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Chapter 9
Areas of Scientific Uncertainly
The main purpose of the Endangerecl Species Act is to provide protection for species with an
uncertain future, and uncertainty permeates all decisions made under the act. This chapter focuses on
the major areas of scientific uncertainty that exist with respect to applications of the ESA. The
emphasis is on uncertainties that could be resolved with further research, as opposed to intrinsic
uncertainties in species survival. Even in the best of possible worlds, with perfect data and valid
estimation and evaluative procedures, there is always a probabilistic element to any assessment of risk.
Nonetheless, the committee concludes that none of the scientific uncertainties discussed below is great
enough to make the ESA unworkable.
ECOSYSTEM-BASED PROTECTION
A stated purpose of the ESA is "to provide a means whereby the ecosystems upon which
endangered species and threatened species depend may be conserved ...." The means to this end is
the listing of individual species. The major threat to most species is loss of habitat, and therefore
ecosystem protection is of paramount importance to the overall preservation of species. Because the
ESA requires that critical habitat be designated at the time of listing, listing a species has the potential
to protect ecosystems and their unlisted components as well. However, this approach can be effective
only if habitat protection is pursued rigorously.
Less clear is whether listing species, as opposed to a broader based policy of listing ecosystems,
is the best means of achieving this goal. Protecting ecosystems is probably the only way to ensure the
long-term survival of large numbers of species, but the best way to achieve such protection is
uncertain.
Ecosystem Management
Species are relatively easy to identify. Ecosystems are difficult to define and certainly more
difficult to manage (see, for example, Franklin, 1993; Irwin and Wigley, 1993; Naiman et al., 1993;
Wilcove, 1993~. For example, a lake ecosystem can be defined by the boundaries of its shoreline or by
its shoreline and the terrestrial watershed on which it is critically dependent. Ecosystem protection is a
fairly new concept, and policy for implementing it is untested. Nonetheless, it appears to the
committee that enough is known to be helpful. Indeed, several federal agencies have expressed their
desire to adopt ecosystem-management approaches and some have developed task-forces to develop
those approaches.
Definitions of ecosystem management tend to fall into two major categories. The first some
concept of management to achieve various ecosystem goals is the more difficult to implement. The
second category is the idea of keeping other ecosystem components and processes in mind when
managing a particular part of an ecosystem. This would mean, for example, that one would keep in
mind the needs of marine mammals and birds when harvesting fish; one would keep in mind aquatic
ecosystems when managing adjacent uplands (whether for forestry, agriculture, grazing, recreation,
143
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Science and the Endangered Species Act
clevelopment9 or any other goal); ant! one would keep various ecosystem processes ant! components in
mind when managing for protection of endangered species. The second category is already being
developed or practiced by many people in federal and state agencies (e.g., LaRoe, 1993; Quigley and
McDonald, 1993), and it has the potential to help protect endangered species, to help protect the
ecosystems they depend on, and to help reduce social ant! economic disruption and conflict. Therefore,
despite the need for more knowledge, experience, management tools and, in some cases, social
acceptance, ecosystem management offers promise.
INADEQUATE KNOWLEDGE OF SPECIES
AND THEIR ROLES IN ECOSYSTEMS
The Endangered Species Act has been applied almost exclusively to vertebrates, invertebrates,
and vascular plants. For small or inconspicuous organisms, a large fraction of the biota probably has
not been classified. Furthermore, new species even of conspicuous taxa are still being discovered
(Wilson, 1988~. Obviously, organisms that have not been iclentifiec! cannot be evaluated and protected
if warranted.
A fundamental characteristic of an evolutionary unit (see Chapter 3) is that an EU is distinct from
other units. Whether a population segment in the wild is distinct or part of a larger genetic entity is
often unclear because historical and current levels of gene flow are unknown. Furthermore,
evolutionary change is dynamic, ant} tests for ctistinctiveness are most difficult to apply when
populations are diverging into in(lepenclent populations.
On scales of tens to thousands of years, most species expand and contract in number and
geographic distribution in response to environmental change and they evolve. But we do not know how
many species can be lost before an ecosystem itself collapses. The roles of most species in most
ecosystems remain unknown for described and unclescribed species. It is known, however, that
complex ecosystems can exhibit sudden changes in state once a threshold level of stress has been
exceeded (Begon et al., 1986~. Thus, we dare not lose sight of the fact that species currently kept rare
by natural or human-induce~i factors play or could play central roles in the biosphere in the future.
