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121.
Gilpin, M., G.A.E. Gall and D.S. Woodruff.
Ecological dynamics and agricultural landscapes. In: Integrating Conservation Biology
and Agricultural Production. Special Issue of Agriculture, Ecosystems and
Environment 42:27-52. (1992b).
BOOK
CHAPTER
I:U
Agriculture. Ecosystems and Environment. 42 (1992) 27-52
Elsevier Science Publishers B.V.. Amsterdam
Ecological Dynamics and Agricultural
Landscapes
MICHAEL GILPINl, GRAHAM A.E. GALL 2 and DAVID S. WOODRUFfl
I
2
Depanmenr of Bi%Kl, University of California, La Jolla; CA 92093 (U.S.A.)
Depanmenr of Anima/ Science, University of California, Davis, CA 95616 (U.S.A.)
ADSTRACf
Gilpin, M., Gall, G.A.E. and Woodruff, 0.5., 1992. Ecological dynamics and agriculturallandscapcs.
Ecosyslems Environ., 42: 27-52.
Agric.
The planet Eanh is having difficulty under the stresses imposed by the diverse demands of an ever
increasing human population. The problem appears to stem not only Crom population pressures but also
Crom an imbalance between the needs and desires oC society. Agriculture, broadly defined to include
farming. rlShing,forestry and grazing systems. plays a significant role in the management of land, water, and
biological resources. This paper provides an analysis of opponunities for interaction between ecological
science and agriculture. The tong-term stability or agric\lhure is dependent on natural sources or genetiC
material
Many pans oC society see a connict between conservation of biological resources and their
exploitation by agriculture. Agricultural production is essential to SOCietyand also can provide stewardship
of biological resources beyond the limits of those directly associated with production 0( a commodity.
However, interdisciplinary effort is needed in the development of strategies, and societal support of
agriculture nlust take a Corm that encourages and rewards agriculture Cor this stewardship.
INTRODUcrION
There is some truth to the view that something is wrong on planet Earth:
its living systems are out of balance. Biological diversity is threatened.
Natural habitat is being lost. Some predict a spate of extinctions unmatched
since the end of the age of the dinosaurs (Table 1). And it's all the fault of
one species: Homo sapiens. We human beings are too numerous. We are
putting too much pressure on the planet's biological support systems.
Nonetheless this perspective is over-simplified, blame-leveling, explains
little, and offers no hope, save of drastically cutting back the human
population, of reversing these mounting difficulties. Human numbers are
part of the problem, yet there are places, such as northern India, where a
quite dense human population has coexisted with high biological diversity for
millennia. Thus, at least part of the problem is due to human culture, not
human population density per se.
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TABLE
1
~timated
status of species 1
Extinct
Endangered
Vulnerable
Rare
Indeterminate
Totally Globally
Threatened Taxa
384
3325
3022
6749
5598
19078
23
81
135
83
21
343
2
9
9
20
10
50
Reptiles
21
37
39
41
32
170
Invertebrates
98
221
234
188
614
1355
113
111
67
122
624
1037
83
172
141
37
64
497
Plants
Fish
Amphibians
Birds
Mammals
1 Taken
from McNeely et al. (1989)
People do not actually realize that they are against Nature. Worldwide,
in both developed and developing countries, the public overwhelmingly
supports efforts to protect species and to conserve natural systems. Various
religions speak to human beings about their role as stewards or caretakers.
The difficulty is that people have many needs and desires. They ask much
from the earth. It is simply our efforts to provide for ourselves and secure
a better lot for our children that leads to the situation which threatens to
overwhelm us today.
Across the entire surface of the earth, human beings fish, hunt and
harvest, sow and reap, log and mine. Systems are often manipulated in quite
sophisticated ways in order to tilt them to produce even more desired
output. It is the human need for food, fiber, and material that is in conflict
with human stewardship of the planet.
Over-exploitation and over-use of the land have been recognized as
environmental problems since the time of Plato. The basic issues are not
new. What has changed in our time is the scale of the problem. At risk now
are not just the pine forests above Athens but all the trees of the earth. The
problem is coming on so fast it is now apparent that this generation will
decide which features of the natural order will survive and which will be lost
forever. So the ethical feeling for an all-embracing conservation is yielding
,
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to the applied science of conservation biology, which pretends to offer a
scientific basis for the choices we face.
This paper is an analysis, from a systems perspective, of the kind of
thinking the ecological sciences bring to these problems. Particular attention
is given to dimensions of time and space, and an attempt is made to show
which processes dominate or control other processes.
CONSERVATION BIOLOGY
A decade ago, conservation biology had its birth in crisis. Many
vertebrate species were seen as teetering on the brink of extinction. Zoos
became emergency rooms for species survival, with the genetic and
demographic systems of the patient the focus of clinical care. There are now
procedures for handling these cases, yet only one species in ten thousand will
ever be selected for such treatment. The costs are too great, especially the
costs of physiological and natural history knowledge. Somehow, these at-risk
species must be provided for in ways that demand less hands-on care.
Species are placed in zoos only when their population size in nature is
no longer minimally effective. Species populations only drop to such low
sizes when the integrity of the processes of whole communities of species has
not been preserved. And these ecological communities are threatened only
when there is a failure at the level of ecosystems and biogeographic
landscapes.
Thus, mismanagement of the natural order can have a
hierarchical character in which a mistake can compound itself through
successive levels. This is the understanding which must guide the human
response to species risk. At question is not only how to act, but where and
how soon.
Conservation biology is now attempting to meet its challenges at these
higher levels, involving land use and whole ecosystems. It is attempting
constructively to engage the processes that threaten biological diversity. To
do this, conservation biology must draw on the knowledge that has been
accumulated about these higher-level processes.
To a large extent,
conservation biology must simply reach out and borrow knowledge rather
than create its own. In this sense, it is an eclectic discipline, However, there
are processes of "intermediate" size and duration that have not been well
researched by academic ecology and about which critical knowledge is
wanting. At these scales, conservation biology must add to the general store
of knowledge.
