Imbalance of Ecosystems and Its Effect on Public and Livestock Health

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Imbalance of Ecosystems and Its Effect on Public and Livestock Health

Imbalance of Ecosystems and Its effect on				disease remains endemic in India, Bangladesh,
Public and Livestock health				and Africa. Vibrio cholerae has also been
				found in the United States-in the Gulf Coast
Dr.Kedar Karki M.V.St. (Preventive veterinary				region of Texas, Louisiana, and Florida; the
Medicine)				Chesapeake Bay area; and the California
				coast.
Central Veterinary Laboratory Tripureshwor
				The increase in prevalence of V. cholerae has
The health of humans, like all living				been strongly linked to degraded coastal
organisms, is dependent on an ecosystem that				marine environments. Nutrient-enriched warmer
sustains life. Healthy ecosystems are the				coastal waters, resulting from a combination
sine qua non for healthy organisms. Yet there				of climate change and the use of fertilizers,
is abundant evidence that many life-support				provides an ideal environment for
systems are far from healthy, placing an				reproduction and dissemination of V.
increased burden on human health. In some				cholerae. Recent outbreaks of cholera in
areas of the world, gains in life expectancy				Bangladesh, for example, are closely
and quality of life made during the twentieth				correlated with higher sea surface
century are at risk of being reversed in the				temperatures. V. cholerae attach to the
twenty-first century. The consequences of				surface of both freshwater and marine
ecosystem degradation to human health are				copepods (crustaceans), as well as to roots
numerous, and include health risks from				and exposed surfaces of macrophytes (aquatic
unsafe drinking water, polluted air, climate				plants) such as the water hyacinth, the most
change, emerging new diseases, and the				abundant aquatic plant in Bangladesh.
resurgence of old diseases owing to				Nutrient enrichment and warmer temperatures
ecological imbalances. Reversing this damage				give rise to algae blooms and an abundance of
is possible in some cases, but not in others.				macrophytes. The algae blooms provide
Prevention of ecological damage is by far the				abundant food for copepods, and the
most efficient strategy.				increasing copepod and macrophyte populations
				provide V. cholerae with habitat. Subsequent
DEFINING ECOSYSTEMS				dispersal of V. cholerae into estuaries and
				fresh water bodies allows contact with humans
An ecological system may be defined as a				who use these waters for drinking and
community of plants and animals interacting				bathing. Global distribution of marine
with each other and their abiotic, or				pathogens such as V. cholerae is further
natural, environment. Typically, ecosystems				facilitated by ballast water discharged from
are differentiated on the basis of dominant				vessels. Ballast water contains a virtual
vegetation, topography, climate, or some				cocktail of pathogens, including V. cholerae.
other criteria. Boreal forests, for example,
are characterized by the predominance of				Two other examples of how ecological
coniferous trees; prairies are characterized				imbalances lead to human health burdens
by the predominance of grasses; the Arctic				concern the increased prevalence of Lyme
tundra is determined partly by the harsh				disease and hantavirus pulmonary disease.
climatic zone. In most areas of the world,				Lyme disease, sonamed because it was first
the human community is an important and often				positively identified in Lyme, Connecticut,
dominant component of the ecosystem.				is a crippling arthritic-type disease that is
Ecosystems include not only natural areas				transmitted by spirochete-infected Ixodes
(e.g., forests, lakes, marine coastal				ticks (deer ticks). Ticks acquire the
systems) but also human-constructed systems				infection from rodents, and spend part of
(e.g., urban ecosystems, agro-ecosystems,				their life cycle on deer. Three factors have
impoundments). Human populations are				combined to increase the risk to humans of
increasingly concentrated in urban				contracting Lyme disease, particularly in
ecosystems, and it is estimated that, by the				North America: (1) the elimination of natural
year 2010, 50 percent of the world's				deer predators, particularly wolves; (2)
population will be living in urban areas.				reforestation of abandoned farmland has
				created more favorable habitat for deer; and
A landscape comprises a mosaic of ecosystems,				(3) the creation of suburban estates, which
including towns, rivers, lakes, agricultural				the deer find ideal habitat for browsing. The
systems, and so on. Precise boundaries				net result is a rising deer population, which
between ecosystems are often difficult to				increases the chances of humans coming into
establish. Often regions slide into one				more contact with ticks.
