People call them the “Dead Marshes”, but that couldn’t be further from the truth. The marshes are in fact incredibly biodiverse and productive. Admittedly that productivity has a lot to do with the slow decomposition of the corpses of men and elves, but they are productive nonetheless. The marsh vegetation also plays an important part in detoxifying the ecosystem. There is a slight problem with the level of mithril contamination in the marshes, but there are several marsh plants that sequester this trace element.
The marshes are also important sinks for carbon. Climate change is increasingly a concern in Middle Earth, what with the dramatic rise in dragon-related emissions and the felling and burning of Fangorn Forest, not to mention the carbon dioxide plume from Mount Doom.