Ecosystems

ECOSYSTEMS

The health of humans, like all living organisms, is dependent on an ecosystem that sustains life. Healthy ecosystems are the sine qua non for healthy organisms. Yet there is abundant evidence that many life-support systems are far from healthy, placing an increased burden on human health. In some areas of the world, gains in life expectancy and quality of life made during the twentieth century are at risk of being reversed in the twenty-first century. The consequences of ecosystem degradation to human health are numerous, and include health risks from unsafe drinking water, polluted air, climate change, emerging new diseases, and the resurgence of old diseases owing to ecological imbalances. Reversing this damage is possible in some cases, but not in others. Prevention of ecological damage is by far the most efficient strategy.

DEFINING ECOSYSTEMS

An ecological system may be defined as a community of plants and animals interacting with each other and their abiotic, or natural, environment. Typically, ecosystems are differentiated on the basis of dominant vegetation, topography, climate, or some other criteria. Boreal forests, for example, are characterized by the predominance of coniferous trees; prairies are characterized by the predominance of grasses; the Arctic tundra is determined partly by the harsh climatic zone. In most areas of the world, the human community is an important and often dominant component of the ecosystem. Ecosystems include not only natural areas (e.g., forests, lakes, marine coastal systems) but also human-constructed systems (e.g., urban ecosystems, agroecosystems, impoundments). Human populations are increasingly concentrated in urban ecosystems, and it is estimated that, by the year 2010, 50 percent of the world's population will be living in urban areas.

A landscape comprises a mosaic of ecosystems, including towns, rivers, lakes, agricultural systems, and so on. Precise boundaries between ecosystems are often difficult to establish. Often regions slide into one another gradually, over a protracted "transition" zone, as for example between the boreal forest and the Taiga regions of Canada.

ECOSYSTEM HEALTH

It is important to recognize the inherent difficulties in defining "health," whether at the level of the individual, population, or ecosystem. The concept of health is somewhat of an enigma, being easier to define in its absence (sickness) than in its presence. Perhaps partially for that reason, ecologists have resisted applying the notion of "health" to ecosystems. Yet, ecosystems can become dysfunctional, particularly under chronic stress from human activity. For example, the discharge of nutrients from sewage, industrial waste, or agricultural runoff into lakes or rivers affects the normal functioning of the ecosystem, and can result in severe impairment. Excessive nutrient inputs from human activity was one of the major factors that severely compromised the health of the lower Laurentian Great Lakes (Lake Erie and Lake Ontario) and regions of the upper Great Lakes (Lake Michigan). Unfortunately, degraded ecosystems are becoming more the rule than the exception.

The study of the features of degraded systems, and comparisons with systems that have not been altered by human activity, makes it possible to identify the characteristics of healthy ecosystems. Healthy ecosystems may be characterized not only by the absence of signs of pathology, but also by signs of health, including measures of vigor (productivity), organization, and resilience.

Vigor can be assessed in terms of the metabolism (activity and productivity) of the system. Ecosystems differ greatly in their normal ranges of productivity. Estuaries are far more productive than open oceans, and marshes have higher productivity than deserts. Health is not evaluated by applying one standard to all systems. Organization can be assessed by the structure of the biotic community that forms an ecosystem and by the nature of the interactions between the species (both plants and animals). Invariably, healthy ecosystems have more diversity of biota than ecologically compromised systems. Resilience is the capacity of an ecosystem to maintain its structure and functions in the face of natural disturbances. Systems with a history of chronic stress are less likely to recover from normal perturbations such as drought than those systems that have been relatively less stressed.