another gradually, over a protracted
"transition" zone, as for example between the				By 1995, in the southwestern United States,
boreal forest and the Taiga regions of				hantavirus infection was confirmed in
Canada.				ninety-four persons in twenty states, with 48
				percent mortality. Variants of the strain
ECOSYSTEM HEALTH				that causes hantavirus pulmonary syndrome
				have also been found in other areas of the
It is important to recognize the inherent				country, as well as in Asia and Europe. The
difficulties in defining "health," whether at				virus is apparently asymptomatic in rodents,
the level of the individual, population, or				and it is transmitted in their saliva and
ecosystem. The concept of health is somewhat				excreta. In humans it has a flu-like
of an enigma, being easier to define in its				presentation, which is followed by acute
absence (sickness) than in its presence.				respiratory distress syndrome. The primary
Perhaps partially for that reason, ecologists				reservoir in the Four Corners area of the
have resisted applying the notion of "health"				southwestern United States is the deer mouse.
to ecosystems. Yet, ecosystems can become				Climatic disturbances, which in recent years
dysfunctional, particularly under chronic				are thought to be exacerbated by human
stress from human activity.Example for this				activity (e.g., global warming), appear to
can be cited the discharge of nutrients from				set up conditions that trigger outbreaks. In
sewage, industrial waste, or agricultural				the early 1990s, ENSO events initially caused
runoff into lakes or rivers affects the				drought conditions to develop in the
normal functioning of the ecosystem, and can				southwestern United States. This led to a
result in severe impairment. Excessive				decline in plant and animal populations,
nutrient inputs from human activity was one				including natural predators of the deer
of the major factors that severely				mouse. Heavy rains followed the drought in
compromised the health of the lower				1993, resulting in a bumper crop of piÃ±on
Laurentian Great Lakes (Lake Erie and Lake				nuts, a major food supply for the deer mouse.
Ontario) and regions of the upper Great Lakes				Subsequently the deer mouse population
(Lake Michigan). Unfortunately, degraded				greatly increased, bringing about increased
ecosystems are becoming more the rule than				contact with humans and triggering the
the exception.				outbreak of hantavirus.

The study of the features of degraded				Antibiotic Resistance and Agricultural
systems, and comparisons with systems that				Practice Antibiotic resistance is a growing
have not been altered by human activity,				threat to public health. Antibiotic resistant
makes it possible to identify the				strains of Streptococcus pneumoniae, a common
characteristics of healthy ecosystems.				bacterial pathogen in humans and a leading
Healthy ecosystems may be characterized not				cause of many infections, including chronic
only by the absence of signs of pathology,				bronchitis, pneumonia, and meningitis, have
but also by signs of health, including				greatly increased in prevalence since the
measures of vigor (productivity),				mid-1970s. In some regions of the world, up
organization, and resilience.				to 70 percent of bacterial isolates taken
				from patients proved resistant to penicillin
Vigor can be assessed in terms of the				and other b-lactam antibiotics. The use of
metabolism (activity and productivity) of the				large quantities of antibiotics in
system. Ecosystems differ greatly in their				agriculture and aquaculture appears to have
normal ranges of productivity. Estuaries are				been a key factor in the development of
far more productive than open oceans, and				antibiotic resistance by pathogens in farm
marshes have higher productivity than				animals that subsequently may also infect
deserts. Health is not evaluated by applying				humans. One of the most serious risks to
one standard to all systems. Organization can				human health from such practices is
be assessed by the structure of the biotic				vancomycin-resistant enterococci. The use of
community that forms an ecosystem and by the				avoparcin, an animal growth promoter, appears
nature of the interactions between the				to have compromised the utility of
species (both plants and animals).				vancomycin, the last antibiotic effective
Invariably, healthy ecosystems have more				against multi-drug-resistant bacteria. In
diversity of biota than ecologically				areas where avoparcin has been used, such as
compromised systems. Resilience is the				on farms in Denmark and Germany,
capacity of an ecosystem to maintain its				vancomycin-resistant bacteria have been
structure and functions in the face of				detected in meat sold in supermarkets.
natural disturbances. Systems with a history				Avoparcin was subsequently banned by the
of chronic stress are less likely to recover				European Union. Another example is the use of
from normal perturbations such as drought				ofloxacin to protect chickens from infection
than those systems that have been relatively				and thereby enhance their growth. This drug
less stressed.				is closely related to ciprofloxacin, one of
				the most widely used antibiotics in the year
Healthy ecosystems can also be characterized				2000. There have been cases of resistance to
in economic, social, and human health terms.				ciprofloxacin directly related to its
Healthy ecosystems support a certain level of				veterinary use. In the United Kingdom,
economic activity. This is not to say that				ciprofloxacin resistance developed in strains
the ecosystem is necessarily self-sufficient,				of campylobacter, a common cause of diarrhea.
but rather that it supports economic				Multi-drug-resistant strains of salmonella
productivity to enable the human community to				have been traced to European egg production.
