Ecosystems, Biomes, and Watersheds:
Definitions and Use

M. Lynne Corn

Specialist in Natural Resources Policy

Environment and Natural Resources Policy Division

July 14, 1993

93-655 ENR

SUMMARY

Due to conflicts in many areas such as old growth forests in the Northwest, the Everglades in south Florida, Chesapeake Bay, and the Rio Grande watershed, many people are advocating a broader approach to management of land and water resources and biological diversity. They argue that existing laws run at cross purposes, or that the plethora of requirements under many laws and regulations means that no land use decision is ever final. As a result, and from a surprising range of philosophical platforms, many in Congress are advocating "ecosystem management" in various land use and conservation issues. Yet, there is little agreement, even in the scientific community, on the meaning of the term ecosystem.

This paper describes the meaning and applications of ecosystem and of the related terms watershed and biome. It discusses the pros and cons of all three as organizing principles for land management, and the major issues that are likely to arise in the debate over ecosystem management.

TABLE OF CONTENTS

INTRODUCTION
ECOSYSTEMS: DEFINITION AND EXAMPLES
BIOMES: DEFINITION AND EXAMPLES
WATERSHEDS: DEFINITION AND EXAMPLES
POTENTIAL PITFALLS FOR ECOSYSTEM MANAGEMENT
Boundaries
Dynamic Ecosystems, Biomes, and Watersheds
Biological Diversity in Different Ecosystems
CONGRESSIONAL QUESTIONS IN THE ECOSYSTEM DEBATE
ECOSYSTEMS AND MANAGEMENT



INTRODUCTION

There is a great deal of criticism of current land management in the United States, not only of Federal lands but of other lands as well. Interior Secretary Babbitt has been among those stressing the need for greater coordination and the avoidance of "train wrecks." Interestingly, criticism comes from a broad ideological spectrum, ranging from loggers, miners, and cattlemen, to wildlife biologists, fishing enthusiasts, and the greenest of environmentalists. While nearly all groups refer to ecosystems, ecologists realize that some groups actually focus on watersheds and biomes as well. When nearly all of these disparate groups are advocating "ecosystem management" as a solution to current issues one thing is clear: there is not enough agreement on the meaning of this concept to hinder its popularity.

This paper does not attempt to address the meaning of this concept nor even whether ecosystem management has clear principles on which to base management. Rather, it focuses only on more basic questions: what is an ecosystem? What are the differences among ecosystems, biomes, and watersheds? What are the pros and cons of any of these three as organizational units for land and resource management? Finally, what questions is Congress most likely to address in its debate on ecosystem management?

ECOSYSTEMS: DEFINITION AND EXAMPLES

A typical definition of an ecosystem is a "[c]ommunity of organisms interacting with one another and with the chemical and physical factors making up their environment."(1) The chemical and physical factors include sunlight, rainfall, soil nutrients, climate, salinity, etc. An ecosystem is inherently leaky: at a minimum, energy and nutrients move in and out. More likely, individual organisms (such as seeds, spiders, and sparrows) move in and out as well. This definition does not directly state that the organisms tend to interact with other organisms in the same ecosystem more than with those of different ecosystems, but the concept is implied.

Like most definitions of the term, this one has several implications. First, all parts of the planet, from the highest microbe drifting in the atmosphere, to the cockroaches in a walled garden, to the most exotic species in deep ocean trenches, are all parts of ecosystems. Moreover, all of these species can be considered parts of other ecosystems ranging up to the entire planet, each defined to suit the convenience of the observer. In general, the larger the particular ecosystem is considered to be, the less interaction individual parts will have with all the others. Beyond some point, consideration of too large an ecosystem may become simply unwieldy.

Second, ecosystems are not necessarily stable, nor are all of their components species necessarily native to the area. Some ecosystems may be threatened or destabilized by new species or changing physical factors, perhaps enough to risk the continued existence of some resident species. The brown tree snake of Guam and the zebra mussel in the Great Lakes are examples of damaging species whose introduction is destabilizing their new ecosystems; the voracious snake has already caused the extinction of some bird species, and the mussel is clogging streams and crowding out native species.