Healthy ecosystems can also be characterized in economic, social, and human health terms. Healthy ecosystems support a certain level of economic activity. This is not to say that the ecosystem is necessarily self-sufficient, but rather that it supports economic productivity to enable the human community to meet reasonable needs. Inevitably, ecosystem degradation impinges on the long-term sustainability of the human economy that is associated with it, although in the short-term this may not be evident, as natural capital (e.g., soils, renewable resources) may be overexploited and temporarily enhance economic returns. Similarly, with respect to social well-being, healthy ecosystems provide a basis for and encourage community integration. Historically, for example, native Hawaiian groups managed their ecosystem through a well-developed social cohesiveness that provided a high degree of cooperation in fishing and farming activity.

Another reflection of ecosystem health lies directly in the public health domain. In spring 2000, a deadly strain of the bacterium E-coli (0157:H7) entered the public water supply in Walkerton, Ontario, Canada, causing seven deaths and making thousands sick. This small town, with a population of five thousand, is in a farming community. Inadequate manure management from cattle operations was the likely source of this tragedy.

HOW HEALTHY ECOSYSTEMS BECOME PATHOLOGICAL

Stress from human activity is a major factor in transforming healthy ecosystems to sick ecosystems. Chronic stress from human activity differs from natural disturbances. Natural disturbances (fires, floods, periodic insect infestations) are part of the dynamics of most ecosystems. These processes help to "reset" ecosystems by recycling nutrients and clearing space for recolonization by biota that may be better adapted to changing environments. Thus, natural perturbations help keep ecosystems healthy. In contrast, chronic and acute stress on ecosystems resulting from human activity (e.g., construction of large dams, release of nutrients and toxic substances into the air, water, and land) generally results in long-term ecological dysfunction.

Five major sources of human-induced (anthropogenic) stresses have been identified by D. J. Rapport and A. M. Friend (1979): physical restructuring, overharvesting, waste residuals, introduction of exotic species, and global change.

Physical Restructuring. Activities such as wetland drainage, removal of shoals in lakes, damming of rivers, and road construction fragment the landscape and alter and damage critical habitat. These activities also disrupt nutrient cycling, and cause the loss of biodiversity.

Overharvesting. Overexploitation is commonplace when it comes to harvesting of wildlife, fisheries, and forests. Over long periods of time, stocks of preferred species are reduced. For example, the giant redwoods that once thrived along the California coast now exist only in remnant patches because of overharvesting. When dominant species like the giant redwoods (arguably the world's tallest tree—one specimen was recorded at 110 meters tall with a circumference of 13.4 meters) are lost, the entire ecosystem becomes transformed. Overharvesting often results in reduced biodiversity of endemic species, while facilitating the invasion of opportunistic species.

Waste Residuals. Discharges from municipal, industrial, and agricultural sources into the air, water, and land have severely compromised many of the earth's ecosystems. The effects are particularly apparent in aquatic ecosystems. In some lakes that lack a natural buffering capacity, acid precipitation has eliminated most of the fish and other organisms. While the visual effect appears beneficial (water clarity goes up) the impact on ecosystem health is devastating. Systems that once contained a variety of organisms and were highly productive (biologically) become devoid of most lifeforms except for a few acid-tolerant bacteria and sediment-dwelling organisms.

Introduction of Exotic Species. The spread of exotics has become a problem in almost every ecosystem of the world. Transporting species from their native habitat to entirely new ecosystems can wreck havoc, as the new environments are often without natural checks and balances for the new species. In the Great Lakes Basin, the accidental introduction of two small pelagic fishes, the alewife and the rainbow smelt, combined with the simultaneous overharvesting of natural predators, such as the lake trout, led to a significant decline in native fish species. The introduction of the sea lamprey, an eel-like predacious fish that attacks larger fish, into Lake Erie and the upper Great Lakes further destabilized the native fish community. The sea lamprey contributed to the demise of the deepwater benthic fish community by preying on lake trout, whitefish, and burbot. This contributed to a shift in the fish community from one that had been dominated by large benthics to one dominated by small pelagics (fish found in the upper layers of the lake profile). This shift from bottom-dwelling fish (benthic) to surface-dwelling fish (pelagic) has now been partially reversed by yet another accidental introduction of an exotic: the zebra mussel. As the zebra mussel is a highly efficient filter of both phtyoplankton and zooplankton, its presence has reduced the available food in the surface waters for pelagic fish. However, while the benthic fish community has gained back its dominance, the preferred benthic fish species have not yet recovered owing to the degree of initial degradation. Overall, the increasing dominance by exotics not only altered the ecology, but also reduced significantly the commercial value of the fisheries.