meet reasonable needs. Inevitably, ecosystem
degradation impinges on the long-term				Food and Water Security. Agricultural
sustainability of the human economy that is				practices are also responsible for a growing
associated with it, although in the				number of threats to public health. Some of
short-term this may not be evident, as				these are related to inadequate waste
natural capital (e.g., soils, renewable				management, which has resulted in parasites
resources) may be overexploited and				and bacteria entering water supplies. Others
temporarily enhance economic returns.				are of entirely different origins and involve
Similarly, with respect to social well-being,				apparent transfer across species of pathogens
healthy ecosystems provide a basis for and				that affect both animals and humans. The most
encourage community integration.				recent and spectacular example is mad cow
Historically, for example, native Hawaiian				disease, known as variant Creutzfeldt-Jakob
groups managed their ecosystem through a				disease in humans, a neuro-degenerative
well-developed social cohesiveness that				condition that, in humans, is ultimately
provided a high degree of cooperation in				fatal. The first case of Bovine Spongiform
fishing and farming activity.				Encephalopathy (BSE), the animal form of the
				disease, was identified in Southern England
Another reflection of ecosystem health lies				in November 1981. By the fall of 2000, an
directly in the public health domain. In				outbreak had also occurred in France, and
spring 2000, a deadly strain of the bacterium				isolated cases appeared in Germany,
E-coli (0157:H7) entered the public water				Switzerland, and Spain. More than one hundred
supply in Walkerton, Ontario, Canada, causing				deaths in Europe were attributed to what has
seven deaths and making thousands sick. This				come to be commonly called mad cow disease.
small town, with a population of five
thousand, is in a farming community.				Improper manure management was the likely
Inadequate manure management from cattle				source of the outbreak of E. coli 0157:H7 in
operations was the likely source of this				Walkerton, Ontario, Canada. Other health
tragedy.				risks associated with malfunctioning
				agroecosystems include periodic outbreaks of
HOW HEALTHY ECOSYSTEMS BECOME PATHOLOGICAL				cryptosporidiosis, a parasitic disease that
				is spread by surface runoff contaminated by
Stress from human activity is a major factor				feces of infected cattle. This parasite
in transforming healthy ecosystems to sick				causes fever and diarrhea in immunocompetent
ecosystems. Chronic stress from human				individuals and severe diarrhea and even
activity differs from natural disturbances.				death in immunocompromised individuals.
Natural disturbances (fires, floods, periodic
insect infestations) are part of the dynamics				ECOSYSTEM RESTORATION
of most ecosystems. These processes help to
"reset" ecosystems by recycling nutrients and				Ecosystem pathology in some cases can be
clearing space for recolonization by biota				reversed simply by removing the source of
that may be better adapted to changing				stress. In cases, for example, where
environments. Thus, natural perturbations				ecosystem degradation is the result of
help keep ecosystems healthy. In contrast,				point-source additions of nutrients or toxic
chronic and acute stress on ecosystems				chemicals, removal of these stresses may
resulting from human activity (e.g.,				result in considerable recovery of ecosystem
construction of large dams, release of				health. A classic case is Lake Washington
nutrients and toxic substances into the air,				(near Seattle, Washington). This lake had
water, and land) generally results in				become highly anoxic (oxygen-depleted) owing
long-term ecological dysfunction.				to a sewage outfall entering the lake.
				Redirecting the sewage outfall away from the
Five major sources of human-induced				lake reversed many of the signs of pathology.
(anthropogenic) stresses have been identified
by D. J. Rapport and A. M. Friend (1979):				In cases where it is not feasible to remove
physical restructuring, overharvesting, waste				the source of stress, more innovative
residuals, introduction of exotic species,				engineering solutions have been tried. For
and global change.				example, in the KyrÃ¶njoki and Lestijoki
				Rivers in western Finland, spring and fall
Physical Restructuring. Activities such as				runoff leads to sharp pulses of acidity.
wetland drainage, removal of shoals in lakes,				Spring runoff from snowmelt, which releases
damming of rivers, and road construction				acid from tilled or dug soils, has been
fragment the landscape and alter and damage				particularly damaging to fish, during the
critical habitat. These activities also				critical time of year for spawning. Fish
disrupt nutrient cycling, and cause the loss				reproduction is severely curtailed, if not
of biodiversity.				all together eliminated in highly acidic
				water. Further there have been massive fish
Overharvesting. Overexploitation is				kills resulting from the highly acidic
commonplace when it comes to harvesting of				waters. One possible remedy is to replace the
wildlife, fisheries, and forests. Over long				original drains which take runoff from the
periods of time, stocks of preferred species				land to the rivers with new limed drains that
are reduced. For example, the giant redwoods				can neutralize the acidity. This solution has
that once thrived along the California coast				been implemented on an experimental basis and
now exist only in remnant patches because of				appears to substantially reduce acidic
overharvesting. When dominant species like				runoff.
