Introduction

Pressures on Marine Biodiversity
World Resources Institute

An estimated 60 percent of the global population lives within roughly 100 kilometers of the shore. This means that about 3.4 billion people rely heavily on marine habitats and resources for food, building materials, building sites, and agricultural and recreational areas and use coastal areas as a dumping ground for sewage, garbage, and toxic wastes (59). Moreover, much of the remaining noncoastal population is concentrated along rivers and other waterways. Pollution and poor land use practices within these watersheds affect downstream marine habitats because sediments and pollutants are ultimately washed into coastal waters.

Pressures on marine ecosystems include coastal population density and continued population growth, which are accompanied by increased consumer demand for marine products, increased waste disposal, rapid alteration of coastal habitats, uncontrolled industrial pollution, inadequate institutional structures for managing marine resources, lack of property rights and management regimes within international waters, and lack of understanding and awareness of marine ecosystem processes and the effects of human actions on marine biodiversity.

Most of the world's marine ecosystems--particularly nearshore habitats--are stressed by a combination of these factors. The Black Sea, for example, is dying under the weight of pollution and overfishing. Land-based pollution in the form of industrial wastes, sewage, and runoff of pesticides and fertilizers, combined with oil and other wastes from ship traffic, have contaminated the entire basin. Eutrophication has left 90 percent of the Black Sea facing critically low oxygen levels (60). The total fish catch within the region declined by 64 percent between 1986 and 1992 (61). The cost of this damage is estimated at $500 million annually to the fishing and tourism industries alone (62).

The direct factors (pressures) leading to the loss of marine biodiversity can be broken into five categories:

habitat loss,
intense overexploitation,
pollution and sedimentation,
species introductions, and
climate change.


Habitat Loss

Habitat conversion and degradation are generally thought to be the most significant threats to terrestrial life. Within marine ecosystems, they rank along with overexploitation and pollution as major causes of biodiversity loss (63).
Coastal development contributes to habitat loss in a number of ways. These include conversion of mangroves and other wetlands as a result of urbanization and agricultural expansion, the building of shoreline stabilization structures such as breakwaters, mining, oil drilling, and dredging and filling. These result both in the destruction of wetlands and other habitats and in the degradation of nearby areas (through siltation and changes in water temperature and flow, salinity, and other physical factors).

Damming of rivers and water diversion projects lead to changes in downstream estuarine and marine communities, because interruption of freshwater flow changes the physical environment of such areas and the amount of nutrients that they receive. Completion of the Aswan Dam on the Nile River in 1965 led to the erosion of delta habitats and is considered an actor in the subsequent collapse of eastern Mediterranean fisheries (64). In addition, dams can cut off species access to spawning areas--this includes not only species that live in saltwater and reproduce in rivers (such as salmon) but also freshwater species that breed at sea (such as freshwater eels).

Intense exploitation of marine resources can indirectly lead to habitat loss. For example, trawling disturbs bottom-dwelling communities--both adjacent to shorelines and in deeper coastal waters--as nets scour seabeds and smother burrowing creatures and other species with sediments. Fishing with dynamite and harvesting of corals are major threats to coral reef areas.


Intense Overexploitation

According to a 1995 report, from 1988 to 1991, humans removed about 8 percent of all annual primary production (the total amount of living carbon) within aquatic ecosystems. This figure is lower than the ratio of primary production co-opted for human use in terrestrial systems; however, it masks exceptionally high removal rates within some of the most productive and species-rich ecosystems. For example, more than one fourth of all production occurring within ocean upwellings and tropical marine shelf areas is consumed by humans; in temperate shelf regions, it is about 35 percent (65). Continued exploitation at such levels is leading to changes in species composition, loss of biodiversity (66), and shifts in dominance and survival ability (67).

Much of the global fishing effort is targeted at a few species, located primarily near the top of the food chain. Overexploitation of these species has three effects.

First, as discussed earlier, it results in the loss of genetic diversity as fish populations decline.
Second, overfishing affects the relative abundance of individual species or the mix of different species within an ecosystem. Often, populations of both the target species and the predators that feed on these species decline and are replaced by stocks of lesser commercial value.
Third, depleted fisheries have direct economic impacts, including reduced income (and unemployment) and higher consumer prices (68).
Overfishing affects other marine species, not just fish stocks. Overharvesting, along with habitat degradation, is a key factor that contributed to a 95 percent decline in native Chesapeake Bay oyster populations (69). Overhunting has decimated many marine mammal populations. By 1994, 90 marine mammal species were listed as threatened or endangered (70). Poor management practices, subsidization of the fishing industry, uncontrolled harvests within international waters, and destructive and wasteful capture methods are to blame for the overexploitation of most marine species. (See Chapter 13, "Water and Fisheries," for a more detailed discussion of the underlying causes and effects of overfishing.)


Pollution and Sedimentation

Dumping and discharging of pollutants into the sea, oil spills, nutrient- and silt-laden runoff from land and rivers, fallout of chemicals carried by the wind from land-based sources, and noise from ships and other machinery (which disrupts communication among whales and other species) are some of the major contaminants affecting marine species and ecosystems (71). As Figure 11.4 shows, air pollution and runoff and point discharges from the land (and rivers) account for some three fourths of the pollutants entering marine ecosystems.

Contaminants affect marine biodiversity in a number of ways. Untreated sewage, oil, heavy metals, and other wastes may be directly toxic to some marine organisms. Their effects may be instantaneous or cumulative. For example, oil has lethal and almost immediate effects on a wide range of marine life--from algae to seabirds--resulting in death through asphyxiation, poisoning, and, among mammals and birds, loss of the insulating functions of feathers and fur, causing hypothermia. Eggs and larvae are particularly sensitive to the toxic effects of pollutants, as are organisms living at the ocean surface and on the seabed, where wastes tend to accumulate (72).

