When Trisha Atwood of the University of British Columbia studied the effect of removing predator fish from ponds and rivers in Canada and Costa Rica, she found a consistent pattern – carbon dioxide emissions increased more than tenfold after the predators were removed.
Wiping out the top predator results in a "trophic cascade" in which the top predator's prey proliferate, which puts pressure on the species that the prey eats and so on down the food chain. Changes to species at the bottom of the food chain, in this case photosynthesising algae, can dramatically increase the amount of CO2 that the ecosystem releases to the atmosphere.
In another study, Christopher Wilmers of the University of California, Santa Cruz, found that the disappearance of sea otters from North American coastlines, resulted in sea urchins thriving and eating out kelp forests, resulting in major CO2 releases.
A study of island ecosystems around New Zealand found the reverse effect. On those islands where there were introduced rats, the rats had become the top predator, wiping out seabird colonies. The ecosystems of those islands without rats stored 40% more carbon than those with rats.