the giant redwoods (arguably the world's
tallest tree-one specimen was recorded at 110				More radical treatments for damaged
meters tall with a circumference of 13.4				ecosystems involve "ecosystem surgery." In
meters) are lost, the entire ecosystem				some cases, invading exotic vegetation (such
becomes transformed. Overharvesting often				as mangroves in Hawaii) have been removed
results in reduced biodiversity of endemic				from regions, and native vegetation has been
species, while facilitating the invasion of				replanted. In areas of North America where
opportunistic species.				wetlands have been severely depleted owing to
				farming, urbanization, and industrial
Waste Residuals. Discharges from municipal,				activity, efforts have been made to establish
industrial, and agricultural sources into the				new wetlands.
air, water, and land have severely
compromised many of the earth's ecosystems.				More often than not, however, reversing
The effects are particularly apparent in				ecosystem pathology is not possible. Efforts
aquatic ecosystems. In some lakes that lack a				to restore the indigenous grasslands in the
natural buffering capacity, acid				Jornada Experimental Range in the
precipitation has eliminated most of the fish				southwestern United States provide an
and other organisms. While the visual effect				example. Overgrazing by cattle has severely
appears beneficial (water clarity goes up)				degraded the landscape and has lead to
the impact on ecosystem health is				replacement of the native grasses by largely
devastating. Systems that once contained a				inedible shrubs, dominated by mesquite.
variety of organisms and were highly				Erosion by wind and episodic heavy rains have
productive (biologically) become devoid of				left areas between shrubs largely bare, and
most lifeforms except for a few acid-tolerant				subsequently underlying sands have developed
bacteria and sediment-dwelling organisms.				in dune-like fashion over a large part of the
				area. The resulting mesquite dunes have
Introduction of Exotic Species. The spread of				proven highly resistant to efforts to restore
exotics has become a problem in almost every				the native grasslands, although almost every
ecosystem of the world. Transporting species				intervention has been tried, including highly
from their native habitat to entirely new				toxic defoliants (Agent Orange), fire, and
ecosystems can wreck havoc, as the new				bulldozing.
environments are often without natural checks
and balances for the new species. In the				Even where it has been possible to restore
Great Lakes Basin, the accidental				some of the ecological functions of degraded
introduction of two small pelagic fishes, the				ecosystems, and thus improve ecosystem
alewife and the rainbow smelt, combined with				health, the restoration seldom results in
the simultaneous overharvesting of natural				reestablishment of the pristine biotic
predators, such as the lake trout, led to a				community. The best that can be achieved in
significant decline in native fish species.				most cases is reestablishment of the key
The introduction of the sea lamprey, an				ecological functions that provide the
eel-like predacious fish that attacks larger				required ecosystem services, such as the
fish, into Lake Erie and the upper Great				regulation of water, primary and secondary
Lakes further destabilized the native fish				productivity, nutrient cycling, and
community. The sea lamprey contributed to the				pollination. In all such efforts, key
demise of the deepwater benthic fish				indicators of ecosystem health (vigor,
community by preying on lake trout,				productivity, and resilience) are essential
whitefish, and burbot. This contributed to a				to monitor progress. Standard ecological
shift in the fish community from one that had				indicators can be used for this purpose
been dominated by large benthics to one				(e.g., measures of productivity, species
dominated by small pelagics (fish found in				composition, nutrient flows, soil fertility)
the upper layers of the lake profile). This				along with socioeconomic and human health
shift from bottom-dwelling fish (benthic) to				indicators.
surface-dwelling fish (pelagic) has now been
partially reversed by yet another accidental				Experience in efforts to restore highly
introduction of an exotic: the zebra mussel.				damaged ecosystems suggests that
As the zebra mussel is a highly efficient				ecosystem-health prevention is far more
filter of both phtyoplankton and zooplankton,				effective than restoration. For marine
its presence has reduced the available food				ecosystems, setting aside protective zones
in the surface waters for pelagic fish.				that afford a sanctuary for fish and wildlife
However, while the benthic fish community has				has considerable promise. Many countries are
gained back its dominance, the preferred				adopting policies to establish such areas
benthic fish species have not yet recovered				with the prospect that these healthy regions
owing to the degree of initial degradation.				can serve as a reservoir for biota that have
Overall, the increasing dominance by exotics				become depleted in the unprotected areas. Yet
not only altered the ecology, but also				this remedy is not without its limits.
reduced significantly the commercial value of				Restoring ecosystem health is not simply a
the fisheries.				matter of replenishing lost or damaged biota.
