, a population could be in a perpetual state of exponential growth with periodic massive mortality events.
• Population oscillations can occur such as with snowshoe hares & lynx.
These two examples of population bursts & busts are not the norm. The logistic growth pattern describes the general growth of populations better than any other model.
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Human population growth• Human population growth is limited by the same
factors as other species• Human populations vary in distribution of space
– dense in cities, more sparse in rural areas• Human populations vary in distribution of age
– there may be more individuals in one age group than another
• Two populations of the same size with different age distributions will have different features– a 10 year old not likely to reproduce; a 30 year old has a
relatively high likelihood of reproducing; an 80 year old has a high likelihood of dying before younger people
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Human population growth cont.• A population can be divided
into cohorts– percentages of individuals in
each age grouping• 0-5 years, 6-10 years, etc.
• Cohorts can be “stacked” into age pyramids
• It is helpful for a society to know how many 10, 30, & 80 year olds are in its population– Does the society need to build
more schools? Hospitals? Convalescent homes?
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Around the world, countries vary in age pyramids describing their pops.
• Industrialized countries– pyramids more like
rectangle– most growing slowly or
not at all– low birth rates & low
death rates in higher age groups (good health care)
– most of the population is middle age or old
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Around the world, countries vary in age pyramids describing their pops.
• Developing countries– pyramids are more
triangular– high birth rates but
also high death rates in older age groups (poor health care)
– most of population is in younger age groups
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Age pyramid of the USA• What do you see? Why is this problematic?
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Changing demographics• Demographic transition
– when populations move from high birth & high death rates >> high birth & low death rates >> low birth & low death rates
– (moving from developing country to industrialized)
– illustrates how health, wealth & education can lead to reduction in birth rate
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Changing demographics, cont.
• ¾ of the world’s population live in developing countries that are transitioning into industrialized– population will
continue to rise quickly & at an unsustainable rate
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How high can human population go?• Humans are a very successful species• Human environment also has a carrying capacity
beyond which the population cannot be maintained• Carrying capacity related to an ecological footprint
– the amount of food, water, fuel & space needed per person per country
– ecological footprint varies by country, but as a whole, the worldwide population is living at an unsustainable level
• Carrying capacity hard to determine; we keep moving it
• living in new habitats• increasing agricultural production• circumventing problems of living densely (disease control & waste
management)
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Tools that help population ecologists• A life history is the summary of statistics of a
species– age of first reproduction – litter size & frequency – probability of survival at each age– longevity
• Different species reach sexual maturity at different ages & produce different # of offspring with each mating– among mammals, litter size tends to increase as
distance from the equator increases. Why would that be?• Knowing the life history of a species can help
ecologists manage resources & predict population growth
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Tools that help population ecologists
• Survivorship curves– graphs that show the
proportion of individuals of a certain age that are alive in a population
– indicate an individual’s likelihood of surviving through an age
– show at quick glance whether most offspring die shortly after birth or if live into adulthood
– curves can change over time