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AORICULWRE
Agriculture, broadly construed to include fishing, forestry, and grazing
systems, is the dominant interface between human beings and nature. It is
the fundamental mechanism by which nature is manipulated. Agriculture is
the direct cause of the loss of natural habitat. When misguided, agriculture
can lead to both over-exploitation and the transport and introduction of
exotic species. And because it is central to the problem of threatened
biological diversity, it is pivotal to the solution.
Agriculture can be divided into three types, graded by the intensity and
expense of the human inputs. First, requiring essentially no human inputs,
are fisheries, grazing systems dominated by domestic animals, and forests not
subject to clear cutting and replanting. These systems can yield their product
indefinitely if exploited properly. Second, there are the low input farming
systems which, with proper management, might also be sustainable;
developed-world forestry often fits in here. Third, there are the high input
systems seen in the developed world where the system is dependent on
human management and costly inputs of energy and chemicals.
Human beings can fail in the management of any of these three classes
of systems and can rapidly degrade them to ecological uselessness. How the
failure happens, however, is different.
The zero-input exploitation systems are the most complex from a
biological perspective and the simplest from a management perspective.
They are all essentially one-dimensional. That is to say, to maximize the
yield of lumber per year is the goal in forest exploitation systems; and the
steady-state stocking rate, in sheep equivalents, tells one what is most
important about grazing systems. These systems, since they involve species
with long life-times and relatively slow growth rates, are slow to respond to
and recover from management action and disturbance.
The other two classes of agriculture partially or wholly supplant the
natural landscape. They are, therefore, ecologically less complex and have
fewer species. Because of human action on the landscape - plowing,
flooding, seeding, harvesting or burning at the end of the growing season its structure can change rapidly. That is, land that was in soybeans one year
can be turned to corn the next year, with great consequence for animals
dependent on that system. Despite these features, agricultural systems are
more complex than natural systems from the management standpoint, since
the farmer can control and manipulate them in a variety of different ways,
such as selecting different soil treatment strategies, fertilizers, herbicides,
pest control tactics, timing of activities, and product removal techniques.
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Because the fundamental time scale of agriculture is the growing season,
usually a year, many kinds of failures can quickly be corrected. Given
suboptimal performance from a system, a farmer can switch to a different
cultivar or application of herbicides in a subsequent season. Thus, modern
agriculture has the appearance of a successful, progressive enterprise and,
in fact, western agriculture is one of the most economically efficient systems
ever invented by humans.
One example of a series of failures leading to success is seen in the
British experience in Ceylon in the late nineteenth century. They first
attempted to grow coffee in their plantation system. However, the combined
attacks of Hemileia tastatrix and the native golunda rat wiped it out. The
next step was to try cinchona, the source of quinine. The climate proved
suboptimal and the system was unable to compete with the Javan crop.
Finally, they tried tea. This turned out to be a successful plantation crop.
When managers of high input agricultural systems try to sustain the
system in a particular state for a long period of time, a feedback loop can
develop which requires more inputs with each passing year. This dependency state is similar to the drug addiction into which a human being may fall.
Modern agriculture suffers from its own successes. Outside of Africa,
the triumph of the green revolution has produced surpluses in most
countries: mountains of butter, lakes of wine and milk. Until human
population growth catches up with its food supply or distribution problems
are solved, such conditions offer a window of opportunity for adaptive
experimentation.
One final category of the landscape consists of those areas of earth that
are now unproductive or otherwise degraded because of past mistakes in
exploitation or agriculture. Degraded ecosystems are often quite simple,
sometimes to the point of virtual sterility, and may have a permanence not
understood by the public. The loss of soil or nutrients, the presence of
biocides, or a dearth of proper colonists species may so constrain these
systems that they must be lifted as it were, with great human effort and
understanding, from the stable configuration of processes into which they
have descended. Thus, these systems may have very long time scales of
recovery.
BIOLOGICAL DIVERSITY
In a word, the goal of conservation is biological diversity. Yet biological
diversity is many different things. At different levels of biological organization, it involves features running from genes through species to ecosystems
32
and landscapes. Biological diversity can be measured over various spatial
scales, and it may be quantified in a variety of ways.
At the level of species diversity, ecologists and biogeographers have three
distinct categories of measures: alpha diversity, the number of species at a
point in space; beta diversity, the rate of addition and subtraction of species
as one moves along an environmental gradient; and gamma diversity, the
difference of species lists from site to site. We might extend this scheme and
call the total number of species on the planet or a continent omega diversity.
It is important to understand that one can increase alpha diversity while
lowering gamma and omega diversity. It should not be the goal of conservation biology to supplant rare endemics with a suite of otherwise common
species.
The biological significance of diversity among landscapes and ecosystems
has not been well quantified, though they are doubtless of great significance,
at least aesthetically. One important biological aspect of a landscape is
patchiness. But it is often the species that operationally defines this or
provides our perception of patches.
Biological diversity is not stationary in time and space. There are ebbs
and flows. The major features of these fluctuations are driven by changes
in climate. Pollen studies in lakes of North America have shown how great
the change of ecological communities has been since the Pleistocene
glaciation. And in Africa, which did not suffer glacial intrusion, the change
has been no less profound. The Kalihari and Sahara deserts were joined
20,000 years ago and the tropical systems were squeezed onto a couple of
mountain ranges to the east and west. Lakes in the Rift Valley were
hundreds of meters lower during the drought. This helps to explain the
greatly reduced beta diversity of the Zaire Basin compared to Amazonia.
In the Zaire Basin, the species of plants and animals have extended their
range a hundred-fold since the time of the great deserts, and have kept
pretty much the same ecological structures through the range of this
expansion. Amazonia has far greater heterogeneity from place to place.
The origin of this is still uncertain.
Biological diversity and agricultural interactions
Although agriculture is concerned directly with only a small fraction of
the genetic and biological diversity of the planet, as presently practiced it
influences the entire biosphere.
There is widespread agreement that
environmentally destructive and exploitative practices (with their increasingly
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unacceptable societal and environmental costs) must be replaced by more
sustainable food, fiber, and timber production systems.
To further complicate issues, this shift in agricultural practice will have
to be accomplished during a period of unprecedented change. First, the
physical environment is being transformed at a rate' hitherto unknown.
