Biodiversity Changes Due to Anthropogenic Effects: Critical Environmental Issues
"The ocean comprises a large portion of Earth's biosphere [and] hosts a vast diversity of flora and fauna that are critical to Earth's biogeochemical cycles and serve as an important source of food and pharmaceuticals.... Human influence on marine biota has increased dramatically, threatening the stability of coastal ecosystems."
National Research Council (1992, p. 3)
Human activities, directly and indirectly, are now the primary cause of changes to marine biodiversity. Natural perturbations have always occurred in the oceans—ranging from seasonal climatic events (such as hurricanes, typhoons, and storm tides) leading to local habitat destruction, to El Niño-Southern Oscillation events, to natural oil seeps—but the resulting changes in biodiversity were frequently reversible or have been long integrated into the larger spatial and temporal patterns of ecosystem structure and function. Effects of many human activities, however, are frequently irreversible, at least over the span of a human life.
Human activities that affect biodiversity are here referred to as critical environmental issues. These issues provide the focus for prioritizing research on the patterns, processes, and consequences of marine biodiversity. In turn, intimate knowledge of how human perturbations affect biodiversity ultimately provides clearer insight regarding the symptoms of changes in the sea caused by human activities.
A vast literature has addressed the types, causes, and significance of anthro-
pogenic stresses in the oceans (e.g., Kinne, 1984; Sherman et al., 1990; Beatley, 1991; GESAMP, 1991; Clark, 1992; Thayer, 1992; Norse, 1993; C.H. Peterson, 1993; Suchanek, 1994). Significant criteria in recognizing those stresses most important to changes in biodiversity include (1) their ubiquity across many different marine habitats, (2) their duration and magnitude, and (3) their degree of reversibility. Moreover, the strength and impact of many human perturbations will vary because of interactions with natural environmental conditions and changes. In turn, alterations to biodiversity may result from (1) the cumulative effects of one or more stresses, (2) the synergistic effects of two or more stresses, or (3) the cumulative or synergistic interactions between natural and human stresses.
Given the above criteria for identifying critical environmental issues, the committee believes the most important agents of present and potential change to marine biodiversity at the genetic, species, and ecosystem levels are the following five activities:
- fisheries operations,
- chemical pollution and eutrophication,
- alteration of physical habitat,
- invasions of exotic species, and
- global climate change.
Singly or in combination, these human perturbations can lead to transformation of energy flow patterns and many other fundamental alterations in system structure and function (e.g., Boxes 5 and 6). Human activities have further led to the global extinction of marine mammals, birds (Vermeij, 1993), and invertebrates (Carlton, 1993), although little is known about the number of threatened, endangered, or extinct marine invertebrates or fish (e.g., Lovejoy, 1980). Many species of marine animals have been hunted to commercial and ecological extinction (Norse, 1993), with potential genetic consequences and concomitant implications for management and conservation programs (Ehrlich and Ehrlich, 1981). Particularly difficult management issues and questions emerge for species that are sliding along the continuum from threatened to endangered.
There is no one operational definition of "serious change" in the oceans—rather, the seriousness of a change is a function of the balance between the magnitude and persistence (endurance) of a perturbation on the one hand, and the ability of a given system to recover from that disturbance when its effects are reduced or stopped on the other. It is evident that a thorough understanding of the composition and functioning of an ecosystem is fundamental to recognizing changes in that system. However, it will not always be possible to separate long-term natural variation or cyclic changes from human impacts and the potentially large synergistic interactions between them. The ubiquity and magnitude of human perturbations have already reduced—and in some areas eliminated—opportunities to study pristine habitats or communities within habitats.
