Lecture 19 - BIO220

statistical problem 1 they are taking obser ved data

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Unformatted text preview: ases K (ppl s urvive longer to reproductive age) -birth control will reduce r. -Statistical Problem 1: They are taking obser ved data and trying to project it for ward i nto the future -Statistical Problem 2: Extrapolating beyond obser ved data is always a risk -Biological Problem 1: No option for oscillation to settle into K capacity (shoots up, crashes b ack down, eventually settles at K) The real human trajectory can’t be modeled simply • Growth (r) depends on balance of fertility rates (bx) and mortality (inverse of Lx) • Birth rates have been high for much of human history, but death rates dropped radically during the 19th 20th centuries • Changes in health care raised r – First vaccination & hygiene, then antibiotics, insecticides (Locke Rowe’s lectures) • Changes in agriculture raised K Ricklefs Ch. 12 -Differing degrees of oscillation due to the lag time from when D dependence is introduced -Green: approaches K, never gets above it Red&Blue: Density D occurs, but it has a time lag. Increased D effects will be felt in the f uture, not immediately. -Perhaps Allee was right.. we are just in the middle of a massive overshoot! -Perhaps there will be a correction and then it will come back down (We have no evidence to s uggest or refute this). Current guesses about trajectory • UN: rate is slowing, might level off at about 9 billion • In 1970, growing at about 2% per year, now down to about 1.2%, a big change… • …but still growing exponentially! • At present, adding 74 million per year • Leveling off requires less birth or more death Three scenarios (United Nations) Optimistic: In lower projections, declines are due to drops in fertility – Genetics, machines,...
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