Alterations of global climate and sea level will have a major impact on the
future geographic distribution of agricultural activities. At the same time,
the demand for agricultural products will likewise be changing at ever
increasing rates.
If the primary imperative in the management of agricultural lands is the
provisioning of our own species, then in the next 20-30 years we will have to
develop sustainable agricultural systems capable of producing double or triple
today's yields. This will be necessitated because in the coming decades, the
human population will double and most tropical forests could be destroyed.
These occurrences will further exacerbate both the human and agricultural
predicaments.
Yet in this apparent conflict between agriculture and the environment,
there are seeds of hope. Our need to produce more food, fiber, and forestry
products is not necessarily incompatible with the conservation of biological
diversity. There is as much to be learned from the positive aspects of the
interaction between agriculture and nature as there is from the negative
aspects. Much is to be gained from examining agriculture's biological and
historical roots. And there are many reasons to believe that future
agricultural systems can be designed to alleviate the problems encountered
throughout the world today.
Traditionally, agriculture was totally dependent on nature. Natural
populations of plants and animals were exploited directly or, to varying
degrees, domesticated. Future sustainable agricultural systems will likewise
be dependent on nature; the crucial linkage in the future, as in the past,
involves genetics. Natural biological diversity does not stand outside of
agricultural diversity as it is broadly defined but, rather, is part of it. And
spatial and temporal dimensions are important considerations of this mix.
Agriculture is presently based on the genetic resources of a handful of
highly select species and their wild relatives. The genetic dependence of
domesticated plant and animal development on their wild relatives is
increasing as we recognize the wealth inherent in hitherto unincorporated
natural germplasm. With rapid progress in genetic engineering and the
foreseeable ability to move genes between unrelated species, the genes of
wild plants and animals become the capital which must be banked or
conserved if they are to return dividends in agriculture.
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34
Agriculture and conservation biology are not mutually exclusive
enterprises. In fact, they can be linked positively on both local, regional, and
global scales. Unfortunately, the populations and species that carry the
genes upon which future agriculture depends are rapidly disappearing just
when the tools which can make effective use of them are being developed.
This adds a great urgency to our attempts to shift "to more sustainable
agricultural systems that are more compatible with biological conservation.
One of the most important ways agriculture can contribute to conservation is through its legacy of research. Much of what we know about the
management of plants and animals and their communities is a result of
agricultural research.
Just as basic science promises to revolutionize
agriculture in the near future, so too the legacy of hundreds of years of
applied research and experience by agriculture promises to contribute
positively to the conservation of nature and natural resources. Agriculture
and conservation biology should not be adversaries; each is important in
biosphere management. The human species is dependent on both disciplines
for its prosperity and its future.
Natural biological diversity and agriculture interact dynamically, whatever
their spatial configuration. Each directs outputs to, and receives inputs from,
the other. In some cases, these input/output relationships may be between
closely adjacent landscape elements, but they may also have a far greater
spatial dimension. For example, at the short spatial range, the tropical
rainforests surrounding Gatun Lake in the middle of the-Panama Canal have
buffered the runoff of seasonal rains; their removal is causing more extreme
variation resulting in water shortages during the dry season. At the greatest
spatial reach is the general and gradual build up of carbon dioxide in the
earth's atmosphere, which is produced in part by the burning of tropical
forests to clear land for agriculture and also by the consumption of
petrochemicals necessary to supply and operate the system.
Neither natural biological diversity nor agriculture is neutral to the other.
Concerning the damage spoken of above, it is important to remember that
sometimes one may benefit the other. Natural elements of biological
diversity within exploited and agricultural systems may yield a number of
beneficial services to these systems. These have relevance to the vital
question of agricultural sustainability. Biological diversity may provide a
refuge for biological control agents such as the predatory insects that prevent
outbreaks of herbivorous pest insects.
Biological diversity stabilizes soils and prevents their loss. Biological
diversity regulates water flow and can also attenuate the effects of wind,
thereby greatly lowering rates of erosion. Biological diversity generally
buffers disturbance and gives human beings greater time to deal with it.
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Biological diversity also provides early warning of building toxicity, as the
birds of Rachel Carson's Silent Spring bore witness. At the genetic level,
biological diversity is the source of novelty. The wild relatives of domestic
cultivars have survived eons of change and challenge and have the answers
to innumerable environmental problems stored in their genetic heritage,
Sources of ecological knowledge
Like other scientists, ecologists observe and compare. They accumulate
data on pattern and process. But their study systems span a spectacular
range of scale, from molecules to the biosphere. They know most at the
genetic and population levels, where there is more replication and where
time scales are shorter. They know least at the level of the biosphere where
there are a limited number of biomic types, but which have behaviors that
occur over millennia.
Ecological experimentation in the laboratory is difficult and for field
studies, the attainment of the various statistical desiderata is often lacking.
Even at small spatial scales, the replication of natural systems is often not
controlled. Each island in an archipelago differs in numerous ways from the
others. For the student of bird diversity, for example, these differences can
only be ignored as noise or else factored out in the roughest statistical
manner. All ecological knowledge of natural systems is subject to much the
same limitations.
Ecologists, like other scientists, build models. These models, however,
are normally not intended as the basis for quantitative prediction. Rather,
they summarize the categories of behavior that have been seen repeated
over many different systems. The models are .for logical prediction, not
temporal or spacial prediction.
The temporal and spatial scale of ecological knowledge acquisition has
been limited to the graduate student life-time because few systems have been
studied in complimentary and overlapping fashion by teams of scientists.
This one-person, one-question approach to ecological science produces
strong biases. Scientists want to study processes that behave fast and are
spatially common because they need to harvest some novel publication
before their financial support dries up.
Some of the best knowledge in ecology comes from after-the-fact reviews
of major human disturbances and manipulations. These can be viewed as
unwitting perturbation experiments. For example, the Smokey the Bear
approach to fire suppression in North America has revealed more about fire.
ecology than ever could have been learned from the study of undisturbed
36
systems, that is, systems where fires had their pristine temporal and spatial
pattern. There are many other such processes about which more and more
knowledge is being gained as human beings interfere with them. The
prospect of restoring degraded systems and working within agricultural
systems to optimize biological diversity can certainly be seen as offering
unmatchable opportunities for the acquisition of new' knowledge.