The Decline of the Seaweed Fucus in the Baltic Sea:
The status of the brown seaweed Fucus vesiculosus reflects the dramatic alterations that are now occurring in the abundance and distribution of life in the Baltic Sea. The Fucus community provides shelter, spawning, and foraging for many economically important fish. Fucus is now, however, greatly diminished: in southwest Finland it nearly disappeared in the late 1970s; near Kiel, Germany, Fucus is now found no deeper than 2 m with a 95 percent decline of biomass since the 1950s; and in Sweden it is now limited to 3–4 m depth having once occurred to 6 m in the 1940s. Eutrophication from human activities appears to be the basis for these changes, although synergistic interactions with natural upwelling may have led to the disappearance of Fucus in Finland. Eutrophication led to increased phytoplankton and thus decreased light penetration, and also stimulated dense growths of epiphytic algae on the Fucus, greatly reducing fucoid growth rates and increasing their drag, making the plants more susceptible to storm removal. In different regions the kelp Laminaria saccharina, the mussel Mytilus edulis, and several species of filamentous algae are now replacing Fucus. The Baltic Sea may be an ideal system in which to assess the consequences of distinct species replacements on ecosystem function because of its well-known hydrographic history and relatively low species diversity.
The Functional Extinction of Oyster Reefs in Chesapeake Bay:
A combination of the anthropogenic effects that are now widely altering marine biodiversity is demonstrated by the virtual elimination of the once vast oyster reefs of the Chesapeake Bay. The native oyster Crassostrea virginica once supported an enormous fishery that began to show significant declines a century ago due to overfishing and associated activities (such as the nonreplacement of shell for larval oysters to settle upon). Pollution closed local beds. Dredging, siltation, and marinas physically destroyed oyster habitat. Beginning in the 1950s, two disease agents, "MSX" (caused by the protozoan Haplosporidium nelsoni) and "Dermo" (caused by the protozoan Perkinsus marinus) led to the final demise of many remaining populations. These epidemics may represent resurgences of native species, or these protozoans may have been introduced. A similar fate with the same causes had earlier befallen the oysters of adjacent Delaware Bay. It has been calculated that Atlantic oysters in the Chesapeake Bay were historically abundant enough to filter much of the Bay's water once a week. Today the remaining oysters would require an entire year to do so.
"'O Oysters', said the Carpenter,
'You've had a pleasant run!
Shall we be trotting home again?'
But answer came there none—
And this was scarcely odd, because
They'd eaten every one."
Lewis Carroll (1872)
Fisheries activities are especially ubiquitous across marine habitats, directly affecting virtually every habitat except the deepest sea floors. Even with management practices now in place, fisheries have major impacts on ocean environments, ranging from direct harvest to by-catch effects, habitat destruction, genetic changes, and food web changes. The challenge is to pursue a balanced view that incorporates ecological considerations and societal needs—that is, for long-term sustainable use of marine organisms for food and other products.
Direct Fisheries Effects
The extraction of large numbers of wild organisms (e.g., of finfish, sea urchins, seaweeds, shrimp, and scallops) from marine habitats through commercial fishing may be the most important impact of any current fishing activity. Cod and haddock (NOAA, 1992), Atlantic bluefin tuna (Safina, 1993), and many other fish populations of economic importance to the U.S. (Norse, 1993) have experienced dramatic declines due to overfishing. Both commercial and sport fisheries have intensively removed large populations of other edible, bait, aquarium, and curio trade organisms (such as mussels, abalones, limpets, clams, tropical seashells and corals, and polychaete worms) from coastal habitats. Little noticed when first surveyed by the present generation of scientists, most coral reefs were poised on the edge of profound change from overfishing of large predaceous and herbivorous fish (e.g., see Fig. 5).
Empirical and theoretical studies of trophic structure indicate that these removals do and will have a major effect on the composition and abundance of other species in these habitats and to the overall function of the ecosystem. Thus, on Georges Bank (United States/Canada), the original cod, flounder, and haddock fish populations, largely removed by fisheries, have been replaced by shark and ray populations, resulting in substantially different patterns of energy flow through the system (Backus and Bourne, 1987). In the Chesapeake Bay, a combination of overfishing, pollution, and eutrophication has greatly reduced the abundance of oysters and thus the role of the oysters in filtering the Bay's water (Box 6). Hunting of whales and seals in the nineteenth and twentieth centuries resulted in well-documented crashes of populations of these mammals (Evans, 1987), thus inevitably altering their roles in the food web.
