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Human population growth From here - end of course: Ecology - the study of the distribution and abundance of organisms Today - population growth w/ humans as focus Wednesday - Life histories - comparing demographic parameters (e.g. birth rates, timing, mortality patterns) across species Friday- Population regulation and competition Then 3 more lectures on interactions between species, mutualism, parasitism, herbivory Then, community ecology, biogeography, conservation ecology, invasive species, ecosystem ecology and the natural history of the Sonoran desert Human Population Growth Human Population Growth I. Concepts of population growth A. Unlimited, exponential growth B. Limited, logistic growth II. Human population growth A. History of population growth B. Projecting into the future I. Unlimited population growth A. Exponential growth Page ‹#› 1. Example of bacterial growth A. Exponential growth Generation 0 1. Example of bacterial growth Bacteria reproduce by binary fission - 1 cell 2 cells No. 1 1. Example of bacterial growth 1. Example of bacterial growth Generation 0 1 Generation 0 1 2 No. 1 2 Page ‹#› No. 1 2 4 1. Example of bacterial growth 1. Example of bacterial growth Generation 0 1 2 3 Generation 1 2 3 4 5 No. 1 2 4 8 No. 1 2 4 8 16 1. Example of bacterial growth 1. Example of bacterial growth Generation 0 1 2 3 4 5 Generation 0 1 2 3 4 5 6 4000 3000 2000 12 10 6 8 4 0 2 1000 0 No. 1 2 4 8 16 32 Page ‹#› No. 1 2 4 8 16 32 64 1. Example of bacterial growth 1. Example of bacterial growth Generation No. Generation 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 1 2 4 8 16 32 64 128 256 4000 3000 2000 1. Example of bacterial growth Generation 0 1 2 3 4 5 6 7 8 9 Generation 0 1 2 3 4 5 6 7 8 9 10 Generation Page ‹#› 12 2000 12 8 Generation 10 6 0 4 1000 0 12 10 6 8 4 0 2 1000 3000 2 2000 4000 Number bacteria 3000 0 Number bacteria 4000 No. 1 2 4 8 16 32 64 128 256 512 1024 8 Generation 1. Example of bacterial growth No. 1 2 4 8 16 32 64 128 256 512 10 4 6 0 2 1000 0 Number bacteria No. 1 2 4 8 16 32 64 128 1. Example of bacterial growth 1. Example of bacterial growth Generation 0 1 2 3 4 5 6 7 8 9 10 11 Generation 0 1 2 3 4 5 6 7 8 9 10 11 12 Generation 2000 12 8 10 0 4 1000 6 12 Generation Features of simple exponential growth 1. Example of bacterial growth In this example, the generation time = the doubling time (time until population doubles) • A constant doubling time Constant growth rate? 4000 3000 2000 Page ‹#› 12 10 6 8 4 0 0 1000 2 8 10 6 4 0 0 2 1000 3000 0 2000 4000 2 3000 No. 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 Number bacteria 4000 Number bacteria No. 1 2 4 8 16 32 64 128 256 512 1024 2048 Features of simple exponential growth A. Exponential growth A constant doubling time An accelerating growth rate i. Example of bacterial growth ii. The importance of generation time When plotted against generations, they seem to be the same... Imagine that 3 populations, bacteria, mice, and elephants, all doubled their population size at each generation. Each bacterium produced 2 offspring, each mouse and elephant couple produced 4. 40 Bacteria Mice Elephants 20 Generation Would their populations grow at the same rate? Page ‹#› 6 5 4 3 2 0 1 10 0 Number 30 When plotted against minutes, the difference is apparent But of course, the generation time of these organisms is rather different! 100000000 10000000 Minutes (log scale) 1000000 100000 0 10,512,000 10000 10 1000 Number 132,480 Generation time: the mean length of time between birth of parents and birth of offspring Elephants 20 100 Elephants (20 yrs) 30 10 Mice (3 months) Mice 30 Generation time in minutes: Some species of bacteria Bacteria 40 (190 yrs!) We need an equation that will let us compare the population growth of organisms with different generation times General differential equation for unlimited population growth: dN/dt = rN N is the population size r is the instantaneous growth rate (the net population growth at any one instant) Page ‹#› The population growth rate, r, is determined by how many individuals are coming into the population at any one time (the instantaneous rate of births) Age distribution Projections of population growth use instantaneous rates of birth, rates of death, and the age distribution reduced by those leaving i.e. t h e instantaneous rate of deaths The age distribution is the way in which a population is distributed among different age