The importance of diversity
Diversity can be measured at different levels of biological complexity
(genetic diversity, species diversity, ecosystem diversity), all of which are
important to the future of agriculture on the planet. The reader unfamiliar
with the basic arguments for the importance of conserving biological diversity
may want to consult volumes prepared by the U.S. Congress, Office of
Technology Assessment (1987) and Wilson (1988). To see the importance
of the biological issues in national and international perspective refer to
Myers (1984), Repetto (1985), and Western and Pearl (1989).
Genetic diversity involves the natural variability of the genes of individual
organisms and the collective variability of the genes of all members of a
population, strain, race or species. A healthy population of a typical plant
or animal species is characterized by genetic diversity; within it, there are
individuals of many different genetic constitutions (genotypes) (Table 2).
This variety of genetic types is an indication that no single genotype is ideally
suited to all the conditions under which the population lives or all conditions
under which the population might live. The variety of genotypes within the
population enables it to be successful over a range of ecological conditions
and to be able to adapt to changes in these conditions over time.
Animal and plant species used in agriculture typically have a much
narrower genetic base. So much so that in species such as wheat, millions
of acres may be sown with a single homogeneous cultivar. This has
considerable advantages under the highly standardized conditions of
fertilization, growing season, pesticide application, and harvest methods of
modern agriculture.
But it has enormous disadvantages if a virulent
pathogen evolves to attack the population because all individuals in that
population are vulnerable. Similarly, a lack of genetic variability will
diminish a population's ability to survive normal climatic variations.
This reduced ability to adapt or evolve makes the conservation of within
species genetic variability of fundamental importance to future agricultural
practices. For a more detailed discussion of the term "genetic conservation"
as it applies to actions at this level of biological conservation, the reader is
37
TABLE 2
Levels of hetcrozygosity
(H) and proportion
of polymorphic: loci (P) (or various taxa I
Hetcrozygosity
Polymorphism
.'
species
mean
s.d.
species
mean
s.d.
Venebrala
551
0.054
0.059
596
0.226
0.146
Invencbrata
361
0.100
0.091
371
0.375
0.219
Plants
56
0.075
0.069
75
0.295
0.251
t Summarized
from Nevo (1984).
referred to the recent reviews by Frankel and Soule (1980), Oldfield (1989),
and Orians et al. (1990). Brown et al. (1989) contains numerous discussions
of the application of population genetics in forestry and crop improvement,
and accounts of the problems and practices of conserving species level
diversity are given by Ehrlich and Ehrlich (1981), Schon ewald-Cox et al.
(1983). Wilson (1988), Woodruff (1989). and Fiedler and Jain (1991). Every
single. species-level extinction diminishes the options and prospects for
humane management of the biosphere.
THE BIOSPHERE
The scope of interest in conservation biology is immense. The field of
study is best thought of as a great hierarchy of entities and processes (Fig.
1). Within the complete organizational structure of nature, the principal
focus of integrating conservation biology and agricultural practices ranges
from the larger landscape ecosystem down to the individual organism. All
of this is contained within the planet Earth and is dependent on its stability.
The biosphere, the 30 kilometer thick zone of biological activity surrounding the planet, is the environment for life on earth. In the biosphere.
energy balances are crucial for the continuity of its processes. It seems
strange to some scientists that the temperatures on the earth's surface have
remained in a relative tight range for the last three billion years, neither
freezing nor boiling life before it had the chance to produce the species that
became self-conscious of this surprising stability. This global homeostasis is
remarkable because the sun has changed its solar flux by as much as 30%
over this period of time. Some scientists believe that the biosphere has self-
38
W!!.lli
I
I I
I
EARTH
ATMOSPHERE
GEOSPHERE
BIOSPHERE
BIOSPHERE
I
1 ... >
HYDROSPHERE
I
N
BIOMES
1 23
••••••••••••••
_>
ECOSYSTEMS
n
COMMUN ITIES
I
I
QUILDS
I
POPULATIONS
, 23
••••••••••
>
INDIVIDUALS
I
ORGANS
I
n
I
I
Fig. 1. Hierarchical structure of conservation biology. Adapted from Western and Pearl (1989).
regulatory properties (the Gaia hypothesis) with which it can accommodate
disturbance.
Two properties largely control the capture of solar energy by our planet:
the composition of the atmosphere and the reflectance of the surface of the
earth. An atmosphere with elevated levels of carbon dioxide absorbs
reflected infrared radiation and produces a greenhouse effect. A whiter
surface - ice or sand - reflects heat and cools the planet.
Global modelers are now attempting mathematically to mimic these
processes at this ultimate spatial scale. To do so, they need to understand
the inputs from ecological and agricultural processes. The output from such
a model will predict the climate of the future and thereby point to what
changes ecological and agricultural systems will need to make in response.
Unfortunately, the scales of knowledge in these enterprises are different.
The unit in a climatological model might be 300 to 400 square kilometers,
a size dictated by the capacity of computers available. This is considerably
larger than units normally studied by ecologists - so processes that occur
,
39
at such a scale will be referred to as "mesoscale" processes; their study is
crucial for linking ecology to climatology.
If climate changes equal anything like some of the predictions now being
advanced - e.g. a 9°e increase in average temperature and a 50% drop in
rainfall in the North American Midwest - there will be massive displacement of systems to different locations on the earth's surface where
temperature and rainfall patterns are more conducive to their survival. At
this point in time, no one can identify these locations. Today's storehouses
of diversity may be tomorrow's deserts.
Populations and species
The new applied science called conservation biology seeks to integrate
the findings of traditional agriculture and resource management with recent
advances in population genetics and ecology. In conservation biology,
traditional population concepts are typically replaced by metapopulation
models, and population structure and subdivision are recognized as having
crucial effects on demography.
A local population of a species is typically at equilibrium with its
surroundings when it is at a level that its total density cannot exceed. When
the number of individuals falls below this level, there-is an average per .caplta
rate of growth. But all local populations receive from their environment,
random shocks of varying intensity, and these stochastic inputs subject all
populations to a probability of extinction. Only by being part of a gridwork
of similar local populations, from which immigrants and colonists can arrive,
can a population persist for an appreciable length of time. The collection of
local populations over a gridwork of environmental patches is called a
metapopulation.
