Other land-based activities

Over-exploitation

Marine and coastal ecosystems and the biological diversity that comprise them provide significant physical, economical, social and cultural benefits to humankind. Some of the direct benefits include sources of food and medicine. However, human overexploitation of many of ocean living resources has placed these very ecosystems under severe threat.

Widespread over-harvesting of many target species is well documented, and has resulted in severe stock collapses. The removal of breeding age individuals has a significant and direct impact on the sustainability of populations; and traditional single-species management approaches have largely failed for many fisheries. With reduced numbers of these target species, fishing pressure often increases as fishers search previously untouched and remote areas, or develop more aggressive fishing methods. Additionally, bycatch, or the incidental catch of non-targeted species, is discarded at sea. This provides a supplemental food source for scavengers and opportunistic predators, including fish, crabs seabirds and raptors, and may in turn impact populations of competitors or prey and influence community structure.

Increasing evidence indicates that removal of these top predators, or human-induced perturbations such as bycatch, can have significant cascading effects across the marine food web. Ultimately, this can affect a habitat's ability to sustain the community. For example, in the Caribbean, decades of over-fishing have led, in many places, to very low levels of grazing fish species. Because of this, herbivorous sea urchins have played an increasingly important role in keeping down algal growth. In the 1980s, huge numbers of these urchins succumbed to disease. Without grazing fish or urchin populations, and spurred on in many areas by organic pollution, algae quickly dominated the reefs, inhibiting coral settlement and sometimes overgrowing living corals. Thus, ecosystem function was transformed so that the habitat no longer supports the same assemblage of species. Long-term, such cascading effects could greatly inhibit future human use of marine and coastal living resources.