Once many years ago I read the hypothesis that methane from dinosaur farts caused climate change. Here's a link suggesting the same thing.
Now recent research tells us that the growing population of beavers in marshes is also contributing to greenhouse gases [via Jack]:
“It has long been known the release of methane from beaver ponds is more intense than for other types of wetlands. With the species’ re-establishment and population growth in regions where beavers have been introduced, we set out to quantify whether the methane produced would be large enough to be significant...”
“We found that valuable habitat area has been established by beavers over the last century,” Mr. Whitfield said.
“While this habitat contributes to the global methane gas emissions, the magnitude of this methane source is lower than many other natural sources and unlikely to be a dominant climate change driver.”
Contributing? Yes, but not much apparently.