Rich coastal habitats around the world have been destroyed by landfills, dredging, damming or channelling rivers, and other coastal developments. Half the world's mangrove forests, for example, are already gone. Most of the world's commercial fisheries are under intense pressure, threatening not just the relatively few species targeted but the many more that are taken as by-catch or whose habitats are disrupted. For example, trawling for cod and other species has driven once abundant skate on the east coast of North America to the brink of extinction. Parts of the North Sea floor are trawled eight times a year, presumably with dramatic impacts on seabed communities.
Pollution gets everywhere.
Persistent toxic substances like DDT and PCBs have poisoned seabirds and marine mammals and may have broader, more subtle effects on ecosystems. The massive alteration of ecological cycles is even more alarming. The introduction of species to places far from their home waters, where unchecked by natural controls they sometimes crowd out native biota, is an ever increasing problem. Coral reefs, for example, may be directly affected by increased CO 2 in the atmosphere as well as by the rising temperatures it brings. Biologists once thought that marine species were practically immune to extinction because currents spread their drifting larvae over huge areas, making it hard to wipe them out.