ESTIMATION OF THE RISK OF EXTINCTION
Current Limitations of Existing Theory
Nearly all of what is now a substantial bocly of theory for predicting the risk of extinction has
been developed since the ESA was initiated in 1973. The major accomplishment of the theory to ciate
is the identification of ways in which expected times to extinction scale with population size when a
single factor is the dominant source of risk (e.g., demographic versus environmental stochasticity
versus episodic catastrophes). Even this level of work has been very clifficult, and numerous
assumptions have been macie to obtain reasonably simple analytical solutions. For example, almost all
existing analytical models ignore the age, spatial, and genetic structures that are inherent in most
natural populations.
Although they have heuristic value, unifactorial models of extinction have limited utility in the
real world of risk assessment for the simple reason that small populations always are confronted
simultaneously with threats from demographic, environmental, and genetic stochasticity. Factors that
could reduce population size can be highly synergistic, and each one may spawn further stochasticity in
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the other. Such interactions can lead to greatly elevated risks of extinction. Thus, unifactorial models
might provide us only with the lower limits of the risk of extinction. Of course, no mocle! can ever be
expected to produce perfect estimates of risk. However, from the standpoint of species protection,
upwardly, rather than downwardly, biased estimates of the risk of extinction are preferred so that
errors in risk assessment would tent] to be on the sicle of species.
Intrinsic Limits of Extinction Models
Biological moclels that jointly incorporate demographic ant! environmental (spatial and temporal)
variation, age structure, ant! genetics can be analyzed by computer simulation. However, predictions
emanating from these models will always be subject to uncertainty Most notahlv some asnect.s Of the
r . ~r ~·. ~. ~. ~. . ~
structure of the model te.g., mode or density clepenclence, temporal and spatial patterns or
environmental variation, and frequency and magnitude of catastrophes) will almost always be in doubt.
Even for rare cases in which the essential information is available, its relevance to predictive moclels
can be limited. For example, fundamental features of population structure and dynamics of a species in
jeopardy because of environmental change might be altered in unanticipated ways. To a certain extent,
those types of uncertainty can be dealt with by using a mode} structure ant! conservative enough
parameter estimates that the predicted risk of extinction will most likely be an overestimate. In
adclition, evaluation of the sensitivity of a moclel's predictions to variation in its parameters can be used
to identify the features of a population for which accurate estimates are most critical to the decision
process. These sensitivity analyses should be conducted routinely, and the results should be used to
direct future research.
LACK OF BASIC INFORMATION
Whether explicitly or implicitly, all decisions concerning rare, threatened, or enclangered species
are based on assessments that have at least some quantitative basis, even if that basis is not explicit.
Yet, critical tiara to make informed decisions on proposals for listing, to designate critical habitat, and
to develop recovery ant} management plans are usually lacking. Our biological unclerstanding of many
rare, threatened, or endangered species floes not extend far beyond a taxonomic description and a
coarse geographic distribution. That lack of data should not be the basis for failure to list a species if
other information is available to indicate that listing is otherwise warranted. The act calls for the use of
the best scientific data available in the decision-making process. It does not, and shouic! not, require
that all desirable data be available at the time of listing.
Dynamics of Natural Populations
Recovery plans often set goals based on target population sizes. Equally important is the need to
stabilize the mean population density. One of the largest gaps in our knowledge of the population
biology of most species concerns the natural temporal and spatial variation that exists in key
demographic factors. That information is critical to evaluating the risk of extinction, regardless of the
mean population size.
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Systematics
Protection of ecosystems is becoming recognized as an attractive option for conserving biological
diversity. However, ecosystems are composed of species and populations, and those components of
ecosystem structure must be understood. Yet the vast majority of species in the United States are
unknown and unnamed. Even for many of the named species, virtually nothing is known of their
geographic ranges, population structure, demography, ecology, or practically any aspect of their
biology. We have only the roughest of estimates of how many species of organisms reside in this
country. A recent NRC report on the National Biological Survey (now the National Biological Service)
(NRC, 1993a) recommended a commitment to a detailecI study of a significant portion of our biota.
