There are two types of stresses associated with reef systems: natural and human-induced. The effects of these stresses can range from negligible to catastrophic. Reefs display a surprising adaptation to short-term natural catastrophic events, such as hurricanes, and usually recover to normal community structure. These natural events can even be considered beneficial in regards to biological diversity. Severe storm events on land can topple large trees. This opens up the forest to recolonization and results in a greater diversity of plants. This same process occurs with storm impacts to reefs. The damaged area of the reef is often recolonized by a greater diversity of organisms than existed before the storm. In the long term this event benefits the ecological integrity of the reef.
However, reefs are not well adapted to survive exposure to long-term stress. Some examples include agricultural and industrial runoff, increased sedimentation from land clearing, human sewage and toxic discharges. Many land-based activities have important implications for reefs. Agricultural activities can introduce herbicides, pesticides, fertilizers and runoff from animal feed lots. Sewage discharges can introduce nitrogen and phosphate compounds along with pathogens and mixtures of toxics. Uncontrolled land clearing can result in erosion, with the resultant increase in sediment loads to surface waters. Roadways, parking lots and buildings consist of impervious surfaces. These surfaces increase runoff rates and carry with those waters mixtures of dissolved substances to surface waters. The surface waters in any watershed eventually discharge into coastal or near-coastal waters. These waters can then impact coral communities associated with these discharge points. Thus, activities occurring in distant locations have impacts to reefs which are far away from these activities.