5.2 Limits To Population Growth


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UBC Bio 111 - Intro to Biology

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5.2 Limits To Population Growth

  1. 1. 5.2 Limits to Population Growth Learning Outcomes: 2. Compare real population growth curves to models. 4. Apply a model of population growth to predict changes in population size. Reminders: Midterm 1 Friday October 9th (Kingdoms to Eukaria is fair game. Put it on your cheat sheet!) Modelling species carrying capacity: Humans can modify the environment to enhance the growth and survival of select species. This can make predictions of the carrying capacity for these species more difficult. e.g., we make more nutrients available to producers by - adding fertilizer to soils, - polluting waterways with effluent e.g., we remove predators would initially remove a limit on the abundance of the prey (removing wolves leads to and increase in the deer population) Real populations do not always conform to the models Environmental conditions can change dramatically as do community interactions. The size of real populations will be impacted by these changes. QUESTION: 1.Which segments of this figure could represent the following scenarios: 1) disease or extreme climatic event? - Density dependant (corresponds to fast decreases in population) 2) rapid growth due to ample resources and lack of predators? – exponential growth. no decreases in population. Lasts till production RATE goes down 3) competition for resources and a gradual reduction of reproductive rate? Figure 1. Historic record wild sheep population 4) population grows beyond resources available? growth 5) starvation and predation cause population crash? Population cycles Historic records from the Hudson Bay Company of the number of pelts traded by trappers indicated a pattern of regular cycles. Figure 2. Population cycles of snowshoe hare and lynx in northern Canada 1. At high densities hares decimate the available shoots and twigs (reduce the food supply), predators remove hares from the population. 2. Hares starve, are physiologically stressed and decrease in fertility. Their weak condition makes them easy prey for the lynx. There is a further decrease in the number of hares.
  2. 2. QUESTION: 2. What type of factors are causing this decrease in the hare population? 3. Lynx prefer to feed on hare, and as the hare population decreases lynx need to search longer for prey. They may switch to other less nutritious prey. The lynx find less suitable food and some starve. Their weakened condition makes them more susceptible to parasites and disease as well as decreases their fertility. Few lynx survive. 4. Vegetation recovers from herbivory. The surviving hares now have an abundant supply of food, and the population begins to increase. One to two years later the lynx population also recovers. 3. Label when each of these numbered events (1 – 4) occurs within the “boom-bust” population cycle on the graph at left. - Red = Lynx - Black – Hares Step 1 – at beginning of hare decline Step 2 – middle of decline Step 3 – bottom of decline Step 4 – beginning of incline Human population growth QUESTION: 4. What has caused great increases in our population growth? - Advances in medicine - Agriculture advances - Advances in science - Labour previously performed by animals is now performed with machines: Steam engine, assembly line, electricity -> Fossil Fuels - Urbanization - Improvements in sanitation
  3. 3. Disparity among nations/societies in population growth In major industrialized countries where societies are prosperous - increases in levels of sanitation, nutrition and health care leads to a decrease in the death rate. - the birth rate continues for several decades and then declines. - the result is that initially there is an increase in population, but eventually leads to a decrease as people develop different attitudes about family size (children needed for farm work or to ensure survival). In developing countries - birth rates and death rates are both high - death rates decrease faster than birth rates - the gap between birth and death rates means a significant and rapid increase in population.
  4. 4. - the adjustment of birth rates takes a significant period of time, during which high birth rates continue and death rates are much lower. QUESTION: 5. As a nation becomes more prosperous (transitions from developing to industrialized) what factors contribute to a decline in human population growth rates? The adjustment of birth and death rates are fundamental to demographic transition - a positive correlation exists between high incomes and low birth rates. - but birth rates can drop even with modest incomes as long as the economic gains are directed. QUESTION 6 : Directed toward what end? What particular changes will cause lower birth rates? - making birth control more available - More educational and economic opportunities (eg: female literacy) - maneipation (religious beliefs) - Government Regulation (eg: China) 5-Stage Demographic Transition Model http://www.geographyalltheway.com/igcse_geography/population_settlement/population/imagesetc/demographic_transitio n_detailed.jpg Predictions of global population change A world clock located at this web site has continuously updated statistics on the human population. http://www.poodwaddle.com/worldclock.swf
  5. 5. Since a peak in 1962 (2.19%) the rate of global population growth has decreased. The world population passed the 6 billion mark in 1999 UN estimates indicate that it will take until the middle of next century before the impact of the post-World WarII population explosion (baby boomers) relaxes. Total world population is projected to reach more than 10 billion by 2100. UN experts expect world population to stabilize between 11 and 12 billion people. The world population will still be growing but the The rate of growth of the human population is correlated with the increase substantially lower thanfuels and growth rate will be in the burning of fossil today. CO2 emissions. Current scientific thinking suggests that energy resources will limit human endeavours and population growth. BIG QUESTIONS: 7. Where should we draw the line on the graph for carrying capacity? Have we exceeded it already? 8. Are parts of the world already facing density dependent factors? – Disease, war, famine 9. Is habitat being degraded? – yes: clearcut logging polarbears -> ice break up pollution (disruption of food chain) Find your place in our population growth: http://www-popexpo.ined.fr/english.html Decide your contribution. Age-structure pyramids Third world countries will account for more than 90% of world population growth. In the developed regions, the changing age structure within an almost stable population will constitute the main problem rather than growth. QUESTIONS: 10. What accounts for the changes in Mexico’s age structure pyramid from 1975 to 2005? – high mortality particularly in children. – high birth rate. – lower infant mortality. – decreasing birth rate. – improved health care. 11. As Mexico’s population becomes more stable how will its shape continue to change? Draw on a predicted line. (predicted line looks like a bulb. Everything seems to even up)
  6. 6. 12. How would you describe the country with this particular population? Is it more likely a developed or developing country? How might you characterize this country in terms of birth rate, medical care, economics and political stability? - Hole in men population due to war.