Although some of the more visible vertebrate groups, such as birds and mammals, are well known,
many plant groups and most invertebrates, except for some commercially important ones, remain
virtually unstudied. Some large, ecologically important groups have few or no systematists studying
them. Any realistic attempt to provide even a basic inventory of our biota will need significant new
resources for training and supporting systematic biologists. Wise understanding, management, and
conservation of our biota need a much better picture of what organisms inhabit our country.
Do Minimum Viable Population Sizes Exist?
A popular heuristic concept in conservation biology is that of a minimum viable population size
(MVP), i.e., a threshold population size below which rapid extinction is virtually guaranteed. Should
MVPs exist in reality, numerical guides to them would be useful as listing criteria. At this point, there
is little compelling evidence that general guidelines can be made in this regard. Certainly, small
populations are more vulnerable to extinction than large ones, but it remains to be seen whether there is
some critical population size below which the vulnerability to extinction increases suddenly. It is
perhaps more useful to estimate extinction probabilities as a function of time for different population
sizes than to identify some specific MVP, as discussed in Chapter 7.
THE PROTECTION OF GENETIC DIVERSITY
In previous chapters, we described the importance for the survival of species of maintaining
genetic diversity for adaptive characters within and between populations. All species are now, and
perhaps always have been, confronted with a globally changing environment. Many rare species have
the additional burden of being confined to habitats that are changing rapidly in response to local human
activity. Although all species have evolved behavioral and physiological mechanisms for coping with
environmental change, the range of environments within which such homeostatic mechanisms are
operative is normally confiner! to the conditions experienced over long periods. For species
encountering entirely new environmental conditions, evolutionary flexibility is essential for long-term
survival. Thus, the preservation of diversity at the species level is intrinsically dependent on the
maintenance of genetic diversity within species. The difficulty lies in the identification and
quantification of this genetic diversity.
Uncertainty Regarding Future Adaptive Challenges to Species
If information on all quantitative-trait variation could be obtained for an endangered species, it
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i47
would still be clifficult to identify which characters should be evaluated, because we would be unsure of
the selective challenges that would confront species in the future or the characters that will contribute to
adaptive change. With this uncertainty, the best strategy for the maintenance of genetic diversity
within species is the implementation of protection programs that are likely to maximize genetic
variation for all characters. Programs clesigneci to maximize effective population size will naturally
maximize the expecter! amount of genetic variation as well. Because individual genomes are mutable,
populations that are devoic! of useful quantitative-genetic variation can replenish that variation over tens
of generations and should not be ruled out as viable evolutionary lineages.
FEASIBLE MANAGEMENT STRATEGIES
Perhaps the paramount challenge to future managers of endangered species concerns the degree
to which management and recovery plans can be developed within a framework that incorporates a
range of continuing human activities. Numerous issues remain unresolved, such as the design of
reserves, reconstruction of habitat, the usefulness of captive breeding and supplementation programs,
and the effects of environmental change.
The Spatial Structure of Reserves
A major challenge for conservation biology is the need to clevelop methods for ascertaining
optimal strategies for moving species towards recovery goals when resources and critical habitat are in
limited supply, as they always are. The spatial arrangement of habitats can have substantial effects on
the persistence of metapopulations, but our understanding of even the most basic issues is still
undeveloped (NRC, 1993b). Analogous to the concept of minimum viable population size, there may
be a threshold number of subpopulations or a threshold degree of isolation beyond which a
metapopulation becomes highly vulnerable to extinction, although this would certainly be expected to
vary from species to species, depending on their biological features.
Corridors and Edge Effects
In principle, corridors between local clemes can allow metapopulations of the demes to serve as
buffers from extinction in a stochastically varying environment. However, because of their large edge
effects, corridors often contain inhospitable habitat through which migration is risky. Consequently,
corridors can be sinks as well as sources of inclivicluals in a metapopulation context. Attempts to
evaluate whether management of a species should involve a few very large reserves versus many
smaller ones will be short-sighted if they do not take into account the demographic consequences of
corridors (NRC, 1993b).
