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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
• 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
• At present, adding 74
million per year
• Leveling off requires less
birth or more death Three scenarios
In lower projections,
declines are due to
drops in fertility – Genetics, machines,...
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- Winter '14
- Annotated Lecture S