Third, ecosystems are difficult to separate from each other, and some would argue that there should be no attempt to do so. For those who question the legitimacy of making the attempt, a physical boundary around an ecosystem makes as much sense as a physical boundary around photosynthesis or the Federal budget process. Others find it acceptable to draw lines for some purposes, such as studying a few species in a given ecosystem. They would be willing to draw one set of boundaries between ecosystems in an area in order to study salamanders, and possibly a different set in the same area to study beetles, recognizing that they have compromised among several competing criteria.

Still others are not only ready to accept lines, but are also willing to select county lines, National Park boundaries, and other legal designations, even though such lines are rarely boundaries that the previous group might select.(2) Noting that, they argue that these lines do occasionally track ecologically relevant boundaries (e.g., major ridge lines) and that to facilitate management and data gathering, these lines are essential. The boundaries of the Southern Appalachian Biosphere Reserve, for example, follow county and State lines, because practically all Federal and State agencies collect data on that basis, and only a few collect it at a more detailed level.(3) This view recognizes that this choice severely compromises the definition of ecosystem given above, but they see few other realistic options if they must make land management decisions.

Two examples may help to clarify both the concept of ecosystem, and the dilemmas in applying that concept to the living world.

Consider cockroaches living in a walled garden on Key West, Florida. Whether the cockroaches are native or introduced is irrelevant under the above definition: they are still interacting with one another, with other species, and with the chemical and physical factors making up their environment. They could be considered just part of the garden ecosystem where, if they are weak fliers, the cockroaches will necessarily interact more with others in the same yard than in neighboring yards or the terrestrial e cosystem of that island.(4) Yet the land is so intimately connected to the nearby sea, depending on it for the moderate climate, humidity, and food sources for terrestrial predators, that the island and its nearby coral reefs also comprise an equally suitable, interdependent ecosystem worth considering. Moreover, many of the birds and fish using the island area depend on Florida Bay for a part of their life cycles: feeding, egg-laying, juvenile development, etc. The richness of Florida Bay in turn depends on not only the correct amount of microscopic nutrients flowing out from the Everglades, but also on those very flows to maintain a salinity suitable to the current mix of species. The cockroaches could therefore be part of many ecosystems simultaneously, all overlapping or contained within others.

Next consider some insects in a stream at high elevation on the Olympic Peninsula in Washington State. The watershed of that stream could be considered their ecosystem, where they interact with other insects, find food and concealment from predators, etc. The area could appear at first glance to be a fairly self-contained ecosystem.(5) Even so, it is intimately tied to its surroundings. Anadromous fish are the clearest link to the outside. After a year or more at sea, perhaps as far as the western Pacific, these fish return (now in far lower numbers than a century ago) high into the mountain streams where they spawn and die. The adult fish are taken not only by humans but other predators as well. The nutrients of their decaying bodies fertilize these streams, providing food for microorganisms that feed still more organisms such as insects, that in turn supply food for the young salmon as they make their way back to the sea. In a sense, the insects harvest the bounty of the sea thousands of miles away via the bodies of decaying salmon; their ecosystem might extend to the Sea of Japan. And birds feeding these insects to their young may winter in Latin America, harvesting resources there; their ecosystem might extend to Argentina. Some might want to consider adjacent watersheds part of the same ecosystem based on similar rainfall, flooding patterns, soil structure, and other physical or chemical factors in defining the ecosystem. So, should there be an ecosystem boundary in this case? If so, should it reflect the ecosystem of the insects only? Of the insects and anadromous fish? Of the insects and birds? Or all three and other species, and rainfall and soil nutrients as well? The answer is "it depends."

As the word "ecosystem" is now used by ecologists, an ecosystem can be large or small, and can include both pristine and highly developed areas. However, the word is rarely, if ever, applied to disjunct spaces: two similar mountaintops would ordinarily be considered similar ecosystems rather than an ecosystem, unless the land between them is also included. Instead, the word biome is usually applied to visually similar but not necessarily connected areas.

BIOMES: DEFINITION AND EXAMPLES

A biome can be defined as a "major regional community of plants and animals with similar life forms and environmental conditions. It is the largest geographical biotic unit, and is named after the dominant type of life form, such as tropical rain forest, grassland, or coral reef." (6) The dominant life forms are usually conspicuous plants, or plant-like species such as corals. A single biome can be widely scattered about the planet. Due to similar pressures of natural selection, species in differ ent parts of a biome may converge in their appearance and behaviors, even when they do not share the same ancestors.

