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Exponential Growth ! ie: Doubling ! 1 2 4 8 16 32 ! J-shaped curve ! B > D, then exponential growth Limits to pop growth? Nesting Sites: A Limiting Factor Limits to pop growth ! Resources, living space, nest sites, parasites, predators, wastes etc. ! Carrying Capacity (K) – Maximum number of individuals of a population that a given environment can sustain ! Logistic growth ! S-shaped curve ! (G) = r (N)*(U) ! U = proportion of resources not yet used ! Can be stable Logistic growth Density-dependant limits to population growth ! Rate of population decrease increases as population gets larger ! Related to crowding and competition ! Disease: more individuals = greater density. Proximity increases, so rate of transmission increases ! Predation: more individuals = fewer hiding places, so predation increases Density-independent limits to population growth ! Rate of population decrease is unrelated to population size ! Abiotic influences – fire, avalanche, tsunami, etc. Limits to population growth ! Example: A population of reindeer introduced to St. Matthew island in 1944 peaked, then collapsed by 1966 ! Food became scarce, then thousands of reindeer starved to death during an unusually harsh winter Overshoot and Crash Survivorship Curves ! A survivorship curve is an age-specific plot that shows how many members of a cohort remain alive over time ! Three types are common in nature ! Type I: Survivorship is high until late in life ! Type II: Death rate varies little with age ! Type III: Death rate peaks early in life Survivorship curves ! Shows how many members of those born at the same time remain alive over time ! Type 1 – Death rate is low until later in life. ! Annual plants, large animals with few offspring ! Type 2 – Death rate is constant through life. ! Lizards, small mammals, birds ! Type 3 – Death rate is high early in life. ! Organisms with lots of offspring and little parental care Survivorship curves Life History Strategies ! A life history pattern shows how an organism allocates its resources between growth, maintenance, and reproduction over the course of its lifetime ! An r-selected species is adapted to conditions that change rapidly and unpredictably – favors traits that maximize number of offspring ! A K-selected species is adapted to life in a stable environment – favors traits that improve offspring quality r-Selected Species ! Shorter development times ! Early reproduction ! Fewer breeding episodes ! Many young per episode ! Less parental investment ! Higher mortality rate & shorter lifespan K-Selected Species ! Longer development ! Later reproduction ! More breeding episodes ! Fewer young per episode ! More parental investment ! Lower mortality rate & longer lifespan Human population growth No limits to human population growth? ! Large brains allowed us to master a variety of skills: start fires, build shelters, make clothing, manufacture tools, and cooperate in hunts ! Language ensured knowledge of such skills did not die with the individual ! The invention of agriculture about 11,000 years ago provided a more dependable food supply No limits to human population growth? ! Energy of fossil fuels led to mechanized agriculture and to improved food distribution systems ! Invention of chemical fertilizers and synthetic pesticides increased agricultural productivity ! Improvements in food safety, sanitation, and medicine lowered death rates and shifted the survivorship curve ! The population is currently 7 billion and the United Nations estimates that it will reach 9 billion by 2050 Population growth: Is a decrease in death rates the whole story? Demographic Transition Model What is human K? ! Birth rates declining, but currently only in developed nations ! People living longer ! More getting access to health care, sanitation Comparing Age Structure Diagrams ! Age structure affects a population’s growth rate ! The world’s three most populous countries – China, India, and the US – differ in age structure ! More than one-third of the world population is in a broad pre-reproductive base ! Even if fertility rate declines to replacement level worldwide, population will continue to increase Figure 44-15 p804 Figure 44-15 p804 Figure 44-15 p804 Different consequences ! Ecological footprint – amount of resources (square feet of Earth’s surface) required to support a given population ! Developed world – 5-8 hectares per person ! Developing world – 0.5-3 hectares per person Table 44-2 p805 Consequences ! If every couple has 2 children, still take 60 years for population growth to slow ! Because nearly 2 billion still to reach reproductive age ! Massive food shortages? ! Fossil fuels? ! Environmental issues?