Population Growth CENV 110

Topics Basic Population Dynamics Human Population Dynamics Sustainable Harvesting of Wild Populations

G.F. Gause 1934 “The Struggle for Existence”

Basic results If birth rate is greater than death rate a population will increase Populations will increase until the competition for resources reduces birth rate, increases death rate, or both Populations will level off when they reach an equilibrium (births=deaths) known as carrying capacity Caution: many populations don’t reach a nice smooth equilibrium but fluctuate

Wildebeest in the Serengeti

Natural populations will grow until births=deaths Birth rate declined – 2 year olds didn’t give birth Death rate went up

Human population dynamics

Trends in the world population

What changed Survival increased dramatically as a result of increased public health (clean water), immunization, antibiotics Then birth rates declined in most of the world as a result of better education, increased wealth and access to birth control

Public health and vaccination

Small group discussions List 3 factors that will limit the ultimate size of the human population in order of what you think their importance will be

mortality

The demographic transition

Human population: future trends Most of the world has undergone the demographic transition Human population is expected to level off at 9 billion people Can we feed them?

Sustainable harvesting of wild populations

Fit a smooth curve through the data

Calculate net increase Increase = population next year – population this year

Optimum harvesting If we held the population at 650 k we could harvest about 65k each year 65k is known as maximum sustainable yield (MSY) We would harvest 10% each year known as uMSY or FMSY The population size would be 650k known as BMSY BMSY MSY

Population Size Harvest Pressure Maximum Sustainable Yield

S Stock Size Stock Size above target level Stock Size below target level

Stock Size Fishing Pressure Fully Exploited Overfishing Under exploited Overfished and overfishing Over fished MSY

The developed world fisheries

Bristol Bay sockeye stand out as a success story in sustainable biological management A single management agency with clear biological objectives A limited entry program to reduce investment Good ocean conditions after 1977

Management by allowing a specific number of fish to reach spawning grounds For each river there is a target “escapement” the number of fish allowed to pass the fishery to the river The fishery is turned on and off to achieve this goal How is the number determined

Spawner recruit analysis

Calculate sustainable yield =return-spawners

Study Questions What did Gause’s experiments demonstrate? Why did Gause’s paramecium increase initially then level off? What is the relationship between Gause’s experiments and the reading from Malthus? What is meant by carrying capacity? Why were the Serengeti wildebeest at low abundance in 1960 What changed to cause them to increase? What changed to stop the increase of the Serengeti wildebeest in the 1980s? When did the human population start to expand rapidly? What was the primary cause of the rise in human population? Draw a graph showing the relationship between children per female and life expectancy at birth. On that graph show the trend through the last 50 years for China and India. Draw the relationship between a countries income and the number of births per woman. On that graph show the trend of China and India in the last 50 years. What has happened to Brazil’s birth rate in the last 50 years. What factors caused this change What is the demographic transition? What are the stages of the demographic transition? What is maximum sustainable yield. In relation to carrying capacity, at what population size will maximum sustainable yield be achieved. Why is maximum sustainable yield not achieved at carrying capacity? What is the status of US fisheries on average in relation to the biomass that produces maximum sustainable yield?