What group of organisms continue the clean up process after scavengers have moved on?

Probably. Ecosystems are connected. While scientists do break the world down to study it in smaller pieces because we can't comprehend the ecosystem of the whole world at one time, the whole world is one big ecosystem. Some parts of it are more directly connected to others, and some parts are less directly connected. If something catastrophic happened in one ecosystem, it would affect other ecosystems. But also remember that the bigger the network, the less any one species affects it. Because there are so many species in the world, the world is a HUGE ecosystem. The affect would be somewhat absorbed by the vastness of the ecosystem. However, if the extinction of one species (such as fish) has a large impact on one ecosystem, it's impact on surrounding ecosystems might be noticed. When you look at the world as a whole, it's not the extinction of any one species that is very, very dangerous but the extinction of many species. If enough species (like the fish) died in "separate" ecosystems, eventually the ecosystem of the whole would be affected.