Question

What happens to the population size of each of the pond organisms when the perch population size is decreased? How many of the species in the food web are affected by a population change in a single species?

Answer 1

I found a presentation of Food web of a pond that will greatly connect with the above problem.  http://www.eduweb.com/portfolio/earthsystems/food/foodweb4.html

Normal setting:
Species        Pop. size
Blue heron   Medium
Perch            Medium
Bass              Medium
Minnows      Medium
Inverts          Medium
Algae           Medium

If PERCH population size decreases, 
Species        Pop. sizeBlue heron   LowPerch            LowBass              MediumMinnows      HighInverts          HighAlgae           Low

As you can see, 4 other species are affected when the Perch population size decreased. Bass is not affected.