Both local populations and metapopulations have thresholds of size and
spatial arrangement below which they suffer catastrophic increases in their
probability of extinction. Because local populations under such conditions
are not indefinitely elastic, at some point they snap quickly to extinction.
Many such cases have been studied with a variety of techniques, and semiquantitative predictions can be made of where these catastrophic thresholds
- involving the joint action of genetics, demography, and spatial arrangement - are likely to be crossed. The thresholds are different for different
species and, unfortunately, there are no magic minimum viable population
numbers that apply equally to all cases.
Local extinction.of components of a metapopulation are now viewed as
normal occurrences. The field of population vulnerability analysis (PYA)
40
focuses on the relative significance of genetic, demographic, and habitat
fragmentation processes in causing such extinctions. Increasingly, predictions
about the survivability of populations must be made with reference to a
specific time scale: single generation, ten generations, a hundred generations, a thousand generations. Proposals to manage -populations for very
short-term gain can have exceedingly deleterious long-term consequences.
In practice, agriculturalists conserve populations and species rather than
specific genes of economic importance. In addition, there is widespread
recognition of the need to protect many domesticated varieties and breeds
and other threatened natural species of great potential value to forestry and
medicine.
Agriculturalists should be aware that conservation and evolutionary
biologists occasionally employ a different taxonomy for the population units
of concern. Agriculturalists have traditionally recognized wild species,
subspecies, land races, breeding lines, varieties, stocks, strains, and cultivars.
Recently, genetically engineered lines have been added to this lexicon. In
contrast, conservation biologists recognize demographic and evolutionary
units clones, outbreeding populations and species. The traditional
concepts of the wild-type genotype and of the subspecies as an evolutionary
category have been progressively abandoned.
Biological species are
envisioned as groups of populations that share a genetic and evolutionary
cohesion based on the ability of individuals to discriminate between members
of their own species and members of other species. Such evolutionary
concepts are directly applicable to agricultural situations.
The difficulty in implementing conservation programs is a consequence
of the number of species involved. There are presently 30,000 threatened
plant species and an additional 10,000 plant species of potential economic
value that may require u situ conservation unless suitable in situ conservation measures are developed. The National Marine Fisheries Service
(U.S.A.) has implemented 30 management plans in a move toward
conserving stocks of 300 species of fish, crustaceans, and molluscs, the
majority of which are still being overfished.
At the international level, conservation efforts for a few hundred
especially appealing animal species have been organized by such nongovernment organizations as the I.U.C.N. (Species Survival Commission with
100 specialist groups) and the American Association of Zoological Parks and
Aquariums (40 Species Survival Plans).
The number of animal and plant populations and species whose survival
depends on intervent.lve management far exceeds present levels of resource
commitment. Woodruff (1989) has reviewed the problems of conserving
'lo'i3
41
animal genes and species noting the strengths and weaknesses of existing
management plans.
Communities
Within the mesoscale regions of the earth, there are smaller, more easily
understood units. Communities are objects of study in which species
identities are known and in which the densities of their members may be
determined. In fact, sometimes the community is represented as a set of
equations of motion for the densities of the individual species populations.
Yet communities have many emergent properties that are also the proper
study of community ecology.
The web of feeding relationships is one area of active study. Feeding is
not random but has a structure along which members of the community can
be arrayed in sequential fashion - feeding patterns tend to form discrete
"intervals" rather than overlapping on their prey species in a random
arrangement.
Alternative and mutually exclusive collections and arrangements of
species may be possible at any point in space. That is, the character of the
community is not completely set by extrinsic factors. The forces generated
between the species themselves_can_haveoverridingimportance. The history
of the system is also important. Shifts between communities can be very
sudden, while the time spent in such "domains of attraction" may be quite
long.
Communities are networks of balanced forces. Though their appearance
may be static, they are fully dynamic. When a force is applied to one species
or one.component in a community, actions and interactions spread to the
other components through many direct and indirect linkages. Often the final
consequence of the removal of one species on the condition of another
species may be different from what would be predicted from cursory
examination of the system. For example, the removal of one competitor can
lead to]the extinction, not the increase, of another competitor.
There is another consideration that leads to the conclusion that the
prediction of community dynamics will be difficult. Outside of their stable
configuration (if one exists), communities are likely to be governed by
nonlinear relationships. This makes communities very sensitive to small
changes in conditions; trajectories of community behavior that are initially
quite similar can diverge to very different final behaviors.
All communities have species components that provide structure for the
system. These are sometimes called "keystone" species. That is, the great
I
42
majority of the species in the community adjust their densities and spatial
distributions to features of the keystone species. The keystone species
provide scope and stability to other species, but not conversely. The
communication of information and adjustment among species is typically
rapid. In most cases this allows ecologists to ignore .the presence of nonkeystone species and to focus their research and dynamic modelling on the
few, key species.
Ecosystems and landscapes
Ecosystems differ from communities most importantly in the way they
are perceived by the scientists who study them. Simultaneous equations of
motion for hundreds and thousands of species populations cannot be studied
profitably. Consequently, ecologists lump, or aggregate the individual
components, and knowledge of community processes is used as a guide to
how this aggregating should be carried out.
Ecosystem ecologists use the language of systems analysis common to all
science where complexity is a fundamental reality. Almost indispensable
tools provided by systems analysis are those allowing for control and
monltoring.of.Inputs and outputs, stresses and responses. There is.very little
that can be learned by simply observing the steady state values of a system
at equilibrium.
The characterization of complex behavior is necessarily subjective. One
tries for a simplicity that is consistent with an accurate description of
behavior; the description is adjusted when confronted with surprising
behavior. Unfortunately, all too often the behavior of ecosystems is
unexpected, especially when the ecosystem is stressed from external activities
associated with exploitation and agriculture. However, much is learned from
such confrontations and the knowledge gained will guide the management of
similar situations in the future.