classes. r=b-d b - instantaneous birth rate d - instantaneous death rate Examples of human a g e distributions 1) Cambodia, 1998 Examples of human a g e distributions 2) United States, 2000 Page ‹#› Which population is likely growing more rapidly? Cambodia, 1998 United States, 2000 I. Unlimited population growth A. Exponential growth If the number of children per mother and mortality rates were held constant in these two countries, would population growth be the same ? Cambodia, 1998 United States, 2000 In nature, we don’ t generally see exponential growth for very long B. Limited (logistic) growth Thrips (flower feeding insects) on rose bushes 1932 Page ‹#› 1938 B. Limited (logistic) growth B. Limited, logistic growth I. The carrying capacity What sorts of factors cause population growth to slow or decline? We observe that most populations in nature have some limit, called the carrying capacity Limited food supply Limited habitat available Deterioration of habitat Predation, disease The carrying capacity (K) is the maximum stable population size the environment can support. Logistic growth Logistic growth, the equation K d N/dt = rN[(K-N)/K] logistic function Same equation as before, with logistic function Page ‹#› W h a t’s the logistic function? d N/dt = rN[(K-N)/K] What happens to the sign of the right hand side of the equation when N=K? When N>K? When N< K? A handy, but somewhat arbitrary mathematical function that returns the population to a line, K What would happen if the habitat for a particular population was reduced? What would happen to the population if the habitat for a particular population was reduced? New K Page ‹#› What would happen if the habitat for a particular population was reduced? I. Unlimited population growth II. Limited (logistic) growth III. Pop. would decline to new K Human population growth As of April 3, 2006 (17:19 GMT), the world population was 6,507,485,579 Every second: 4.1 births, 1.8 deaths A net increase of 2.3 individuals II. Human population growth A. History of human population growth What kind of growth? Year Global population 1800 1930 1975 2000 1 2 4 6 billion billion billion billion } W h a t’s the doubling time between 1 and 2 billion? Page ‹#› Year Global population Year Global population 1800 1930 1975 2000 1 2 4 6 1800 1930 1975 2000 1 2 4 6 billion billion billion billion } W h a t’s the doubling time between 2 and 4 billion? If a constant doubling time is a feature of simple exponential growth, and human populations show a decreasing doubling time… billion billion billion billion What can we say about the doubling time between 1800 and 1975? In recent years human populations have grown faster than simple exponential growth t h e n… ? Page ‹#› II. II. Human population growth A. History of human population growth A. History of human population growth B. Projecting into the future Will it continue to grow like this indefinitely? What is the capacity planet Earth? What population parameter would we like to know? B. Projecting into the future i. Human carrying capacity, K Been the subject of speculation since van Leeuwenhoek (1679) imagined a world like 17th century Holland (13 bill) Estimates range from 900 mill to “a billion billions” Dependent on many assumptions, value systems Unlike other animals, humans have the potential to move towards a sustainable population size Human population growth carrying of the i. Human carrying capacity What should be the standard of living? Hardin (1986): 1 average American uses 300 times the amount of energy as an average Ethiopian. If everyone lived as Ethiopians, the earth could support 300 billion people… Page ‹#› i. Human carrying capacity II. Human population growth How does one view sustainability? Many estimates are based on maximum agricultural capacity and fossil fuels, no consideration for degradation of farm land, pollution, global warming, or loss of biodiversity Pimentel et al. (1994) assumed solar and other renewable energy at increased rates, soil conservation in agriculture. K=3 billion, 1-2 billion in relative prosperity B. Projecting into the future i. Human carrying capacity ii. Population growth projections United Nations world population predictions to 2050 (2002) ii. Population growth projections Estimates of future populations do not take into consideration the environment or resources, or any explicit estimate of K - just project trends in birth rates and death rates into the future 1.4E+07 