The tundra biome, for example, is characterized by low grasses and sedges or in drier areas by low woody plants. Growth rates of perennial plants are slow, and nearly all life forms are highly specialized to grow and reproduce during a short growing season. Examples range from the North Slope of Alaska along the Arctic Ocean to the high peaks of the Cerro de la Muerte in Costa Rica or Mount Kilimanjaro in Tanzania, to the Antarctic Peninsula.

The grassland biome is characterized by grasses and their relatives, and often abundant flowering annual plants. Plants are often adapted to fast, scattered fires that burn the tops of plants but leave seeds, roots or other resistant structures intact. Examples include the tallgrass prairie of the United States and Canada, the llanos of Colombia and Venezuela, the steppes of Central Asia, and the plains of Africa. Because these areas are often suitable for cultivation or livestock grazing, a great deal of this biome around the world has been highly modified, often for many centuries or millennia.

It is a well-recognized scientific principle that many species may be found at the interface between biomes (the "edge effect"). This richness is particularly clearly documented in the interface between terrestrial and ocean environments and between forest and grassland.(7) The interface between grassland and forest, for example, is the preferred habitat of the indigo bunting, a bird species whose iridescent blue males are a fairly common springtime sight around Washington. Nests occur at the forest margin, and the birds forage in the more open areas. This species, like many others, is not part of one area or the other, but both.

WATERSHEDS: DEFINITION AND EXAMPLES

A watershed is the "Entire region drained by a waterway that drains into a lake or reservoir; total area above a given point on a stream that contributes water to the flow at that point; the topographic dividing line from which surface streams flow in two different directions."(8) Clearly, watersheds are not just water. A single watershed may include combinations of forests, glaciers, deserts and/or grasslands. Watersheds can be defined for a small rivulet in the Rockies, or the Colorado watershed, including parts of Wyoming, Colorado, Utah, New Mexico, California, Arizona, Sonora, and Baja California; or the Amazon River, including parts of Brazil, Venezuela, Colombia, Ecuador, Peru, and Bolivia.

In contrast to biomes, a single watershed may include a considerable diversity of habitats: desert, marsh, glacier snowfields, forest, and others. Compared to ecosystems and biomes, watersheds are unique in one respect. Except where two watersheds are separated by very flat areas, it is relatively easy to draw their boundaries. The U.S. Geological Survey has mapped watershed boundaries for major rivers and many small streams. Watershed boundaries could be mapped for the smallest streams.(9) Many States in the western United States organize their water management on a watershed basis.

POTENTIAL PITFALLS FOR ECOSYSTEM MANAGEMENT

Current congressional interest in ecosystem management is intense but nascent. If it focuses on the management of specific areas of Federal or non-federal lands, one approach may be to select portions of States or regions along lines intended to approximate ecosystems, biomes, or watersheds. If so, the issues of setting boundaries, addressing dynamic change in these areas, and weighing relative biological diversity, will probably all be addressed.

Boundaries

Among ecologists willing to draw any lines between ecosystems, no two are likely to draw the same ones. Even if two agree, they would recognize the inherent artificiality of their effort, and probably make the attempt with only a few species in mind. Moreover, one of them might prefer to draw a band or zone of separation rather than a line. Different lines are not surprising, but rather are entirely expected, because of the intrinsic interconnectedness of living systems: the discrepancies between scientists accurately reflect the diversity of the real world.

Further research will not make ecosystem boundaries less fuzzy. Even so, a fuzzy boundary is not a random boundary. The boundaries of ecosystems may not be perfectly definable, but lines can still meet a test of reasonableness. One ecologist might separate terrestrial from marine ecosystems at the high tide line while another might defend a low tide line, but neither is likely to choose a line running from the top to the bottom of a seaside cliff. A boundary separating the Olympic Peninsula from the rest of Washington would share the same problem, but the zone of dispute would be measured in sections, rather than square feet. One school of thought, as mentioned above, would probably resolve this problem by bowing to human convenience. This group might pick a county line, the limits of land managed by some agency (e.g., a National Forest boundary), the line of 47°N latitude, or some combination of these criteria, as a convenient approximation of an ecosystem boundary. When the purpose of the distincti on is human convenience rather than scientific precision, such a line may be logical, but would still unacceptably compromise the definition of ecosystem to some scientists.

