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What do you think happens when sea otters are hunted? Sea urchins increase or decrease? Kelp forest increase or decrease? Sea otters are then placed on the endangered species list So now what happens to the population of sea otter? Starts to increase How does this affect the kelp and the sea urchins? Sea urchins start to get eaten again=decrease in # Kelp increases b/c less sea urchins to eat them
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What happens to the sea otter, kelp, and sea urchins?
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What does this tell us about population? Population changes There are many factors that influence a population Natural Unnatural Population density has a great impact on ecosystems
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3 importantcharacteristicsof Population Geographic Distribution Density Growth Rate Birthrate, death rate, individuals entering/leaving ***Population Age structure is also an important characteristic
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Geographic Distribution AKA Range Describes an area inhabited by a population Can vary Few cubic centimeters Kilometers of the ocean
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Math Time Formula for calculating population density Population density=Number of individuals units area Problem: Suppose there are 150 bullfrogs living in a pond that covers an area of 3 square kilometers. What is the density of the bullfrog population? 50 bullfrogs per square kilometer
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Sampling Techniques How would we measure the population of a species? Impractical to count each and every one Variety of sampling techniques Quadrants Indirect counting Mark-Recapture
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Quadrants Involves marking off specific area, boundary Count specific species within the boundary Repeat in several locations within desired ecosystem Average the results to determine population density More quadrants sampled=more accurate
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Indirect Counting Used for species that are too difficult to see or move around too quickly Does not involve counting organisms themselves Count nests, burrows, tracks
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Mark-Recapture Most common Choose a study area Trap/capture animals Mark the captured animals and release back into habitat Markings are not to disturb organism After a period of time, recapture animals in the same study area Count marked and unmarked organisms
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How to Estimate Population from Mark-Recapture Method Total population= (# in first capture) x (# in second capture) number of marked animals RECAPTURED
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Limits to Accuracy Involve making assumptions about populations Assumptions not valid=estimate not accurate Quadrant Assumption: Organisms distributed evenly in study area Problems “Clumps” Quadrant with clump vs quadrant without clump Minimize problem Analyze how study population is distributed in order to choose appropriate quadrant size Mark-Recapture Assumption: Both marked and unmarked animals have same chance of surviving and being recaptured in second trial Problem After being captured once, how do you think animals will behave? Leads to overestimating population size Minimize problem Minimize effects of trapping on organisms
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3 Factors that affect population size # of births # of deaths # of individuals that enter or leave population Population will increase or decrease depending on # of individuals added or removed
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Have more births than deaths? Population increases Have more deaths than births? Population decreases Have equal amounts of births and deaths? Population remains constant What happens to the population when we….
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Immigration “im”= in Migrate= to move from one place to another Immigration is the individual movement into an area Animals in search of mates and food in new areas
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Emigration “E” means ‘out’ Migrate means to move from one place to another Emigrate means individuals moving out of one place and into another Young wolves and bears leaving as they mature Shortage of food
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Two types of growth Exponential growth Individuals reproduce at a constant rate Population multiplies by constant FACTOR over constant time interval Logistic growth Occurs when a populations growth slows or stops after a period of exponential growth As population approaches carrying capacity, BR may decrease, DR may increase or both, until equal
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Exponential Growth Occurs under ideal conditions with unlimited resources Think about exponents in math…. Starts slowly then sky rockets to infinity Our graph will look like a J Bacteria
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Lets look at bacteria… Bacteria reproduce by splitting in half Bacteria have a doubling time of 30 minutes If you start will one bacterium, how many bacteria will there be after the first 30 minutes? 2 After an hour? 4 After an hour and a half? 8 After two hours? 16 After 15 hours? Over a billion
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Logistic Growth As resources become less available, the growth of the population slows or stops S-shape curve No net increase or decrease in population What we usually see in nature
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Carrying Capacity The largest number of individuals that a given environment can support The part of the logistic graph after the exponential growth…the flattening out The point at which this flat line reaches the y-axis is the size of the population when the growth rate reaches zero This doesn’t mean the population stops growing Many factors slow the growth of plants and animals…
Boom and Bust Growth Cycles Increase rapidly for a period of time followed by a sharp decrease in population for a brief period of time Still not completely understood Hypothesis: Changes in food supply Stress from overcrowding Other organisms influence on population
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Boom-and Bust Population Cycles -Involves more than one population -Rapid increase and decrease in populations
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