Other contaminants such as radioactive waste, pesticides, and other chemicals have cumulative effects, building up within individuals over time, especially within species high on the food chain. Moreover, various contaminants and physical degradation can act together in a cumulative or synergistic fashion.

Between 1987 and 1991, dolphin and seal die-offs were recorded in the North and Baltic seas, were recorded in the North and Baltic seas, off the eastern coast of the United States, in the Gulf of Mexico, and in the Mediterranean Sea (73). The carcasses of these animals were found to contain elevated levels of polychlorinated biphenyls (PCBs), dioxins, and other organochlorines, known to accumulate in the blubber (or lipid tissues) of large species and predators at the top of the food chain. These die-offs and an epidemic of tumors observed within green sea turtles have been linked to the cumulative buildup of PCBs and other chemicals that are believed to weaken immune systems, creating a vulnerability to viral infections (74).

Other contaminants can trigger ecosystem-wide changes, resulting in conditions that are inimical to a range of species. Runoff of sewage from cities and of fertilizers from agricultural areas elevates the levels of nutrients within nearshore waters. Certain algal species capitalize on these conditions, undergoing massive population explosions (known as blooms), which, by lowering water clarity and oxygen content, effectively crowd out other taxa in the community (75). (Algal blooms block the light reaching algae living within corals and other photosynthesizing bottom-dwelling organisms, killing them; then, the decomposition of the bloom algae deoxygenates the water.)

Many bloom species produce toxins. So-called killer blooms have been linked to die-offs of fish, shellfish, and other species that consume or come into contact with toxic algae or that ingest other consumers of those algae (76). Human health can also be at risk. A 1987 toxic bloom occurring off the Guatemalan coast, for example, indirectly resulted in the death of 26 people and produced serious illness in 200 other individuals who consumed poisoned seafood (77). Although small-scale blooms (both toxic and nontoxic) are a naturally occurring phenomenon in most regions, the frequency, magnitude, and toxicity of such events appear to have increased dramatically in recent years (78).

Widespread effects are often noted as a result of sedimentation. Soils eroded from deforested areas and poorly managed agricultural lands often end up at sea, reducing light penetration to seagrass bed, coral, and other communities dependent on the productivity of photosynthesizers living on the sea floor. As sediments settle out, they smother bottom-dwelling organisms and affect filter-feeding species.

In a 1990 report, United Nations marine pollution experts estimated that rivers carry volumes of sediment three times higher than the levels that might be found in undeveloped watersheds, testifying to the magnitude of this problem (79).

Nontoxic solid wastes and marine debris cause significant mortality among marine species. For example, plastic bags, fishing lines, and other debris can entangle seals, seabirds, and other organisms, causing slow but sure deaths. Bits of plastic and other man-made materials are regularly ingested by sea turtles and other species, often with fatal consequences. Abandoned fishing nets, lobster pots, and other equipment continue to catch fish and other marine creatures years after the gear is discarded or lost (80).


Species Introductions

For centuries, ships have served as a means by which organisms can hitchhike to new waters (81). Until recently, such transport was limited mainly to animals that attached themselves to or burrowed into the hulls of ocean-going vessels. Now, however, ships carry an enormous variety of exotic species, including both plankton and larger species in larval form, within their ballast water (82). According to one estimate, about 3,000 species are transported in ships around the world each day. This number reflects both the heavy volume of international shipping and the large size (i.e., large volume of ballast water) of modern ships (83).

Accidental introduction of exotic species may be one factor in the apparent spread of toxic blooms; it is also the suspected cause of a disease affecting corals that has recently appeared in waters off the coast of Asia and the Middle East (84) (85). By feeding on or overrunning dominant native species, exotic species can trigger changes in the species mix within ecosystems. For example, an American jellyfish first observed in the Black Sea in 1982 is now one of the most common animals reported in those waters; furthermore, its predation on anchovy stocks contributed to the collapse of the Sea of Azov anchovy fishery (86).

Although there are no documented marine extinctions caused by exotic species, introduced species have played a major role in threatening or leading to the extinction of numerous inland species (87). Even though a species may not be exotic, transfer from one area to another may cause the mixing of genetic stocks and the transmission of disease. For example, a stock enhancement program that transferred the Atlantic salmon from the Baltic Sea to the Norwegian Sea introduced a parasite to the Norwegian Sea that now threatens Norwegian native stocks (88).


Climate Change

Global warming could be a significant threat to marine biodiversity. Among other effects, rising waters (as a result of melting ice caps) could drown coastal mangrove and other wetland habitats. Even if global warming were to proceed at a pace slow enough to permit species to colonize new coastline boundaries, the presence of existing agricultural and urban development with protective bulkheads and dikes would, in many cases, prevent the establishment of new wetland areas (89).

Projected climate change could have other effects, including changes in ocean currents, salinity (due to changes in river flow), and surface temperatures. These would alter the species compositions found within individual ecosystems today, perhaps triggering local and global extinctions in the process (90).

Some evidence exists that local and regional warming episodes may already be affecting marine ecosystems. It is difficult to determine whether these changes are due to natural, cyclical variations in temperature or to a long-term warming trend. As noted above, localized increases in water temperatures are believed to be one of several factors behind recent episodes of global coral bleaching. In a long-term study off the coast of southern California, researchers found an 80 percent decrease in zooplankton density between 1951 and 1993. This decline was linked to increases in ocean surface temperatures of 1.2 to 1.6 degree C during this time period (91). Other scientists in central California have reported major shifts in bottom-dwelling coastal populations over the past half century.