The equilibria I, balance-of-nature view of ecosystems under which any
shock could be absorbed, and any stress accommodated, is still commonly
held and often implicitly accepted by society and its managers. For example,
if the river system has been able to absorb an amount X of sewage, then it
can absorb amount 2X. Another manifestation of this view is that societies
often take spatial scale as irrelevant since the same kind of equilibrium
obtains everywhere. This view has been proved false in virtually every
situation where it has been tested experimentally.
The new paradigm of ecosystem structure recognizes the existence of
multiple stable states, and it emphasizes variability and spatial heterogeneity.
'J..IO
43
Behavior is only expected to be continuous over small domains of environmental variation. Sharp, discontinuous adjustments are expected at the ends
of these ranges. Most experience with this new paradigm has come from
systems where human beings have played a great role.
Consider, for the moment, changes in climate. Such changes have been
profound even before human modification of biochemical cycles began to
play a role. The second of the two views of systems dynamics mentioned
above demands a simple management prediction: indefinite, slow adjustments in agricultural practices cannot be expected, even under gradual
modification of climate - at some point, very abrupt adjustments will have
to be made. It is precisely at these points where a large pool of alternative
cultivars and management strategies are necessary if the changes are to be
survived with minimum disruption to support processes.
Considering a different level of spatial perception, the instability just
discussed can become the basis for regional stability. Many stable systems
depend on a random pattern of small spatial scale unstable transitions. Each
small spatial patch in the system may take on a variety of behaviors over a
long period of time. But only by playing such different roles over time can
the patch be properly renewed and refreshed to continue in the game. Force
it into anyone role for too long and it will eventually collapse to some
degraded form in which it may remain indefinitely.
MIGRATION AND DISPERSAL
The movement of animals and plants over large distances is a process
that "cuts through" and interacts with all the "horizontal" layers of organization discussed above, from genes to ecosystems, and must be considered
separately.
Organisms move for a variety of reasons. Often they move back and
forth over a set route to take advantage of seasonal patterns of food
availability; this is called migration. Large-scale migration is seen in such
various organisms as butterflies, birds, whales, and savanna grazers. A
second kind of movement is permanent, within the lifetime of the individual
organism; it is called dispersal. Typically, one sex and age class will leave the
place of its origin to settle elsewhere. The evolutionary explanation for this
is that genetic lines survive because they have 'hedged their bet' on long-term
survival by playing in many different and thus uncorrelated environments.
This reduces the risk of 'betting' everything on the constancy of the
environment at orte point in space. Another possibility is that genetic lines
2.1\
44
with the 'urge' to move are successful because this behavior balances some
of the negative values of inherent small population sizes.
Migration presents a management problem at the level of individual
species, because it means that the chain of survival is only as strong as its
weakest spatial link. Over-harvesting, pollution assaults, or habitat loss at
anyone of the areas of a species' occupancy can doom the entire system at
all its locations. Migration also can contribute to dispersal in the sense that
seasonal movement tends to "mix"individuals from various origins within the
population. For example, the monarch butterfly and whooping crane require
protection by different national governments.
When the migrating species plays a keystone role in one of the spatial
systems of which it is a part, the management becomes quite uncertain and
risky. The predatory activity of birds on the larvae of herbivorous insects
can often be a key environmental process, as it is, for example, in the forests
of Canada. The birds suppress the population growth of herbivorous insects
and keep their numbers in balance with regard to food supply, i.e. the forest
itself. However, these birds migrate to tropical areas in Central and South
America to over-winter. With accelerating deforestation in the tropical
areas, there is increased risk of defoliation to Canadian coniferous forests.
The bulldozer in Bolivia threatens economic and ecological disaster in
Canada (Terborgh, 1990).
SMALL POPULA nONS
Many populations of agricultural importance are either numerically small
and geographically isolated or numerically large but derived from very few
founding individuals. Both types of populations are effectively small in a
genetic sense. The genetic behavior of effectively small populations is
qualitatively different from that of large out-breeding and geographically
widespread populations, and more active intervention is required for their
management.
The genetic vulnerability of rare or threatened species is often a direct
consequence of habitat destruction and range fragmentation. The eventual
loss of the species may be due either directly to the lack of adequate habitat
or indirectly due to genetic consequences of range fragmentation and
reduction in population size (demographic bottlenecks). The analogies
between natural species and agricultural strains or cultivars are not
accidental, from a management perspective; both are based on the same set
of genetic principals.
'2.1'2..
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45
Inbreeding - the mating of closely related individuals - is rarely a
problem in nature. But, under some management regimes, it can effectively
cripple small populations. Outcrossing populations that suddenly decline in
numbers usually experience reduced viability and fecundity known as
inbreeding depression. Inbreeding produces increased homozygosity of
recessive deleterious mutants and, by chance in small populations, these
alleles become fixed. Breeding programs for managed populations must be
designed to counter the effects of exposing lethal and deleterious recessive
genes in homozygous form.
In large natural populations, close inbreeding rates are typically less than
2% per generation and over centuries of experience animal breeders have
learned to keep the rate at approximately half that level. The consequences
of closer inbreeding are tragically illustrated in isolated human populations
or in mammals in zoos where juvenile mortality is greatly increased for
offspring of related rather than unrelated parents.
In contrast, management involving gradual inbreeding or reduction in
numbers allows selection to purge deleterious recessive alleles as they
become homozygous. These populations, and those of species that normally
reproduce by self-fertilization, suffer little inbreeding depression. However,
the hybrid vigor seen when inbred lines are crossed shows that they still carry
some genes with poor performance when in the homozygous state. This is
a continuing problem for managers of agriculturally important populations.
The second major problem associated with small populations involves
their inexorable loss of genetic variability. In small populations, random
fluctuations in allele frequencies lead to the occasional loss of rarer alleles
and the fixation of commoner ones. Such generation-to-generation fluctuations are the result of chance: different individuals play out their lives with
or without reproducing successfully. Such genetic drift, as it is known
technically, reduces genetic variation and leads to increased homozygosity
and loss of ability to evolve.
