Worksheet #3
Predation
Distinguish between additive and compensatory mortality
Additive mortality is when predators eat the prey above their carrying capacity and below.
Compensatory mortality means that predators eat the prey when they are above the K which means that they will die anyway.
Read the excerpt from the Seattle times editorial. Viewing sea lions and spawning access as resources (limited by dams), frame the argument of
those against killing sea lions in terms of additive vs compensatory mortality. Why do they believe killing sea lions would not be effective in
restoring salmon?
They said that its habitat destruction, dams and overharvest have far greater impacts.
Killing sea lions in order to compensate the salmon population would be considered as compensatory mortality. They believe that killing sea lions
would not be effective because there’s other factors to take into consideration, like other predators, and the effects of killing sea lions would have
on the environment. Since this is a top-down regulation where the predator controls the prey population. If there wasn't enough salmon for sea
lions to eat, sea lions would eventually die without its resources, so.
Now frame the argument for those in favor:
If sea lion populations wouldn’t be affected by the increased death rates, then it would be an additive mortality where it would help the salmon
population to stabilize.
Both sea lions and birds eat salmon- sea lions eat adult salmon migrating upstream (mostly), birds eat small salmon migrating
downstream(mostly). For a given fish that is eaten, which act of predation (done by bird or by sea lion) is more likely to represent compensatory
mortality? Explain with reference to survivorship curves.
Birds (blue) Sea Lions(black)
There's an overall effect of predation, there might be different outcomes from different fish.
Wolves clearly eat elk, and by 1926 wolves were eliminated from Yellowstone Park partly out of a desire to
increase elk populations. This caused a variety of ecological problems, which led ecologists to propose
reintroducing wolves. This was opposed by some groups who felt that elk populations would be reduced to the
point that food availability was limited for wolves.
Consider the passage below from: Boyce, M.S. 2018. Wolves for Yellowstone: dynamics in time and space.
Journal of Mammalogy, 99(5):1021-1031
Density dependence for wolves was debated, and my earlier models were questioned for assuming density
dependence in wolves based on their well-known territorial behavior. Yet, this assumption clearly was supported
after wolf populations had stabilized and intraspecific interactions were shown to be the most common cause of
mortality among wolves
Does this sound like wolf populations will ultimately be limited by the availabilityofelkasfood? Explain.
Wolfs are density dependent! And their populations will be limited by the availability of ealk food because if
there’s a limitation of resources they will end up having infraspecific interactions which means that they will end
up killing each other to survive. This shows that by being density dependent there’s a bottom up regulation
where resources availability affects populations sizes of wolfs.
D
2)
3)
-)
5)
A
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