				It is also a matter of reestablishing the
Global Change. Rapid climate change (or				complex interactions among ecosystem
climate warming) is an emerging potential				lifeforms. Having a ready source of healthy
global stress on all of the earth's				biota that could potentially recolonize
ecosystems. In evolutionary time, there have				damaged ecosystems is important, but it is
of course been large fluctuations in climate.				only part of the solution.
However, for the most part these fluctuations
have occurred gradually over long periods of				PREVENTION OF ECOSYSTEM DISRUPTIONS
time. Rapid climate change is an entirely
different matter. By altering both averages				Given the difficulties in reversing ecosystem
and extremes in precipitation, temperature,				degradation, and the many associated human
and storm events, and by destabilizing the El				health risks that arise with the loss of
NiÃ±o Southern Oscillation (ENSO), which				ecosystem health, the most effective approach
controls weather patterns over much of the				is simply the prevention of ecosystem
southern Pacific region, many ecosystem				disruption. However, like many common-sense
processes can become significantly altered.				approaches, this is easier said than done. In
Excessive periods of drought or unusually				both developed and developing countries there
heavy rains and flooding will exceed the				is a strong inclination to continue economic
tolerance for many species, thus changing the				growth, even at the cost of severe
biotic composition. Flooding and unusually				environmental damage. Apart from selfish
high winds contribute to soil erosion, and at				motivations, the argument is made that
the same time add to nutrient load in rivers				economic growth has many obvious health
and coastal waters.				benefits, such as providing more efficient
				means of distributing food supplies,
These anthropogenic stresses have compromised				providing more plentiful food, and providing
ecosystem function in most regions of the				better health services and funding for
world, resulting in ecosystem distress				research to improve standards of living.
syndrome (EDS). EDS is characterized by a				These are indeed benefits of economic
group of signs, including abnormalities in				development, and have led to substantial
nutrient cycling, productivity, species				increases in health status worldwide.
diversity and richness, biotic structure,
disease prevalence, soil fertility, and so				However, at the dawn of the twenty-first
on. The consequences of these changes for				century, the past is not necessarily the best
human health are not inconsiderable.				guide to the future. The human population is
Impoverished biotic communities are natural				at an all-time high, and associated pressures
harbors for pathogens that affect humans and				of human activity have led to increasing
other species.				degradation of the earth's ecosystems. As
				ultimately healthy ecosystems are essential
ECOSYSTEM HEALTH AND HUMAN HEALTH				for life of all biota, including humans,
				current global and regional trends are
An important aspect of ecosystem degradation				ominous. Under these circumstances, a
is the associated increased risk to human				tradeoff between immediate material gains and
health. Traditionally, the concern has been				long-term sustainability of humans on the
with contaminants, particularly industrial				planet may be the only option. If so, the
chemicals that can have adverse impacts on				solution to sustaining human health and
human development, neurological functions,				ecosystem health becomes one of devising a
reproductive functions, and that appear to be				new politic that places sustaining life
causative agents in a variety of carcinomas.				support systems as a precondition for
In addition to these serious environmental				betterment of the human condition.
concerns (where the remedies are often
technological, including engineering				BIBLIOGRAPHY
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Ecological Imbalances. Cholera is a serious				Hutchinson, T. C. (1985). "Ecosystem Behavior
and potentially fatal disease that is caused				under Stress." American Naturalist
by the bacterium Vibrio cholerae. While not				125:617-640.
nearly so prevalent as malaria, cases are
nonetheless numerous. In 1993, there were				Reeves, W. C.; Hardy, J. L.; Reisen, W. K.;
296,206 new cases of cholera reported in				and Milby, M. M. (1994). "The Potential
South America; 9,280 cases were reported in				Effect of Global Warming on Mosquito-Borne
Mexico; 62,964 cases in Africa; and 64,599				Arboviruses." Journal of Medical Entomology
cases in Asia. Most outbreaks in Asia,				31(3):323-332.
Africa, and South America have originated in
coastal areas. Symptoms of cholera include				Ruiz, G. M.; Rawlings, T. K.; Dobbs, F. C.;
explosive watery diarrhea, vomiting, and				Drake, L. A.; Mullady, T.; Huq, A.; and
abdominal pain. The most recent pandemic of				Colwell, R. R.. (2000). "Global Spread of
cholera involved more regions than at any				Microorganisms by Ships." Nature 408:49-50.
previous time in the twentieth century. The

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