Global Change. Rapid climate change (or climate warming) is an emerging potential global stress on all of the earth's ecosystems. In evolutionary time, there have of course been large fluctuations in climate. However, for the most part these fluctuations have occurred gradually over long periods of time. Rapid climate change is an entirely different matter. By altering both averages and extremes in precipitation, temperature, and storm events, and by destabilizing the El Niño Southern Oscillation (ENSO), which controls weather patterns over much of the southern Pacific region, many ecosystem processes can become significantly altered. Excessive periods of drought or unusually heavy rains and flooding will exceed the tolerance for many species, thus changing the biotic composition. Flooding and unusually high winds contribute to soil erosion, and at the same time add to nutrient load in rivers and coastal waters.

These anthropogenic stresses have compromised ecosystem function in most regions of the world, resulting in ecosystem distress syndrome (EDS). EDS is characterized by a group of signs, including abnormalities in nutrient cycling, productivity, species diversity and richness, biotic structure, disease prevalence, soil fertility, and so on. The consequences of these changes for human health are not inconsiderable. Impoverished biotic communities are natural harbors for pathogens that affect humans and other species.

ECOSYSTEM HEALTH AND HUMAN HEALTH

An important aspect of ecosystem degradation is the associated increased risk to human health. Traditionally, the concern has been with contaminants, particularly industrial chemicals that can have adverse impacts on human development, neurological functions, reproductive functions, and that appear to be causative agents in a variety of carcinomas. In addition to these serious environmental concerns (where the remedies are often technological, including engineering solutions to reduce the release of contaminants), there are a large number of other risks to human health stemming from ecological imbalance.

Ecosystem distress syndrome results in the loss of valued ecosystem services, including flood control, water quality, air quality, fish and wildlife diversity, and recreation. One of the major signs of EDS is increased disease incidence, both in humans and other species. Human population health should thus be viewed within an ecological context as an expression of the integrity and health of the life-supporting capacity of the environment.

Ecological imbalances triggered by global climate change and other causes are responsible for increased human health risks.

Climate Change and Vector-Borne Diseases. The global infectious disease burden is on the order of several hundred million cases per year. Many vector-borne diseases are climate sensitive. Malaria, dengue fever, hantavirus pulmonary syndrome, and various forms of viral encephalitis are all in this category. All these diseases are the result of arthropod-borne viruses (arboviruses) which are transmitted to humans as a result of bites from blood-sucking arthropods.

Global climate change—particularly as it impacts both temperatures and precipitation—is highly correlated with the prevalence of vector-borne diseases. For example, viruses carried by mosquitoes, ticks, and other blood-sucking arthropods generally have increased transmission rates with rising temperatures. St. Louis encephalitis (SLE) serves as an example. The mosquito Culex tarsalis carries this virus. The percentage of bites that results in transmission of SLE is dependent on temperature, with greater transmission at higher temperatures.

The temperature dependence of vector-borne diseases is also well illustrated with malaria. Malaria is endemic throughout the tropics, with a high prevalence in Africa, the Indian subcontinent, Southeast Asia, and parts of South and Central America and Mexico. Approximately 2.4 billion people live in areas of risk, with some 350 million new infections occurring annually, resulting in approximately 2 million deaths, predominantly in young children. Untreated malaria can become a life-long affliction—general symptoms include fever, headache, and malaise.