Fragmentation of habitat, in general, is a particularly serious area of uncertainty. Because
ecosystem structure clevelops over several hundreds to thousands of years, several human generations
could pass before the full consequences of habitat fragmentation and the resulting edge effects were
revealed.
Reconstruction of Habitat
In habitat conservation plans involving mitigation, proposals by developers to reconstitute
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Science and the Endangered Species Act
ecosystems at alternative sites are becoming increasingly common. Careful management of species
whose biology is well understood can lead to their protection in altered environments. However,
clevelopment of complex communities for listed species must be approached cautiously, because we
often are clearing in theory rather than proven ability. Reconstituted ecosystems can have very different
internal and external interactions than their predecessors (NRC, 1992~. As a consequence,
maintenance of such ecosystems might require long-term and, perhaps at times, intensive management.
Ecosystems, like species, evolve over time and space as their component species wax and wane.
Artificial manipulation (management) might therefore be necessary if we are to focus on a particular
listed species or species set as a target for management.
Consequences of Captive Breeding and Supplementation
From a genetic perspective, a funclamental issue for which we have almost no empirical
information is the degree to which semi-isolated populations clevelop genomic incompatibilities, which
upon crossing, would be exhibited as reduced fitness of the offspring. This issue is becoming
increasingly important as recovery plans incorporate captive breeding, supplementation, and
sometimes, hybridization procedures into management policies.
Global Environmental Change
In applications of the ESA, the major focus on species protection has been on local issues, such
as dam and road building, logging anal mining, grazing, and housing development. However, evidence
suggests that human activity is causing global changes in temperature and the chemical composition of
the atmosphere (Abrahamson, 1989; Kareiva et al., 1993~. Even before humans had the capacity to
cause environmental changes at larger than local scales, regional and global environments were
changing; indeecl they have changed as long as life has been on earth (see Chapter 2~. Those types of
changes, particularly when combined with habitat fragmentation, could pose major threats to rare and
sensitive species. Policies for managing biodiversity will be short-sightec} if they are developed in a
setting that floes not consider the implications of global environmental change.
VALUING RARITY
Many uncertainties in economics (defined broadly as the science of human choice and valuation)
relate to the Endangered Species Act. One of the largest of these concerns the valuation of rarity.
Valuation is an enormously controversial topic. Some cognitive psychologists argue that strong
environmental values are not represented in monetary form in people's mental moclels (e.g., Gregory et
al., 19931. Tversky et al. (1988) argued that the way people rank and order items depends on the
measure used. Applied to the case of endangered species, it means that people might put a higher
dollar value on one species than another but reverse the ordering if asked to decide which species
should be preserved to make the greatest contribution to genetic diversity. Many issues are contentious
between the fields of economics and ecology. Indeed, some economists and philosophers have doubted
the ability of economics to solve this question because of the diversity of attributes being evaluated, the
lack of information available, and especially the difficulty of including moral and long-term
considerations in the valuation (e.g., Norton, 1988; Norgaard, 1988~. Others have pointed out the
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149
usefulness of having some sort of balanced and complete economic analysis, even if it includes only
short-term considerations (e.g., Randall, 19881. This discussion is limited to the economic perspective.
Value does not inhere in objects. The attribution of values to objects by individuals is motivates!
by cultural and religious underpinnings, but our tastes and preferences are also influenced by more
transitory forces of television, advertising, and the print media. Therefore, explaining taste is presently
very much an exploratory enterprise.
Rare things often are valuer! because ownership is a conspicuous way of displaying superiority.
So some are willing to pay a great deal to possess a private good that few others can afford to have.
Unfortunately, no research has established the relationship between rarity and dollar value. But rarity
and great value are not the monopoly of private goods. Many fee! great exaltation in front of
Michelangelo's Pieta; looking at one of Christo's wraps; or observing natural wonders, such as the
great migrations of~zebras, wilclebeests, and other animals between Kenya and Tanzania, or tens of
thousands of migratory birds taking flight from a lake. The economic analog of these ideas is a
willingness to pay if necessary to enjoy these experiences rather than go without one or more of them.
Great economic value can arise from great quantity and is not limited to things quantitatively scarce, as
long as qualitative attributes are acknowledged.