Biomes, by definition, are more visibly separated from their neighboring biomes than ecosystems are. However, not only is the separation likely to be a zone (rather than a line), it is also likely to be a species-rich zone, relative to the diversity on either side of the zone. Whether a boundary through a species-rich zone is a problem obviously depends on why the boundary is being created. For management purposes, the fact that boundaries of biomes (however they are drawn) rarely follow legal boundaries could be a problem or an asset, just as it could with ecosystems and watersheds.

Watersheds lack some of the boundary problems associated with ecosystems and biomes. While the U.S. Geological Survey has mapped major watersheds, such as the Ohio River, nothing but money and time prevent mapping the watershed of its tributary, the Scioto River, or of its tributary, Darby Creek, and thousands of other smaller watersheds around the country. However, with some exceptions, these lines would rarely follow county or State boundaries, which seem more likely to be lines along the bank of a river than along the top of a watershed. Consequently, data collection could be more difficult.

Some Federal agencies have boundaries that approximate ecosystems, biomes, or watersheds. Many National Forests, for example, have some boundaries at roughly the zone separating a grassland or desert biome from the forest biome. Some wilderness boundaries, such as Trapper Creek in the Gifford Pinchot National Forest in Washington, are virtually identical to the boundaries of watersheds. Many other boundaries are conspicuously not ecosystem, biome, or watershed boundaries: the alternating sections of much of the public domain land in the western States, and most of the international border between the United States and Canada are examples. In other cases, straight lines may have taken on an ecological meaning over time. The intense commercial development at the borders of some National Parks, for example, could produce land use changes that become candidates for ecosystem boundaries for some observers.

Dynamic Ecosystems, Biomes, and Watersheds

Separation of ecosystems and biomes is a problem not only in space but time as well. A given ecosystem boundary (even an admittedly fuzzy one) may vary over time, with changes in stream movements, ecological succession, drought, extinction of species, human intervention, introduction of exotic species, and other factors. Biomes, defined on the basis of key life forms, are equally dynamic, although perhaps on a longer time scale. Their edges may move with changing rainfall, global warming or cooling, movement of glaciers, rise or fall of sea level, etc. Even the boundaries of watersheds, which are comparatively easy to define at any given moment, may change after avalanches, floods, storms, earthquakes, and gradual erosion. Clearly, none of these three categories can simply be marked off on a map and expected to remain indefinitely valid (or at least as valid as they were initially). For ecosystems and biomes that change dramatically and frequently (e.g., barrier islands), lines on a map are even more suspect than in many other areas.

Biological Diversity in Different Ecosystems

Not all ecosystems are created equal nor endowed with equal numbers of species. No two ecosystems are identical, and some will have more species than others. Extremely unusual ecosystems will likely have unusual flora and fauna: the geyser basins of Yellowstone support unique species highly adapted to that environment, desert springs have unique fish, and the remaining portions of the Everglades support exceptionally large numbers of bird species. Conservation of ecosystems is not identical to the preservation of biodiversity, but some configurations of ecosystem management could protect more species than others, depending on the choice of ecosystems to be managed. To illustrate, imagine a conservationist wanting to buy additional land, with the chief criterion that the purchase should protect as many species as possible that are not already protected elsewhere. To do that, the conservationist might well look for areas that are as different as possible from areas that are already protected.(10)

CONGRESSIONAL QUESTIONS IN THE ECOSYSTEM DEBATE

Many Members of Congress, scientists, Federal agencies, and interest groups are all promoting the concept of ecosystem management. As might be expected, there is no consensus on the meaning of either ecosystem or management. While the discussion below does not attempt to resolve the debate on ecosystem management, it does address those questions relevant to a broader understanding of what ecosystems, biomes, and watersheds are.

Which is more relevant to the congressional debate: ecosystems, biomes, or watersheds? If Congress is primarily interested in a different style of management on lands already owned by the Federal Government, any and all of these concepts may be relevant, since existing Federal boundaries reflect these concepts haphazardly at best. If Congress' primary goal is consideration of a larger number of ecological, social, and economic variables across Federal jurisdictions,(11) then the focus of ecosystems on processes and interrelationships may be helpful. But a focus on Federal lands only would commonly omit land that virtually all ecologists would include in the same ecosystem as adjoining Federal lands.