The climate sensitivity of malaria arises owing to the nature of the interactions of parasites, vectors, and hosts, all of which impact the ultimate transmission rates to humans. The gestation time required for the parasite to become fully developed within the mosquito host (a process termed sporogony) is from eight to thirty-five days. When temperatures are in the range of 20°C to 27°C, the gestation time is reduced. Rainfall and humidity also have an influence. Both drought and heavy rains tend to reduce the population of mosquitoes that serve as vectors for malaria. In drier regions of the tropics, low rainfall and humidity restricts the survival of mosquitoes. Severe flooding can result in scouring of rivers and destruction of the breeding habitats for the mosquito vector, while intermediate rainfall enhances vector production.

Ecological Imbalances. Cholera is a serious and potentially fatal disease that is caused by the bacterium Vibrio cholerae. While not nearly so prevalent as malaria, cases are nonetheless numerous. In 1993, there were 296,206 new cases of cholera reported in South America; 9,280 cases were reported in Mexico; 62,964 cases in Africa; and 64,599 cases in Asia. Most outbreaks in Asia, Africa, and South America have originated in coastal areas. Symptoms of cholera include explosive watery diarrhea, vomiting, and abdominal pain. The most recent pandemic of cholera involved more regions than at any previous time in the twentieth century. The disease remains endemic in India, Bangladesh, and Africa. Vibrio cholerae has also been found in the United States—in the Gulf Coast region of Texas, Louisiana, and Florida; the Chesapeake Bay area; and the California coast.

The increase in prevalence of V. cholerae has been strongly linked to degraded coastal marine environments. Nutrient-enriched warmer coastal waters, resulting from a combination of climate change and the use of fertilizers, provides an ideal environment for reproduction and dissemination of V. cholerae. Recent outbreaks of cholera in Bangladesh, for example, are closely correlated with higher sea surface temperatures. V. cholerae attach to the surface of both freshwater and marine copepods (crustaceans), as well as to roots and exposed surfaces of macrophytes (aquatic plants) such as the water hyacinth, the most abundant aquatic plant in Bangladesh. Nutrient enrichment and warmer temperatures give rise to algae blooms and an abundance of macrophytes. The algae blooms provide abundant food for copepods, and the increasing copepod and macrophyte populations provide V. cholerae with habitat. Subsequent dispersal of V. cholerae into estuaries and fresh water bodies allows contact with humans who use these waters for drinking and bathing. Global distribution of marine pathogens such as V. cholerae is further facilitated by ballast water discharged from vessels. Ballast water contains a virtual cocktail of pathogens, including V. cholerae.

Two other examples of how ecological imbalances lead to human health burdens concern the increased prevalence of Lyme disease and hantavirus pulmonary disease. Lyme disease, sonamed because it was first positively identified in Lyme, Connecticut, is a crippling arthritic-type disease that is transmitted by spirochete-infected Ixodes ticks (deer ticks). Ticks acquire the infection from rodents, and spend part of their life cycle on deer. Three factors have combined to increase the risk to humans of contracting Lyme disease, particularly in North America: (1) the elimination of natural deer predators, particularly wolves; (2) reforestation of abandoned farmland has created more favorable habitat for deer; and (3) the creation of suburban estates, which the deer find ideal habitat for browsing. The net result is a rising deer population, which increases the chances of humans coming into more contact with ticks.

By 1995, in the southwestern United States, hantavirus infection was confirmed in ninety-four persons in twenty states, with 48 percent mortality. Variants of the strain that causes hantavirus pulmonary syndrome have also been found in other areas of the country, as well as in Asia and Europe. The virus is apparently asymptomatic in rodents, and it is transmitted in their saliva and excreta. In humans it has a flu-like presentation, which is followed by acute respiratory distress syndrome. The primary reservoir in the Four Corners area of the southwestern United States is the deer mouse. Climatic disturbances, which in recent years are thought to be exacerbated by human activity (e.g., global warming), appear to set up conditions that trigger outbreaks. In the early 1990s, ENSO events initially caused drought conditions to develop in the southwestern United States. This led to a decline in plant and animal populations, including natural predators of the deer mouse. Heavy rains followed the drought in 1993, resulting in a bumper crop of piñon nuts, a major food supply for the deer mouse. Subsequently the deer mouse population greatly increased, bringing about increased contact with humans and triggering the outbreak of hantavirus.

