Perhaps the clearest correlation of population size to species loss is that of human density to wildlife habitat. Nations with densities under 379 per square kilometer retained 45 percent on average, while nations with densities of up to 454 per square kilometer retained only one third of their original habitat. Marine ecosystems are also in decline due to factors such as water diversion and pollution, sedimentation from upstream soil erosion, over-harvesting, and introduced species. While land species receive the most attention, marine species are in many ways more diverse, and more threatened. Almost half of the Earth's animal phyla are exclusively marine, and 31 of 32 known animal phyla are represented in marine environments.
Marine species provide a large percentage of humans' protein supply.
They also absorb tremendous amounts of CO2, and hold the potential for important pharmaceutical advances. Indicators such as shrinking fish stocks and shellfish harvests, extinction of numerous freshwater species, and the decline of coral reefs all demonstrate the increasing threat to marine biodiversity due to human activities. In 1996, the World Conservation Union, completed the first-ever extensive survey of the status of animal life on Earth. Species extinction rates worldwide are now estimated to be 100 to 1,000 times greater than normal, and are rising dramatically. Plants, insects, snails, and many other organisms are also threatened.