Fisheries activities also cause changes in the demography of target species. Selectively removing the largest, and therefore oldest, components of the population can have dramatic effects. In the California sardine, for example, collapse ensued after the fishery had reduced the population from five to two reproducing year classes, and two consecutive years of poor environmental conditions led to spawning failure (Murphy, 1967). Extracting species with great longevity has often led to significant truncation in the age structure (Leaman, 1991), with potential genetic implications (Ryman, 1991; P.J. Smith et al., 1991). In tropical reef systems, a general pattern in the development of fisheries has been the selective removal of first the large species and then the large individuals within smaller species (Gobert, 1994). For species such as groupers with protogynous hermaphroditism, such size-selective removals have greater impacts on males, potentially altering the sex ratio, lowering mate supply, and thereby affecting the reproductive potential of the population (Shapiro, 1987). Fishing may change other life-history characteristics as well, resulting, for example, in earlier maturation and maturation at smaller sizes through the artificial selection regimes imposed on the populations (DeMartini et al., 1993).
Indirect Fisheries Effects
Three of the major indirect impacts of fisheries are by-catch—that is, capture and mortality of nontarget species (NOAA, 1992)—habitat destruction (discussed below, under "Alterations in Physical Habitat"), and ancillary impacts on interacting species or ecosystem effects. In the eastern tropical Pacific, tuna purse seine fisheries began in the late 1950s and incidentally encircled dolphins to capture the yellowfin tuna schools typically found underneath them. High mortality rates ensued for the dolphins, and populations declined by the mid-1970s to near 20 percent of the 1959 population estimate for eastern spinner dolphin and below 50 percent for the offshore spotted dolphin (T.D. Smith, 1983). Public concern about this by-catch was in part responsible for the passage of the Marine Mammal Protection Act. "Ghost fishing" (lost and abandoned nets that continue to capture fish and mammals) may have localized impacts. By-catch in marine ground fisheries can also have major impacts on fish (NOAA, 1992) and macrobenthos (Watling and Langton, 1994). Shell damage to clams has even been examined as an index of fishing activity in the North Sea (Witbaard and Klein, 1994).
Effects on abundance and demographics of interacting species likewise may be important; depletion of the great whales led to an increase in available krill by approximately 147 million tons per year (Laws, 1977, 1985). Marked changes were noted in growth rates and ages at sexual maturity in other marine mammals such as crabeater seals (Bengston and Laws, 1985) and southern minke whales (Kato, 1987). (For additional discussion of this topic, see Box 2.)
Mariculture, the farming of animals and plants in the sea, has a long and rich history in coastal regions around the world, providing food and employment for many people. Fish, molluscan and crustacean shellfish, and seaweeds are among the most common organisms now cultivated. Large-scale, well-managed, and productive mariculture systems may further have the potential to preserve segments of marine biodiversity—at genetic, species, and habitat levels—by shifting attention away from the extraction and hunting of wild stocks.
Mariculture has, however, also had important impacts on coastal and estuarine habitats in many parts of the world (Ritz et al., 1989; Van der Veer, 1989; Tsutsumi et al., 1991; Rönnberg et al., 1992; Everett et al., in press). These impacts include extensive changes to the benthos and altered local nutrient inputs. Mariculture in the form of sea-ranching can reduce the genetic diversity of a species if the individuals in cultivation begin to contribute substantially to the gene pool of the wild population (that is, through external fertilization between gametes from cultivated and wild members of the species [Mork, 1991; Gharrett and Smoker, 1993]). Mariculture also has the potential to subject wild populations to an increased incidence of disease because of the well-known susceptibility of large, homogeneous populations (Lannan et al., 1989; Padhi and Mandal, 1994).
Effects of Scientific Collecting
The extraction of organisms from the sea by scientists, although on a much smaller-scale than the other activities discussed here, may have local consequences. Scientists or their representatives may intensively collect selected species, a variety of species within certain habitats, or all species at a site, for environmental, ecological, or other biological studies. Scientific fervor over exceptional discoveries may have inadvertent consequences in this regard: intensive scientific collecting off South Africa and in the Indian Ocean of the coelacanth Latimeria chalumnae, a primitive bony fish, led to an increased rarity of this already rare fish (Bruton and Stobbs, 1991).
