Lecture+10.ppt

Lecture+10.ppt - Population Dynamics Population dynamics...

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Population Dynamics
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Population size changes as a result of four processes: Birth, death, immigration, and emigration. N t = Population size at time t B = Number of births D = Number of deaths I = Number of immigrants E = Number of emigrants E I D B N N t t 1 Population dynamics : The ways in which populations change in abundance over time.
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Patterns of Population Growth These four patterns are not mutually exclusive. A single population can experience each of them at different times. Populations exhibit a wide range of growth patterns, including exponential growth, logistic growth, fluctuations, and regular cycles.
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Exponential Growth Population increases by a constant proportion at each point in time. When conditions are favorable, a population can increase exponentially for a limited time. When a species reaches a new area, exponential growth can occur if conditions are favorable. The population may grow exponentially until density-dependent factors regulate its numbers.
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Species such as the cattle egret colonize new regions by long- distance or jump dispersal events. Local populations then expand by short- distance dispersal events.
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Logistic Growth These populations first increase, then fluctuate by a small amount around the carrying capacity. Plots of real populations rarely match the logistic curve exactly. “Logistic growth” is used broadly to indicate any population that increases initially, then levels off at the carrying capacity.
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Sheep in Tasmania
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For K (carrying capacity) to be constant, birth rates and death rates must be constant over time at any given density. This rarely happens in nature. Birth and death rates do vary over time, thus we expect carrying capacity to fluctuate.
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Population Fluctuations In all populations, numbers rise and fall over time. Fluctuations can be deviations from a growth pattern, e.g., the Tasmanian sheep population or erratic - In Lake Erie phytoplankton populations, fluctuating abundance could reflect changes in environmental factors such as nutrient supplies, temperature, or predator abundance.
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Figure 10.6 Population Fluctuations
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Figure 10.7 Populations Can Explode in Numbers population outbreak - # of individuals increases rapidly
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An ongoing outbreak of the mountain pine beetle has killed hundreds of millions of trees across British Columbia. This has altered forest composition, and CO 2 is released as the trees decay—17.6 megatons every year.
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Population Cycles Some populations have alternating periods of high and low abundance at regular intervals. Populations of small rodents, such as lemmings and voles, typically reach a peak every 3–5 years.
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Figure 10.9 A Population Cycle
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Different factors may drive population cycles in rodents.
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