ECOL 182R 1st Edition Lecture 16 Outline of Last Lecture I Population II Unlimited growth A Bacteria B Generation time III Limited growth A Carrying capacity B Logistic growth IV Estimating population size Outline of Current Lecture I The present A Malthus II Growth patterns A Exponential growth III Estimates A Population growth B Stable age distribution IV Fertility rates A Demographic transition pattern Current Lecture The present Every second 4 1 births 1 8 deaths A net increase of 2 3 individuals Where does this information come from 1 The UN U S Census Bureau Who collects it Demographers conduct statistical study of human populations Malthus was an economist and early demographer Power of population is so superior to the power of Earth to produce subsistence for man that premature death must visit the human race Kinds of population growth are humans experiencing Exponential is when pops grow exponentially Logistic slows when it reaches carrying capacity The human population is going thru a remarkable transition growth is starting to slow down from the fastest growth in our stay on Earth Growth Patterns What tells you that a population is growing exponentially Constant doubling time an accelerating growth rate If a constant doubling time is a feature of simple exponential growth and human pops showed a decreasing doubling time then it is growing faster than exponential growth Up until recently human populations have grown faster When the doubling time slightly increases growth is now slightly slower than exponential What goes into the U N population growth estimates Population growth births deaths The demographers express births as number of children per female fertility Express deaths as life expectancy Death rates dropped dramatically at the end of the 19th century to the middle of the 20th Why Medical advances World life expectancy increased dramatically from about 1900 1945 has been changing only slightly since Birth rates World fertility rates have been dropping Then why is the world still growing 1 Fertility is still above replacement Just having enough children to replace 2 Population structure Fertility is the number of births per female but it also depends on the number of females reproducing or have yet to reproduce Age structure or age distribution graphs can give a snapshot of population growth at a single time Triangular graphs mean the population is growing rapidly Stable age distribution A pop that is neither growing nor declining The bulge at the bottom of rapidly growing age distributions is called population momentum by demographers Population momentum After fertility drops the population keeps growing untll all the children born at high fertility rates have reproduced themselves But why are fertility rates dropping worldwide Part of a robust pattern without one clear explanation the demographic transition The demographic transition is a pattern Countries go from having small slow growing populations to having large slow growing populations in four steps 1 The initial condition High death rates high birth rates No growth small population 2 Death rate falls but birth rate stays the same rapid growth larger population 3 Birth rate falls death rate stays the same population increase cause of population momentum large population 4 Birth and death rates stay the same no growth large population Most developing countries are somewhere in the process fertility rates are dropping but populations are still growing What causes the dropping fertility rate Somewhat of a mystery but some likely factors are that there is a drop in the ideal family size couples with education for girls access to contraception in some cases costs of children born increasingly by families No one factor appears universal to fertility decline K is the population size that the habitat can be sustain indefinitely The UN population projections are based on everything staying exactly the same Don t take into account health of the environment resources or technology Estimates of K Van Leeuwenhoek imagined a world like 17th century Holland Estimates 500 million to 800 billion Dependent on assumption cvalue system and our planet s response to growing population What should be the standard of living Pimental et al K 3 billion 1 2 billion in relative prosperity
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