Chemical Pollution and Eutrophication
Chemical pollution and eutrophication have altered the biodiversity of estuaries and coastal environments (Hughes and Goodall, 1992; Suchanek, 1993, 1994), and pollution has the potential to alter the biodiversity of deep-sea habitats through current and projected uses of this habitat for waste disposal.
The types and sources of marine pollutants vary widely (Hughes and Goodall, 1992). Organic and inorganic wastes enter the sea through sewage and industrial outfalls, river inputs (and their agricultural wastes), direct dumping, mariculture
activities, spills and accidental losses of cargoes, and through atmospheric deposition of particulate materials. Halogenated hydrocarbons (pesticides, herbicides, and plastic compounds such as polychlorinated biphenols [PCBs]), heavy metals, petroleum products (including compounds such as polycyclic aromatic hydrocarbons [PAHs]), fertilizers (nitrogenous and phosphorous compounds), mining wastes, fuel ash, and radioactive materials are among the primary marine pollutants (Hughes and Goodall, 1992). An increased incidence of tumors and diseases in fish is one of the many consequences of contamination of estuarine and coastal environments by this broad range of pollutants (Myers et al., 1991; McCain et al., 1992; Vethaak and Rheinaldt, 1992).
Coastal eutrophication—nutrient enrichment from agricultural, sewage, and urban sources—has had severe impacts in shallow shelf areas and enclosed estuaries and bays worldwide (Nixon et al., 1986; Mannion, 1992; Turner and Rabalais, 1994). Macroalgal and phytoplankton blooms are frequent results which, in turn, often create conditions of hypoxia (low oxygen concentrations) and anoxia (no oxygen). Extensive invertebrate and fish mortalities may ensue (Norse, 1993). In particular, eutrophication has been linked to a more common occurrence of blooms of toxic algae (Hallegraeff, 1993; Smayda and Shimizu, 1993; Anderson, 1994).
Alterations in Physical Habitat
Coastal zones around the world have undergone significant physical alterations. In many regions large portions of salt marshes have been removed by dredging, filling, and diking to create dry land (Chabreck, 1988). Such activities are manifested today in tropical estuaries through the removal of mangrove communities for shrimp pond aquaculture (Robertson and Alongi, 1992; Norse, 1993). Long stretches of coastline in many regions of the world have been impacted by the emplacement of seawalls, jetties, groins, railroads, and other artificial structures that have altered natural patterns of sedimentation, erosion, and water flow. Mining has directly impacted intertidal and nearshore habitats and is a potential source of stress to the biodiversity of the deep sea. Upland and coastal mining, agriculture, and deforestation have caused extensive land erosion and the subsequent deposition of sediment, at times meters thick, in intertidal and shallow-water systems. In fact, sedimentation has become the major threat to certain coral reefs.
Other coastal habitats have been extensively altered through dredging (e.g., for ship channels) and by commercial dragging of the bottom in nearshore habitats for fish, clams, sea urchins, and other commercial targets (Matishov and Pavlova, 1994). Indeed, trawling and dredging on the seafloor are important indirect effects of fisheries operations. Recent surveys have revealed particularly profound impacts in the Gulf of Maine (Witman and Sebens, 1992; Watling and Langton, 1994) and elsewhere such as the North Sea (de Groot, 1984; see general
review by Jones, 1992). In these regions the "megabenthos" (the largest bottom-dwelling animals and plants) have been entirely lost or significantly altered because of trawling and dredging activities.
By reducing the flow of water into estuaries, dams built throughout the twentieth century have significantly depressed the successful return of anadromous fish such as salmon and shad to their spawning grounds and have had many other effects on the local estuarine habitat by altering the natural salinity gradient (Skreslet, 1986). Although fewer dams are now being built, water diversion projects for agriculture and urban development are succeeding dams as a threat to estuarine biodiversity (Skreslet, 1986).