If Congress wishes to improve coordination of Federal land management practices with those of adjoining non-federal lands, the utility of any of these three concepts depends on the approach intended. For example, would a limited number of ecosystems be federally designated, with boundaries, and with one set of actions occurring only inside those boundaries, and business as usual outside those boundaries? If so, perhaps watersheds (with their relatively clear natural boundaries) would be the unit of choice over ecosystems. Even biomes would lend themselves to clearer boundaries than ecosystems.

Should boundaries be defined, and if so, based on what criteria? If Congress does not focus on management within a block of land or water, but rather on management across the national landscape,(12) boundaries become less important. If all parts of the landscape are affected by new legislation, the choice of ecosystem, biome, or watershed, and the boundaries of any of these ecologically defined areas become less relevant, but not irrelevant. Under such a scenario, all parts of the country would be part of some ecosystem (or watershed or biome (l3)), and coordination could take place within and/or among these units.

Among those willing to define ecosystem boundaries at all, it is common to do so based on specific criteria, usually the needs of a single species or of closely related species, but sometimes on rainfall patterns and other inputs. That approach might be used for Federal land management, but it is not clear how such species or patterns would be chosen.

Who should define ecosystem boundaries? If a single species is used to define an ecosystem, then the issue becomes one of scientific fact and professional judgement, as it is for listing species under the Endangered Species Act, for example. But Congress would have to decide how scientists would chose the species so honored, and by what means they would define a relevant ecosystem. Perhaps more importantly, how would the scientists themselves be chosen? The choice of scientists could well predeterm ine the focal species. Similarly, if biomes or watersheds are the focus of management, who would chose these areas?

Alternatively, Congress itself could choose to designate areas to be managed on an ecosystem basis, perhaps specifying certain criteria on which to base the choice, as wilderness areas are now designated. Or areas could be nominated by State or local governments or Federal agencies. Any of these options could allow areas already designated for some special status under other laws to be included. Existing U.S. Biosphere Reserves are among the obvious possibilities.

Should legislation based on ecosystems be restricted to Federal lands? Boundaries of Federal lands rarely follow lines that could be considered ecosystem boundaries even among those willing to draw such lines. In areas such as Yellowstone National Park and its environs nearly all observers have included non-federal lands in their definitions of the Yellowstone Ecosystem. It is hard to argue against the logic that this inclusion would improve coordination, but landowners would likely complain if they felt that their land management options were being restricted to their economic disadvantage. Opposition to including non-federal lands would therefore depend on the precise consequences of that inclusion: the more restrictive the consequences, the more likely the protest.

Are some ecosystems more important than others? All else being equal, ecosystems exhibiting highly unusual habitats (whether they are unusual naturally or because human activities have eliminated similar habitats) are likely to have species not found in other areas. If a major goal of ecosystem management is to protect biodiversity, science would suggest that some ecosystems, biomes, and watersheds now receiving little attention should have much higher priority. Habitats like old growth, native, eastern hardwood forests and native tall grass prairie for example, are reduced to a few isolated pockets, and the proportion left is less than that of the remaining much more famous Pacific Northwest forests. The geyser habitats of Yellowstone are an example of ecosystems that are naturally extremely rare. Species dependent on these habitats are presumably correspondingly rare as well. If protection is based on objective scientific criteria, this protection may or may not match public opinion and preferences.

Should ecosystem boundaries be designed or permitted to change over time? If Congress does address ecosystem management by somehow specifying coverage of particular areas, then regardless of the suitability at the time, boundaries may become inadequate later. Rivers may change their courses, warming trends may shift the distribution of species, bird feeders may encourage the overwintering of species which previously did not, hurricanes may destroy or create barrier islands, suburbanization may drive out species, rainfall patterns may change, scientific understanding of the area may improve, introduced species may modify the distribution of other species, etc. For these and other reasons, factors that may have caused an already contentious ecosystem boundary to be put in one place may just as rationally cause it to be in another place a few years or decades later. Congress will have to determine whether and how ecosystem management should respond to changing conditions.

ECOSYSTEMS AND MANAGEMENT

Ecosystem management is a concept that so far lacks widespread agreement on the meaning of the phrase. Ecosystem itself is a word whose application is exceedingly difficult. As a result, management approaches requiring clear and more or less permanently fixed or existing boundaries would tend to compromise concepts of what an ecosystem is. Alternatively, management could be based on concepts of watersheds or biomes, since these are at least somewhat easier to define. Even so, biomes and watersheds (like ecosystems) are unstable over time. No one of these three terms is wholly satisfactory for organizing purposes.

