Marine ecosystem degradation involves the loss of ocean benefits (ecosystem services) to society; it can occur either as a result of natural causes or human activities, such as those considered elsewhere in this report. The marine ecosystem is under severe stress; in many areas the conditions necessary to sustain life are either in jeopardy or non-existent. The destruction of marine habitats is especially prevalent along coastlines where human populations have increased. Habitat loss, pollution, overfishing, destructive fishing practices, and global warming are all undermining the marine environment. Habitat loss, pollution, runoff, and increased salinity are destroying coral reefs, sea grasses, and other habitats for birds and fish. As coastal wetlands are filled in to accommodate growing human populations, the damming of rivers decreases the flow of freshwater, slows nutrient runoff, and inhibits fish migration. Less freshwater means increased salinity in wetlands and estuaries, which harms the grasses that purify water as it flows to the sea. Erosion caused by deforestation sends silt into rivers, streams, and eventually the ocean, blocking the sunlight needed for coral reefs to survive.