Perhaps even more important, intact ecosystems perform many vital functions, like purifying the air, filtering harmful substances out of water, turning decayed matter into nutrients, preventing erosion and flooding, and moderating climate. It is not known how many species can be eliminated from an ecosystem without its functioning being impaired. It is likely that an ecosystem with more species is more stable than one that has lost some species. For example, research has shown that grassland plots with a greater number of plant species are better able to withstand drought than those with less species diversity. This stability may well be important in the future, as changes in precipitation brought on by global warming stress ecosystems.
Some species are particularly important to the health of their ecosystems.
A classic example of the consequences of removing a keystone species occurred when fur hunters eliminated sea otters from some Pacific kelp beds. Otters eat sea urchins, which eat kelp. With its major predator gone, urchin populations exploded and consumed most of the kelp. Fish and other animals associated with kelp beds disappeared. In many cultures, humans value animals for reasons other than maintaining ecosystem health.