Congress may wish to use these concepts to help guide any new approaches to land management, but it is hard to imagine how site-specific management laws could do justice to somewhat amorphous concepts like ecosystems and biomes. Congress could be forced simply to declare some mapped, specified area "an ecosystem" (while recognizing that the choice will be open to dispute) and state new principles under which it is to be managed. Or these principles could be directed to apply to some category of lands (e.g., National Wildlife Refuge System, wetlands, public domain lands, National Forest System, Rio Grande watershed, or other categories). Alternatively, Congress could establish some broad set of management principles without specifying the areas to be covered, and a panel could be named to choose the areas.

A second option would be for Congress to require certain prerequisites for some category of Federal action. Current examples of this approach include the consultation requirements under the Endangered Species Act and the Environmental Impact Statements required under the National Environmental Policy Act. Like both of these laws, the new legislation could apply to both Federal and non-federal lands.

A third option could follow the model of the coastal barrier islands protection system. A wide range of Federal subsidy programs, grants, loan guarantees, and other programs could be eliminated for certain categories of actions in selected areas. This legislation could be quite difficult to draft clearly, in terms of both the areas to be covered and the activities to be encouraged or discouraged. It is also hard to image how such a program would apply to Federal lands. Since the focus would be on the removal of Federal spending, this approach could offer important cost saving over the other two options.

The suitability of ecosystems, biomes, and watersheds as organizing principles for land management will likely depend significantly on the purposes or structure of legislation designed to manage them. No one of these concepts will serve all goals. A major issue will clearly be whether to focus on a few selected areas at a regional or nationwide level, or on management of many resources across the national landscape.

Endnotes

1. G. Tyler Miller, Jr. Environmental Science: Sustaining the Earth. Wadsworth Publishing Co, Belmont, CA. 1991. p. A7.

2. However, if severe modification has occurred on one side of a line, both groups might chose it as a boundary. The line separating the heavily cut Targhee National Forest and Yellowstone National Park is an example. Parts of the boundary are easily visible in satellite photographs. See discussion of artificial boundaries below.

3. Personal communication May 4, 1993, with Hubert Hinote, Executive Director, Southern Appalachians Man and Biosphere Program.

4. Whether the factors promoting interaction within the ecosystem are natural or artificial (e.g., a garden wall) is also irrelevant under the definition. An aquarium can be an ecosystem, as long as its organisms can be considered to interact with each other and with the chemical and physical factors making up their environment.

5. There are concrete measures of the degree to which species within the Olympic Peninsula ecosystem have interacted with each other more than with those of neighboring areas. For example, there has been enough isolation for the evolution of species of flowers, frogs, and mammals unique to the area, and it lacks some mammals that are common in the nearby Cascade Mountains.

6. Dictionary of Biology. Elizabeth Tootill, ed. Intercontinental Kook Productions, Ltd. Maidenhead, Berkshire. 1980. p. 29

7. See discussion p.310-312 in Robert L Smith, Elements of Ecology, 3rd ed. Harper Collins. New York 1992.

8. p. G12 in Robert L. Smith, Elements of Ecology, 3rd ed. Harper Collins. New York. 1992.

9. Actually, some watershed boundaries may be so flat that flow into one stream versus another may depend on factors such as wind speed and direction, but this is a rare phenomenon that can generally be ignored.

10. This could present a circular problem, however: does the area have many different species from other areas because it is different, or does it look different because of its different species?

11. An informal survey suggests that the effort to consider more issues in management decisions may be one of the few unifying threads connecting the disparate advocates of ecosystem management.

12. Essentially, such a choice might reflect the difference between resource management and land management. Federal laws with consequences for resource management beyond Federal land boundaries already exist. Examples include the Endangered Species Act, the National Environmental Policy Act, and the Clean Water Act.

13. Biomes could be difficult to define in highly developed areas, and would perhaps have to depend on inferences about what was once the dominant vegetation. Ecosystem boundaries would be no easier. On the other hand, watershed boundaries in developed areas would probably be no more difficult to define than in a forest or grassland.