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Populations How Populations Grow Objectives • Identify the 3 characteristics of a population. • Identify the 3 factors that affect a population‟s growth. • Distinguish between exponential and logistic growth. • Define carrying capacity. Characteristics of Populations 1. Geographic distribution – the total area inhabited by a population • Also called range 2. Population density – the number of individuals per unit area • Number of individuals ÷ Area 3. Population growth – an increase or decrease in a population Population Growth • 3 Factors that Affect a Population‟s Growth 1. Birthrate – number of births 2. Death rate – number of deaths 3. Number of individuals that enter or leave a population • • Immigration – the movement of individuals into an area Emigration – the movement of individuals out of an area Growth Curves • A growth curve is a graph showing the number of individuals in a population over time Exponential Growth • Exponential growth – individuals in a population reproduce at a constant rate • • • Shown on a J-curve Only occurs under ideal conditions with unlimited resources Shows the biotic potential of a population Biotic Potential • Biotic potential – the rate at which a population will grow if all individuals survive and reproduce at maximum capacity • • • • can only happen when individuals are placed in an ideal environment with unlimited resources and space and without hazards such as disease and predators does not naturally occur usually only occurs when a species is reintroduced to a habitat with no other competing species • only lasts for a short period of time (ex: trout reintroduced to a lake with no fish) populations become stable (no large growths or declines) when there is a balance between the number of individuals and the J-Curves • • Lag phase – little or no increase in population growth Exponential phase – population increases rapidly • Number of individuals doubles rapidly in a specific time interval • Population keeps doubling in shorter periods of time Logistic Growth • Logistic growth – a population‟s growth slows or stops following a period of exponential growth • • Shown on a S-curve Occurs as resources become less available Carrying Capacity • Carrying capacity – maximum number of individuals an ecosystem is capable of supporting • • • • Usually represented by „K‟ on a graph Can be found on an S-curve graph on the flat line after the exponential growth ends Stable populations fluctuate near the carrying capacity Fluctuations become more radical in an unstable environment (ex: blizzards and other natural disasters) Notes Review • Identify the 3 characteristics of a population. • Geographic distribution • Population density • Population growth Notes Review • Identify the 3 factors that affect a population‟s growth. • Birth rate • Death rate • Immigration/Emigration Notes Review • Distinguish between exponential and logistic growth. • Exponential growth – individuals in a population reproduce at a constant rate • Logistic growth – a population‟s growth slows or stops following a period of exponential growth Notes Review • Define carrying capacity. • Carrying capacity – maximum number of individuals an ecosystem is capable of supporting Populations Limits to Growth Objectives • Define “limiting factor.” • Distinguish between density-dependent limiting factors and density-independent limiting factors. Limiting Factors • Limiting factor – a factor that causes a population‟s growth to decrease • Examples: • • • • • Competition Predation Parasitism & disease Drought & other climate extremes Human disturbances Density-Dependent Factors • Density-Dependent Factor – a limiting factor that depends on population size • Have a greater effect when a population is large and dense • Examples include competition, predation, parasitism, and disease Competition • When populations become crowded, organisms compete with one another for food, water, space, sunlight (plants), and other resources • Interspecific competition – 2 or more different species compete for the same resources • Intraspecific competition – individuals that are the same species compete for the same resources Interspecific Competition Intraspecific Competition Predation • Predation – the hunting of prey for food • Predator-Prey Relationship – a method of population control in which a population is regulated by predation • • • • • When there‟s not a lot of predators, prey populations increase When prey populations increase, predator populations increase because there‟s more prey to eat Then prey populations decline due to predation Predator numbers decline due to lack of prey The cycle then repeats Predation Parasitism and Disease • Parasites steal nourishment from the organism they are in, weaken it, and can kill it or cause disease • Examples: some types of bacteria, tapeworms Parasitism and Disease Density-Independent Factors • Density-Independent Factors – factors that affect all populations, regardless of population size • Can cause population crashes • Usually abiotic factors • Examples: • Extreme weather or climate • Natural disasters (earthquake, tidal wave, tornado) • Drought Notes Review • Define “limiting factor.” • Limiting factor – a factor that causes a population‟s growth to decrease Notes Review • Distinguish between density-dependent limiting factors and density-independent limiting factors. • Density-Dependent Factor – a limiting factor that depends on population size • Density-Independent Factors – factors that affect all populations, regardless of population size