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For each of the following situations, would you expect a population to
be r-selected, K-selected, or to bet hedge? (a) A species of ant lives
primarily under tree bark. This is a very stable environment not
affected by random changes in aspects of the environment such as weather.
(b) A species of ant lives in sandy soil. Larval ants suffer from
unpredictable, sometimes high mortality because strong winds may cause
them to be crushed in the sandy soil, but adult ants are protected from
crushing by their exoskeleton and are not strongly affected by changes
in weather (c) A species of ant has colonies inside acorns; the abundance
and persistance of acorns fluctuate widely as a result of weather and irregular
fluctuations in populations of mammals that eat the acorns, so that the
ants are subject to irregular, unpredictable bouts of high mortality.
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Bet-hedging and K selection make some similar predictions about life history
traits. What are they? Why do these different models predict
similar traits? What would you want to know about the environment
and its effects on survival of various life stages to test whether these
traits evolved through bet-hedging or through K-selection?
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Birds called blue tits lay average clutches that are smaller than the optimal
clutch size predicted when the number of young leaving the care of their
parents is related to clutch size. Give three possible explanations
for this difference between the experimentally determined optimal clutch
size and the observed clutch size.
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Sometimes invertebrates are classified as r-selected, vertebrates as K-selected.
What are problems with this kind of general classification?
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F.S. Dobson and others have found the following life history differences
between low elevation and high elevation populations of Columbian Ground
Squirrels:
Table 1. Life history traits of Columbian ground squirrels
at different elevations in the Rockies
Low elevation High elevation
|
Low elevation |
High elevation |
| Mean Litter size |
3 |
2 |
| Age at maturity |
1-2 years |
2-3 years |
| % adult females that breed |
90% |
45% |
| Survival |
65% |
75% |
| Body mass |
375g |
400g |
(a) Give three possible explanations of the differences.
(b) One group of researchers found that at high elevation there was
a higher standard deviation in temperature than at low elevation.
Which life history theory hypothesis would this observation support?
Why?
(c) A second group of researchers found that weather was more predictable
at high elevation than at low elevation. What hypothesis of life history
theory is supported by this observation? Does this observation support
the same hypothesis as the previous observation? Which observation
-- standard deviation or predictability -- do you think is a better measure
of the factor that should result in life history trait evolution?
(d) A third group of researchers food supplemented some of the populations
and found that if high elevation populations were given a higher food supply,
they had the same life history traits as low elevation populations.
Which hypothesis of life history theory does this observation support?
Why? What does this account for that the previous studies did not?