week 4- marine food webs

Question 1

what is a food web?

Answer 1

‘In general terms it is a pathway of consumption and energy flow from one trophic level to another”
* Simple
* Linear
* Efficient transfer of energy

Question 2

what levels do a food web contain?

Answer 2

4. Tertiary consumer (top predator)
5. Secondary consumer (heterotroph)
6. Primary consumer (heterotroph)
7. Primary producer (autotroph))

Question 3

In marine systems there are consistently only 4-5 levels…why?

Answer 3

• Eating is inefficient
• 70-99% of energy is lost as heat (respiration)
• Energy cannot be created so diminishes through each
  level
• Primary production in marine systems is very large but
  doesn’t scaffold more levels
• Long food chains are unstable
• Predator design is limited (apex predators can only be so
  effective)
• Omnivory (eating both plant matter and animals) is common

Question 4

The importance of body size-

Answer 4

• Energetic demand of consumers increases with body size
• Marine primary producers are very small and numerous
• But some apex predators begin life as eggs larvae and increase in size by 5 orders of magnitude

Question 5

why do Marine systems generally have more
complex food webs for several reasons?

Answer 5

• Low levels of specialism
• Openness of marine systems
• Large size changes along a life history
• Long lifespans – ontogenetic shifts

Question 6

Food web regulation- Bottom-up control

Answer 6

Bottom up control: resource availability drives food web stability
E.g. More phytoplankton so more copepods, sand eels and seabird breeding success

Question 7

Food web regulation- top down control

Answer 7

driven by predator prey interactions sometimes grazing, important for ecosystem structure and function

Question 8

evidence of top down control?

Answer 8

Overfishing led to depletion of cod stocks in N. Atlantic
* Subsequent increases in the abundance of shrimp, crabs, and lobster
* Evidence of top-down control by cod on benthic macro-invertebrates

Question 9

what s a trophic cascade?

Answer 9

• An ecological phenomenon triggered by
  the addition or removal of top predators
  and involving reciprocal changes in the
  relative populations of predator and prey
  through a food chain, which often results
  in dramatic changes in ecosystem structure
  and nutrient cycling

Question 10

what is a Mesopredator release

Answer 10

• A phenomenon in which populations of medium-sized predators rapidly increase after the removal of predators