lec 26: overharvesting

Question 1

what is overharvesting

Answer 1

the over-exploitation of biological resources

Question 2

what are the 2 hypotheses to the North America extinctions during the late pleistocene

Answer 2

1. climate change hypothesis
2. overkill hypothesis

Question 3

what is the evidence for the overkill hypothesis

Answer 3

1, the extinction occured rapidly (within <1000 yrs)
2. large mammals were decimated (reflecting hunters selection)
3. extinctions followed the spread of humans
4. Large mammals survived in Africa, where humans and megafauna had co-evolved

Question 4

What is the evidence AGAINST the overkill hypothesis

Answer 4

1. some species that were hunted (eg. bison, moose) are still around today
2. Some extinctions started before the Bering Land Bridge formed
3. Australian extinctions started before humans arrived
4. Some herbivorous mammals were not the focus of hunting but also went extinct (this is however explained by secondary extinctions)

Question 5

secondary extinctions example in grasslands

Answer 5

In east africa, african elephants keep grasslands open

where elephants are decimated by poaching, shrubs/trees grow in place of grass

grassland mammals decline

Question 6

catches of whales in the southern hemisphere

Answer 6

decline

Question 7

trends in northern cod abundance (spawning stock)

Answer 7

Decline!

Question 8

Barndoor Skate

Answer 8

one of the largest skates in the northwest Atlantic

once common, but now threatened with extinction from intense trawling

taken in trawlers’ nets as by-catch

Question 9

T of F: Total fish caught has increased 5X in 50 years

Answer 9

True , with a worldwide decline in sticks of large predatory fish (due to fishing down of food webs)

Question 10

Fishing down food webs

Answer 10

• the trophic levels of a species = how far removed it is from the base of the food web
• early fisheries focused on large carnivorous species (i.e. species at hight trophic levels)
• as the larger fish became depleted, fisheries focused on smaller fish (at lower trophic levels)

ex: when Atlantic cod stocks became depleted, fisherman began harvesting snow crab

Question 11

role of top predators in ecosystems

Answer 11

1. predators can regulate productivity through top-down effects
2. Predators can maintain community diversity by controlling dominant species
3. predators can control pest outbreaks by keeping their prey well bellow “carrying capacity”

Question 12

top-down control

Answer 12

system is regulated by consumers

Question 13

bottom-up control

Answer 13

system is regulated by primary production (upward flow of energy)

Question 14

2 trophic levels

Answer 14

plants are controlled

Question 15

3 trophic levels

Answer 15

plants are released

Question 16

4 trophic levels

Answer 16

plants are controlled

Question 17

controlling dominant species (role of top predators) examples

Answer 17

1. starfish in rocky intertidal communities
   (maintain diversity in rocky intertidal ecosystems)
2. sea otters in kelp bed communities
   (maintain kelp bed diversity by controlling herbivores)

Question 18

Carrying capacity (K)

Answer 18

= the maximum size of a population supported by available resources

Question 19

effect of shell harvesting on coral reefs

Answer 19

adds a new layer of predator (human intervention)