1.2E+07 1.0E+07 Const. Fert. 13b We are here “High” 11b “Medium” 9b }between 7 8.0E+06 6.0E+06 4.0E+06 2.0E+06 0.0E+00 Constant Page ‹#› “Low” 7b High Medium Low and 11 bill. considered most likely In “low” estimate, the population drops Note the “constant fertility” projection (i.e. fertility at current rates) is the highest and considered unrealistic. Why? 1.4E+07 1.2E+07 1.0E+07 8.0E+06 6.0E+06 4.0E+06 2.0E+06 0.0E+00 Const. Fert. 13b We are here Constant Note the “constant fertility” projection (i.e. fertility at current rates) is the highest and considered unrealistic. Why? Fertility rates are dropping worldwide Part of a robust pattern without one clear explanation - “ t h e demographic transition” “High” 11b “Medium” 9b “Low” 7b High Medium Low C. Projecting into the future i. The demographic transition C. Projecting into the future i. The demographic transition Countries go from having small, slow growing populations to having large, slow growing populations in 4 steps In the past 200 years, all “developed” countries have undergone this process Most “developing” countries are somewhere in the process - fertility rates are dropping but populations are still growing 1) High death rates, high birth rates fi no growth, small population 2) Death rate falls, birth rates stay same fi rapid growth, larger population 3) Birth rate falls, death rates stay the same fi population increases (because of age distribution), large population 4) Birth rate and death rates stay the same fi no growth, large population Page ‹#› Children per mother Fertility rates 1950 - 2050 (U.N. report 2002) We are here 8 7 6 5 4 3 2 1 0 World More developed Less developed Least developed If fertility rates are dropping why is the world population still growing? 2) Age distribution - if most individuals in a population are young, population will still grow after fertility drops below replacement Most world population growth is coming from developing countries If fertility rates are dropping why is the world population still growing? 1) Fertility is still above replacement 8 7 6 5 4 3 2 per mother 2 - replacement 1 0 Children per mother C. Projecting into the future i. The demographic transition We are here World More developed Less developed Least developed C. Projecting into the future i. The demographic transition Why the demographic transition? What causes the dropping mortality rates? Better public health In developing countries the “public health revolution” has been from 1945- present Page ‹#› C. Projecting into the future i. The demographic transition C. Projecting into the future i. The demographic transition Life expectancy from 1950 - 2050 (U.N. 2002) Why is life expectancy not currently rising in the least developed countries? we are here 90 80 70 60 50 40 30 20 10 0 90 80 70 60 50 40 30 20 10 0 World More developed Less developed Least developed World ? More developed Less developed Least developed Why is life expectancy not currently rising in the least developed countries? C. Projecting into the future i. The demographic transition HIV/AIDs : huge economic and social costs in some of the least developed countries in Africa Some tragic statistics: -In Botswana, 1 in 3 adults is currently infected -Life expectancy has dropped from 63 years to 44 years -Population is expected to be in decline by 2015 Outside of Africa, no. of countries where HIV/AIDS is a problem is growing, especially in Asia, Latin America, Caribbean Peak prevalence has passed in many countries, but prevalence expected to stay relatively high until 2050 Why the demographic transition? What causes the dropping fertility rates? No one really knows. Some likely factors in recent decline in Africa: Drop in the “ideal family size” in many countries coupled with Education for girls Access to contraception In some cases, costs of children borne increasingly by families No one factor appears universal to fertility decline Page ‹#› C. Projecting into the future C. Projecting into the future iv. What else can we see ahead? Where do we fall on this curve? Or possibly up here What is K? We may be somewhere in here 1965-1970 max. world pop. growth For some time, an ageing population A much larger population, but one that seems to be slowing its growth, and may even start a slow decline within 100 yrs. With the prospect of a world population that is not growing exponentially in the future, we can begin to think about what population size the earth can sustain Page ‹#›