Goods and services have high economic value when they are economically scarce, i.e., when the
demand for them is large relative to supply. A key element in explaining whether consumers place a
high or low value on something is the availability of substitutes. The destruction of something we like
enormously is not so bad if we can easily find a substitute. Few substitutes is a necessary but not
sufficient condition for high value. (For example, rare, fatal diseases are not particularly valuable.)
Not all rare things are valuable.
Endangered species are, by definition, rare (or nearly so), but quantitative rareness is not a
sufficient attribute to conclude that any and all endangered species have great economic value. Wilson
(1988) and others effectively have heightened public awareness of the accelerated pace of species
extinction in recent years, three per hour in the rain forests alone, according to Wilson's latest
estimate (Wilson, 19921. Yet we are complacent with that knowledge and with knowledge of threats to
tropical rain forests and other hotspot ecosystems around the world. In the final analysis, allocated
funds reveal how valuable the citizenry thinks endangered species are and how much it is willing to
give up other things to have greater preservation activities. Congress annually appropriates funds for
the Office of Endangered Species in the United States that are not adequate to list more than a small
fraction of the candidate species or to pay for more than a fraction of the possible recovery plans for all
enciangered species. U.S. voters and their representatives in state ant] national legislatures have yet to
demonstrate enthusiasm in support (or willingness to make sacrifices for) of species preservation
despite the belief of many scholars and researchers that the pace of extinction is too rapid.
Preservation efforts might be facilitated if estimates of economic value of rare and endangered
species were available. Estimates of value are elusive because of the nature of the benefits (Brown,
1990~; with few exceptions, credible estimates clo not exist.
Many suggest that a species is worth preserving if it yields products of commercial worth. It is
easy to find specific plants with great commercial value, such as the rosy periwinkle (Catharanthus
roseus) which has been user] in a cure for acute lymphocytic leukemia and Hodgkin's disease. But
generalization of an estimate of economic value to all species is more problematical, particularly since
the chance of finding a product of economic value is so small, perhaps on the order of 1 in 10,000
(Ay~warcl et al., 1993~. And preservation is costly and requires tradeoffs. It is not surprising that
companies are reluctant to make privileged information about specific costs and revenues public so data
do not exist to estimate the commercial economic value of genetic resources in general and any species
in particular. Preserving for commercial value is not a good strategy, unless endangereci species can be
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Science and the Endangered Species Act
ranked according to the chance of successful discovery of commercial products and expected value if
successful.
Even if the expecter! economic return could be accurately predicted, estimating the commercial
value of species correctly will result in a systematic underestimate of species' value to society. If the
species are common property, owned by none, then others can use them directly or inclirectly for
competitive commercial gain so that value of any template or product developed from a species is
ievaluec3 by the first discoverer who knows that a successful rival cannot be far behind. Patents are an
imperfect protection. Removing common property status is tantamount to privatization, in which case
the private "owners," apart from exceptional cases, act as monopolists and exploit their monopoly
power to the detriment of social welfare.
Many species are valuable because they provicle either food or recreation directly to the
consumer. However, this direct consumptive value is an excluded source of value for rare species.
The benefits we clerive from species as goods- either commercial or consumptive are relatively easy
to value compared to another type of value that derives from the services that species perform within
the ecosystems that contain them. Such services include the maintenance of fertile soil and water,
control over the composition of the atmosphere, and regulation of the climate ant! the hydrological
cycle (including floor} control), and pest control. These major benefits to the human economy and to
human well-being are called ecosystem services by ecologists, but that phrase masks the important roles
that individual species or groups of species play in providing those services. For example, some
species of microorganisms (denitrifying bacteria) convert nitrate in soil into a gas, nitrous oxide, that
plays an important role in regulating the concentration of atmospheric ozone. Difficulty in valuing the
roles of individual species in providing these services arise from uncertainty over the actual economic
value of fertile soil, clean water, and other ecosystem-derivecT benefits. Indeed, Norton (1988) equated
that value to "the summer} value of all the GNPs of all countries from now until the end of the woricl."
Another part of a species' value is called non-use value. We can derive value from species by
knowing they exist today for example, the value of viewing and photographing them. An illustrative
study of these values for elk, bighorn sheep, and grizzly bear was reported by Schuize et al. (1981~.