A fundamental concept is that effective population size, N., is closely
linked to this principle of genetic drift. Sewall Wright's notion of effective
population size is somewhat awkwardly defined as the number of individuals
required in an ideal population that would be expected to experience genetic
drift at the same rate as the actual population. Unequal numbers of males
and females, increased variance in family size, and temporal fluctuations in
numbers all cause effective population size to be much less than the actual
census count simply because not all individuals contribute equally to future
generations. In the absence of factors promoting genetic variation, such as
mutation and dispersal, the expected rate of loss of heterozygosity is
1/(2N. + 1) per generation. The same rate of loss is predicted for the
46
variance in polygenic characters, many of which are of agricultural importance. Simply stated, loss of variability increases as the genetic effectiveness
of the population of breeding individuals decreases. The survival of two
individuals would produce progeny with only 25% of the original variation.
Although little genetic variation may be lost in anyone generation, small
numbers sustained for several generations can severely deplete variabilitymost variability should be lost within 2N. generations.
When these
theory-based relationships are translated back into real numbers it becomes
clear that many current management practices strip populations of their
variability within relatively few generations. Thus, genetic drift can imperil
a mismanaged population within tens rather than hundreds of generations.
During the last decade there has been considerable debate about the
optimal values of N. for a managed population. For outcrossing natural
populations, two values, N. = 50 and N. = 500, have been proposed based on
theoretical considerations and different management objectives (Frankel and
Soule, 1981). Studies of mutation rates in a few polygenic characters suggest
that a population with N. =. 500 should maintain its intrinsic genetic
variability indefinitely. Similarly, it is argued that N. = 50 was a requirement for short-term survival. Definitions of minimum viable population size based on such magic
numbers are now seen as unsound (Lande, 1988). The numbers based on
the theoretical consideration of a few characteristics of the organism may not
be relevant to the behavior of the whole genome. Genetic uniformitarianism
cannot be assumed; other types of genetic variation are equally important in
estimating minimum viable population size. Thus, there is no simple answer
to the optimal effective population size question other than the trivial one
that large is better than small. Clearly, a reduction in the degree to which
populations are geographically isolated would effectively increase population
size. Thus, an integration of agricultural practices and biological resource
management strategies offers great possibility. The agricultural landscape
could be the largest nature preserve human beings ever dreamed of.
EXPLOITATION STRESS
This section addresses explanations for the degradation and collapse of
exploitation systems, whether fisheries, forests, or grazing systems. These
systems are one-dimensional from the standpoint of the manager in the
sense, there is a single product to be maximized. The product may not admit
internal variation, but there is usually a linear scaling factor between these
internal classes. For example, an old-growth douglas fir may be twice as
,
47
valuable per unit volume as a six year-old tree. And a skipjack tuna may be
worth 1.3 yellowtail tunas.
Managers of such systems invariably adapt a method of maximum
sustainable yield. One thing managers are especially adverse to is variation
in output. Typically, they have a capital investment in extraction equipment
on which they have to pay interest, a constant per year amount. Disaster
looms if there are years or seasons where the yield, in monetary terms, is
below the carrying costs for capital. Therefore, attempts are made to
homogenize the behavior of the system in time. This does not necessarily
require spatial homogenization, but it often leads inexorably to this state.
The Sahel is the semiarid region between the hyper-arid Sahara Desert
and the regions of the African Savanna. Over the last 40 years, a series of
efforts has been undertaken to make it more productive in the hope of
feeding the growing human populations in the region. Prior to World War
II, most of the region was engaged in migration-based pastoralism involving
cattle. After the war, peanut farming spread north into the drier regions,
displacing pastoralists. With the elimination of cattle in these farming
regions, natural fertilization of the food crops ended. The farmers were too
poor to supply artificial fertilizers so yields started to decline.
A second homogenization process contributed to the problem. The
pastoralists who inhabited the driest regions of the Sahel were forced to
move more often than normal because of a lack of groundwater. Politicians
thought it better to have them stay in a given place so wells were drilled to
provide permanent water. This provided better health services to both
humans and cattle, causing population growth. But the elimination of
seasonal migration meant that the grassland surrounding the permanent
water was soon overexploited.
A train of ecological consequences was set running on a track toward
disaster. Changed grazing patterns led to a succession of plant species
favoring shallow-rooted and unpalatable forms. More soil was exposed, and
there was compaction from human and domestic animal activity making
conditions unfavorable for plant establishment. A process of decertification
was established.
This decertification is apparently changing local weather for the worse.
Sandy, denuded soils reflect a greater fraction of solar energy than do soils
covered with vegetation. Given that local rainfall is produced by the heating
and lifting of moist tropical air, the cooler soil conditions mean even less
rainfall, which only contributes further to decertification. A managementinduced, self-sustaining drought has degraded what once was a stable system
based on a migratory pattern of grazing.
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48
There is possibly no way out of this cycle short of massive intervention.
Small spatial solutions are worthless, for as soon as effort to create them is
relaxed, the area is again engulfed by the regional equilibrium.
Only a
region-wide solution will work. Some propose that the humans and the
grazers would have to be removed from the region that spans 10 different
countries and then the soil would have to be artificially fertilized so that it
would turn green after periods of greater than average precipitation.
Similar stories have occurred in fisheries systems. The great whales,
although not totally eliminated, have been pushed to such low population
densities that their per capita growth rate is reduced to near zero. Part of
the problem no doubt arises because of difficulties finding mates, which is
associated with uneven age structures and reduced rates of social encounter.
The dungeness crab population off the shore of Northern California was
pushed low enough that an egg parasite, which had previously been swamped
by the crab's reproductive output, is now able to hold the crab stably at this
low level.
Overexploitation of multispecies fisheries often leads to the elimination
of all but one of the species. This is the direct consequence of the
celebrated "principle of competitive exclusion" that says two species cannot
be limited by the same factor. Since exploitation by humans becomes the
dominant limiting factor -for all the-species, only the species best able to
respond to this pressure will survive. This is the reason that in densely
populated regions of the earth, carp is the common table fish.
