LEC 31 - TROPHIC CASCADE

Question 1

generally, how do top down effects work?

Answer 1

apex predator –> predator –> prim prod

when the apex predator is removed, the predator is allowed to run unchecked, and the prim prod suffers

Question 2

what is the relationship between top down and bottom up effects? in other words, which one dominates?

Answer 2

they are both important.

nturients set the limit on potential biomass, while predation sets the limit on atcual biomass

Question 3

how does compensation work in trophic cascades? what is an example, perhaps with two species of trout?

Answer 3

because animals fill whatever niche opens up, if an animal is removed from a niche another one will take its place

demonstrated w rainbow trout, who were the main ptoniv in an oligo lake, who were promptly removed and replaced by brook trout

Question 4

what are the effects of oligo- and eu- on trophic cascades

Answer 4

in oligo lakes, zpton biomass is too low to have a significant grazing effect (theres just not that much ppton to go around in the first place)

in eu lakes, grazing is low bc inedible cyanobacteria dominate

however in mesotrophic lakes a clear grazing effect is observed

Question 5

what is the “marble in a cup” theory

Answer 5

we oscillate bw two stable states

turbid:
- lots of sediment = low light pen
- macrophytes inhibited by sediments = ppton dominate
- bc theres a lot of ppton, a lot of zpton want to arise, and so ptoniv fish explode in response = less zpton
- bc lots of ptoniv fish, lots of bioturbation (resuspend sediments)

clear:
- no sediments = lots of light pen
- macrophytes dominate, competing w ppton for nutrients
- bc macrophytes are present, they help keep the sediment steady (can even keep an oxic layer down there)
- macrophytes provide refuge for zpton against zptoniv fish

Question 6

atmospheric CO2 can be a problem. which is the biggest contributor: higher algal biomass, or higher large predator biomass?

Answer 6

actually algal biomass due to decomposers

large predator biomass may reduce atmospheric CO2 by decreasing prey biomass, resulting in less detritus and algae

Question 7

gizzard shad have ontogenetic diets. explain.

Answer 7

as young, eat zpton. as they get older, they develop a gut w a grinding chamber and filters on gills - can eat ppton
weird bc they get even more ptoniv whereas most fish switch to pisciv but go piss girl

Question 8

herbiv stoneroller and pisciv bass. they tied da bass down. what was found

Answer 8

within the radius that the bass was tied down, there was more algae. however, otuside the ring, there was no algae. the answer was that the herbiv stoneroller didn’t enter where the bass was so the algae went unchecked, whereas elsewhere they ate the algae