Antibiotic Resistance and Agricultural Practice Antibiotic resistance is a growing threat to public health. Antibiotic resistant strains of Streptococcus pneumoniae, a common bacterial pathogen in humans and a leading cause of many infections, including chronic bronchitis, pneumonia, and meningitis, have greatly increased in prevalence since the mid-1970s. In some regions of the world, up to 70 percent of bacterial isolates taken from patients proved resistant to penicillin and other b-lactam antibiotics. The use of large quantities of antibiotics in agriculture and aquaculture appears to have been a key factor in the development of antibiotic resistance by pathogens in farm animals that subsequently may also infect humans. One of the most serious risks to human health from such practices is vancomycin-resistant enterococci. The use of avoparcin, an animal growth promoter, appears to have compromised the utility of vancomycin, the last antibiotic effective against multi-drug-resistant bacteria. In areas where avoparcin has been used, such as on farms in Denmark and Germany, vancomycin-resistant bacteria have been detected in meat sold in supermarkets. Avoparcin was subsequently banned by the European Union. Another example is the use of ofloxacin to protect chickens from infection and thereby enhance their growth. This drug is closely related to ciprofloxacin, one of the most widely used antibiotics in the year 2000. There have been cases of resistance to ciprofloxacin directly related to its veterinary use. In the United Kingdom, ciprofloxacin resistance developed in strains of campylobacter, a common cause of diarrhea. Multi-drug-resistant strains of salmonella have been traced to European egg production.

Food and Water Security. Agricultural practices are also responsible for a growing number of threats to public health. Some of these are related to inadequate waste management, which has resulted in parasites and bacteria entering water supplies. Others are of entirely different origins and involve apparent transfer across species of pathogens that affect both animals and humans. The most recent and spectacular example is mad cow disease, known as variant Creutzfeldt-Jakob disease in humans, a neuro-degenerative condition that, in humans, is ultimately fatal. The first case of Bovine Spongiform Encephalopathy (BSE), the animal form of the disease, was identified in Southern England in November 1981. By the fall of 2000, an outbreak had also occurred in France, and isolated cases appeared in Germany, Switzerland, and Spain. More than one hundred deaths in Europe were attributed to what has come to be commonly called mad cow disease.

Improper manure management was the likely source of the outbreak of E. coli 0157:H7 in Walkerton, Ontario, Canada. Other health risks associated with malfunctioning agroecosystems include periodic outbreaks of cryptosporidiosis, a parasitic disease that is spread by surface runoff contaminated by feces of infected cattle. This parasite causes fever and diarrhea in immunocompetent individuals and severe diarrhea and even death in immunocompromised individuals.

ECOSYSTEM RESTORATION

Ecosystem pathology in some cases can be reversed simply by removing the source of stress. In cases, for example, where ecosystem degradation is the result of point-source additions of nutrients or toxic chemicals, removal of these stresses may result in considerable recovery of ecosystem health. A classic case is Lake Washington (near Seattle, Washington). This lake had become highly anoxic (oxygen-depleted) owing to a sewage outfall entering the lake. Redirecting the sewage outfall away from the lake reversed many of the signs of pathology.

In cases where it is not feasible to remove the source of stress, more innovative engineering solutions have been tried. For example, in the Kyrönjoki and Lestijoki Rivers in western Finland, spring and fall runoff leads to sharp pulses of acidity. Spring runoff from snowmelt, which releases acid from tilled or dug soils, has been particularly damaging to fish, during the critical time of year for spawning. Fish reproduction is severely curtailed, if not all together eliminated in highly acidic water. Further there have been massive fish kills resulting from the highly acidic waters. One possible remedy is to replace the original drains which take runoff from the land to the rivers with new limed drains that can neutralize the acidity. This solution has been implemented on an experimental basis and appears to substantially reduce acidic runoff.