Invasions of Exotic Species
Biological invasions have become ubiquitous in virtually all habitats occupied or modified by human activities (OTA, 1993). Many estuarine and nearshore environments have been extensively invaded by exotic (nonindigenous) species, especially through the transport of larvae and spores in ballast water of ships, but also through introductions associated with mariculture. Box 7 focuses on biological invasions via ballast-water transport, and a specific example of ecosystem-level effects of a ballast-water invasion was given in Box 6.
The extent to which exotic invasions have affected the pelagic ocean, the coastal shelf, and tropical ecosystems such as coral reefs is largely unknown because of a lack of baseline information on the composition of these communities and a lack of studies focused specifically on invasions.
Global Climate Change
Atmospheric pollution is altering the exposure of the oceans to ultraviolet (UV) radiation and is increasing the concentration of gases that may lead to long-term climatic changes.
Compounds generated by human activities, including chlorofluorocarbons and brominated compounds rising into the stratosphere, destroy the ozone that shields the atmosphere from the sun's UV radiation. Increased UV-B (the biologically damaging UV) radiation penetrates many meters below the surface of the ocean (Fleishmann, 1989; R.C. Smith et al., 1992). UV-B exposure has increased under ozone "holes" in the Antarctic and elsewhere. In addition, recent satellite data indicate that volcanic activity (e.g., the 1991 eruption of Mt. Pinatubo) has reduced total air column ozone by as much as 10 percent (leading to increases in UV-B exposure of approximately 20 percent) in latitudes as low as Florida and the Bahamas (Gleason et al., 1993).
In turn, studies have confirmed that significant biological and ecological damage to phytoplankton and zooplankton (Hardy and Gucinski, 1989; Kramer, 1990; Behrenfeld et al., 1993a, 1993b), ichthyoplankton (Hunter et al., 1981),
Box 7: Exotic species introduced via ballast-water transport have displaced natural fauna and flora and may be threatening the world's coastal ocean biota.
BIOLOGICAL INVASIONS VIA BALLAST-WATER TRANSPORT
The most important global dispersal mechanism for passively moving shallow-water organisms between and across oceans is ships' ballast water and sediments. One estimate suggests that more than 3,000 species of coastal marine animals and plants are in transit around the world at any given moment in the ballast of ships. The result is that aquatic habitats all over the world are becoming dominated by exotic species. Scores if not hundreds of invasions have occurred during and since the 1980s alone. Examples include:
Introductions occur in all habitats, although they appear to be most common in estuaries and bays. Many invasions have profoundly altered the distribution and abundance of native species and the food webs of the systems they have invaded. Human introductions transcend natural dispersal barriers, bringing into contact organisms with no evolutionary experience between them, thus setting the stage for often dire results. The prospects for future spectacular invasions of coastal waters around the world remain extraordinarily high, as long as ballast water continues to be moved and released.
and benthic organisms (Jokiel, 1980; Jokiel and York, 1984; Kramer, 1990; Bothwell et al., 1994) from UV-B can and does occur in relatively shallow or surface waters. Recent work has further documented the, effects of increased UV-B on corals (Gleason and Wellington, 1994). The species-specificity of Antarctic phytoplankton susceptibility to UV-B damage, for example,
suggests potential changes in the size and taxonomic structure of the phytoplankton assemblage (Karentz et al., 1991).
The burning of fossil fuels and global agricultural practices have increased the amount of carbon dioxide, methane, and other gases in the atmosphere—such gases trap the heat radiating from the Earth, creating a "greenhouse effect." Continued increases in these gases have a strong potential to lead to global warming, and many scientists think that such warming has begun. A warming Earth could affect the sea, from the most inland marshes to the deepest oceans, in predicted ways that range from sea-level rise to modified patterns of rainfall and oceanic circulation (which, in turn, would affect nutrient supply and distribution). Increased sea water temperatures may alter the abundance, distribution, and reproduction of many coastal species (Ray et al., 1992), and may make northern regions more susceptible to invasions by warm temperate and subtropical species (Chapman, 1988).
As with all other environmental perturbations, there exists the potential for synergisms and cascading effects of global climate change that have not yet been considered, and which may interact with the other stresses reviewed here.