Humans are the ultimate weed species.  My 17-year old son is studying environmental science.  After he took the AP exam last week, he explained to me the difference between a keystone species and an indicator species:

The former are often at the top or bottom of the food chain, apex predators like wolves or orcas are keystones because if they disappear from an ecosystem, the animals they eat become too numerous and can overstress their environments.  Thus, they are essential to - or keystones of - a healthy ecosystem.  The same goes for small prey species like voles or mice since their predators may not have an available alternative food source and thus they will disappear shortly after their prey.

Indicator species are species whose health generally correlates well with that of the ecosystem in which they reside.  Polyps in coral reefs are indicator species since a sea with healthy coral reefs is likely to be a healthy sea, i.e., lots of biodiversity with most species thriving.  An unhealthy coral reef signifies just the opposite.

It seems clear that humans are neither keystones nor indicators.  We're not keystones because if you took us out of the web of life, it would immediately start to heal and quickly become stronger.  We're not indicators because, despite the fact that we're living through (actually reponsible for) the 6th great extinction, there are more of us than ever before and individual humans are probably healthier than in the past and more likely to reproduce.