More radical treatments for damaged ecosystems involve "ecosystem surgery." In some cases, invading exotic vegetation (such as mangroves in Hawaii) have been removed from regions, and native vegetation has been replanted. In areas of North America where wetlands have been severely depleted owing to farming, urbanization, and industrial activity, efforts have been made to establish new wetlands.

More often than not, however, reversing ecosystem pathology is not possible. Efforts to restore the indigenous grasslands in the Jornada Experimental Range in the southwestern United States provide an example. Overgrazing by cattle has severely degraded the landscape and has lead to replacement of the native grasses by largely inedible shrubs, dominated by mesquite. Erosion by wind and episodic heavy rains have left areas between shrubs largely bare, and subsequently underlying sands have developed in dune-like fashion over a large part of the area. The resulting mesquite dunes have proven highly resistant to efforts to restore the native grasslands, although almost every intervention has been tried, including highly toxic defoliants (Agent Orange), fire, and bulldozing.

Even where it has been possible to restore some of the ecological functions of degraded ecosystems, and thus improve ecosystem health, the restoration seldom results in reestablishment of the pristine biotic community. The best that can be achieved in most cases is reestablishment of the key ecological functions that provide the required ecosystem services, such as the regulation of water, primary and secondary productivity, nutrient cycling, and pollination. In all such efforts, key indicators of ecosystem health (vigor, productivity, and resilience) are essential to monitor progress. Standard ecological indicators can be used for this purpose (e.g., measures of productivity, species composition, nutrient flows, soil fertility) along with socioeconomic and human health indicators.

Experience in efforts to restore highly damaged ecosystems suggests that ecosystem-health prevention is far more effective than restoration. For marine ecosystems, setting aside protective zones that afford a sanctuary for fish and wildlife has considerable promise. Many countries are adopting policies to establish such areas with the prospect that these healthy regions can serve as a reservoir for biota that have become depleted in the unprotected areas. Yet this remedy is not without its limits. Restoring ecosystem health is not simply a matter of replenishing lost or damaged biota. It is also a matter of reestablishing the complex interactions among ecosystem lifeforms. Having a ready source of healthy biota that could potentially recolonize damaged ecosystems is important, but it is only part of the solution.

PREVENTION OF ECOSYSTEM DISRUPTIONS

Given the difficulties in reversing ecosystem degradation, and the many associated human health risks that arise with the loss of ecosystem health, the most effective approach is simply the prevention of ecosystem disruption. However, like many common-sense approaches, this is easier said than done. In both developed and developing countries there is a strong inclination to continue economic growth, even at the cost of severe environmental damage. Apart from selfish motivations, the argument is made that economic growth has many obvious health benefits, such as providing more efficient means of distributing food supplies, providing more plentiful food, and providing better health services and funding for research to improve standards of living. These are indeed benefits of economic development, and have led to substantial increases in health status worldwide.

However, at the dawn of the twenty-first century, the past is not necessarily the best guide to the future. The human population is at an alltime high, and associated pressures of human activity have led to increasing degradation of the earth's ecosystems. As ultimately healthy ecosystems are essential for life of all biota, including humans, current global and regional trends are ominous. Under these circumstances, a tradeoff between immediate material gains and long-term sustainability of humans on the planet may be the only option. If so, the solution to sustaining human health and ecosystem health becomes one of devising a new politic that places sustaining lifesupport systems as a precondition for betterment of the human condition.

DAVID J. RAPPORT

(SEE ALSO: Acid Rain; Ambient Air Quality [Air Pollution]; Ambient Water Quality; Biodiversity; Cholera; Ecological Footprint; Emerging Infectious Diseases; Global Burden of Disease; PCBs; Pesticides; Pollution; Vector-Borne Diseases)

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