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.