Many studies document that a substantial fraction of species values arises if we can be assured that they
will be around in the future for subsequent generations to enjoy.
The literature on the estimation of the non-use value of species is very modest. All studies
estimating non-use value use a contingent valuation method, discussed below. The value of preserving
the whooping crane population at the Arkansas National Wildlife Refuge in Texas for viewers and non-
viewers has been estimated by Stoll and Johnson (1984) and Bowker and Stoll (1988~. Hageman
(1985) valued blue whales, bottlenose clolphins, California sea otters and northern elephant seals.
Brown and Henry (1989) estimated the value of preserving elephants in Kenya. Boyle and Bishop
(1987) estimated the value of preserving the striped shiner, a Wisconsin endangered species. Boyle and
Bishop (1986) also have estimated the existence value for eagles focusing, in part, on whether
respondents view eagles or not. Brown et al. (1994) estimated the value of the northern spotted owl, as
have Hagen et. se. (19911.
The valuation of non-use by economists is new and controversial. By its very nature, such
valuation is not founded on behavioral observations, which is the source of controversy. Non-use
values cannot be observed from organized markets. The research method is contingent valuation
(Mitchell and Carson, 198S, Cummings et al., 1986~. It involves the design of a survey that elicits
dollar values that, for example, represent a respondent's willingness to pay for the preservation of one
or more rare species. Critics argue that the values estimated from contingent value surveys are
hypothetical and lack credibility. Advocates rebut that socioeconomic factors considered to be
determinants of value have the right sign (i.e., they are positive when expected to be and negative when
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Areas of Scientific Uncertainty
expected to be) ant! are statistically significant in well-designect stuclies and that more than i,000
contingent valuation studies have been done in more than 40 countries.
151
Controversy over the method of contingent valuation was sparked by the Exxon Valdez of} spill,
because the regulations call for such studies. The National Oceanic and Atmospheric Administration, a
federal trustee for natural resources injured by of! spills, creates! a pane} composed of Nobel Prize
winners in economics anti other experts. The pane! approved of the contingent valuation method,
providing certain criteria were met (NOAA Pane! on Contingent Evaluation, 1993~.
The case on economic grounds for preserving endangered species depends crucially on the
magnitude of non-use values for species. Although we may believe that any or all enciangered species
are too valuable to sacrifice, there is an inadequate scientific basis to demonstrate whether citizens are
willing to make the sacrifices necessary to save all endangered species in this country now and in the
foreseeable future. In acidition, economic analyses are less effective when assessing long-term values
than short-term ones. The expected short-term use value in monetary terms of preserving many of the
tens of millions of extant species is likely to be small relative to the short-term, real costs of saving
them (Brown, 1990; Gregory et al., 1993), especially if externalities such as ecosystem goods and
services are not factored into the analysis. In part because of uncertainties in biological knowledge, the
long-term costs and benefits of protecting endangered species and their ecosystems is poorly known.
In our world of limited resources, the harsh fact is that we must give to get. In the absence of
scientific facts, belief, not science, defends the view that endangered species are more economically
valuable to citizens of the United States than the value of resources it will take to save them. However
many policy decisions concerning public goods are macle without compelling economic arguments. It
has also been arguer! that economic and ecological values are consistent with each other and that this
consistency should be recognized by policy makers (e.g., Ashford, 1995), so inasmuch as preserving
species is related to preserving ecosystem functioning, preserving species should leac! to an
enhancement of both ecological and economic values.
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Ashford, W. 1995. The Economy of Nature: Rethinking the Connections Between Ecology and
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Information and its Role in Biocliversity Conservation: Case Studies of Costa Rica's National
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Brown, G. M. Ir., and W. Henry. 1989. The Economic Value of Elephants. International Institute
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LaRoe, E. T. III. 1993. Implementation of an ecosystem approach to endangered species
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Mitchell, R. C., and R. T. Carson. 1988. Fusing Surveys to Value Public Goods: The Contingent
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NOAA (National Oceanic ant! Atmospheric Administration) Pane! on Contingent Evaluation. 1993.
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Norton, B. 1988. Commodity, amenity, and morality: The limits of quantification inbiodiversity.
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Representative terms from entire chapter:
population size