DEPENDENCY
A human culture can become "locked-in" to a particular form of
exploitation. We call this dependency. Among other things, it means that
the human culture is part of a stable ecological system and that any change
of human behavior could produce profound changes in the non-human
system processes, or vice versa. The Irish peasants of the eighteenth and
nineteenth centuries became dependent on the potato. They were not
initially dependent on the potato, but their life style, demographic patterns,
political and economic systems, and land tenure practices adjusted to the
bounty of the potato. They were part of a system that only a disaster of
major proportions seemed able to change.
The nomads of the Sahel were culturally dependent on a wandering
pastoralism.
But; at least in the short run, this dependency was mainly
cultural so that external political forces could greatly alter this pattern. The
49
system is now in a transition phase and what equilibrium it is tending toward
is not yet clear.
Developed world farmers are often dependent on a particular agricultural
system. This dependency is relative and not the product of a lack of
ecological knowledge. In fact, it may be the consequence of an assumption
that we understand more than we think we do about ecological processes.
Many classes of developed world farmers are locked into modes of
production because of a great investment in capital. The consequent need to
pay the interest costs of this investment on a timely basis results in most
management decisions being made at the margin, so to speak. These
farmers are so deeply committed to a practice that the extra cost of a lateseason pesticide application is minor and taken as a matter of course.
This economic dependency is in many ways no better or worse than other
forms of dependency - but in one quite foreseeable way it is probably
unsustainable. If this dependency were true and the economic constraints
valid, there might be no problem. But through governmental policies to
support agriculture, these constraints are often fictional and deny true
economic constraints that ultimately must be felt.
Petroleum inputs are one obvious case. The cost of petroleum-based
fertilizers and herbicides, and also the energy inputs to farm machinery and
transportation, represent only extraction costs of petroleum, not its true
worth. Petroleum extraction rates will soon start to decline and the price of
these resources to farmers will greatly increase, making their application
unprofitable. What replacement will there be for them?
Water is another commodity that is often not costed properly. The San
Joaquin Valley of California, especially the west side, depends on irrigation
water that has to be pumped uphill. The laws of thermodynamics being what
they are, this consumes the bulk of the energy that the same water earlier
produced as it dropped through the turbines of great dams. With rising
energy prices, the public will soon have to face choices on how it consumes
its hydroelectric power.
There are, however, other costs associated with irrigation. Irrigation
invariably leads to the build-up of salts in the soils. The fall of civilizations
has been based on this simple fact. To forestall the accumulation of salts in
the soil, farmers must apply additional water that carries off salts through a
drainage system. This waste water must be collected and disposed. In 1957,
the State of California came to grips with this problem and drew up a plan
to transport such waste water north to the Sacramento River for disposaL
In 1960, Congress expanded the project to encompass a 300 kilometer master
drain for the wh~le valley. In 1964, however, a consultant predicted the
50
"total destruction of wildlife" should the waters from this drain reach the San
Francisco Bay.
Construction on the drain began in 1970. The intention was to discharge
the waste water into the Sacramento-San Joaquin River Delta, but a
reservoir was needed to regulate the flow. Land was purchased in western
Merced County that became the Kesterson Reservolc.Because this was also
sensitive wildlife habitat, the U.S. Fish and Wildlife Service agreed to
participate in the management of the drainage water.
What has happened since is unmitigated disaster. The connection of the
drainage canal to the San Francisco Bay was never completed. Kesterson
Reservoir became a progressively more poisonous evaporation pond, and the
Fish and Wildlife Service now must try to keep wildlife out of it. No
solution is yet in view.
HUMAN SCALES AND HUMAN RESPONSE
In discussing the theory of scale, it is important to recognize the role
played by human beings in natural and agricultural systems. Various human
cultural forms are scattered in a mosaic all over the earth. Different cultures
have different time horizons in viewing nature and formulating political and
management plans. With pressing mortgage payments, a farmer may not be
able to see past the current growing season. The politicians of democracies
often cannot see past the next election. Cultural patience may depend on
religion. Patterns of land tenure are extremely variable. The Japanese
horticulturalist may make a living off a few hectares, while the Texas rancher
may control an area of thousands of square kilometers.
The Irish potato famine of 1845-1848 speaks to temporal scale issues in
human adjustment. The potato was brought to Europe in the early 1600's
by Sir Walter Raleigh. Over the next two and one-half centuries it grew to
pervasive popularity among the Irish peasantry for a variety of reasons,
including: it was well-suited to the deep soil of Ireland; it was not particularly
sensitive to war (of which there were many during this period) since
trampling and burning could only destroy the above-ground part of the plant;
it required a low input of external materials and was, thus, suited to poor
people; its nutrients were complimentary to those produced by dairy cattle,
making a baked potato with sour cream a complete meal; it was even the
source of an alcoholic intoxicant, poteen.
The Irish peasants adjusted slowly to the blessings of this highly
productive crop by increasing their population growth rate, typically through
51
early marriage. Politically, they divided their land holdings to roughly a
hectare per family.
Spatially, however, Ireland was far from homogeneous. A great deal of
the land in Ireland was held by absentee English landlords who practiced
cash-crop agriculture and pasturage. There was also a group of Montenegins
from Serbia who practiced a very different agriculture. Both of these
components of the system were perfectly unaffected by the potato famine.
The potatoes of Ireland, and all Europe, had gone through a small
genetic bottleneck during their introduction and were genetically
depauperate. In 1845, a blight struck and the crop failed. There was widespread hunger. "Bad luck" the peasants thought, and replanted potatoes the
next year. Again, the blight struck. There was starvation. But again they
replanted potatoes.
They were spared the blight in 1847, by which point they had made very
little adjustment, but it returned in 1848. This exceeded the limit of the
resiliency of the system. There was great starvation and mass emigration to
America; 50% of the population died or emigrated. Finally, the episode
greatly altered the subsequent social and political structure of Ireland. Land
holdings were consolidated and passed to the eldest son. The population
growth rate was stabilized, principally through delayed marriage and a slow
rate of emigration. The population level stabilized at 50% of its midnineteenth century-level.
While the adjustment of the Irish has proven to be permanent, it is not
comforting that it took so violent a disaster to achieve it. The great
problem, which is outside the province of ecological theory, is how to
mobilize an adaptive response by a society before a disaster strikes.
Understanding ecological processes is important, but understanding is not
sufficient to bring about change.
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