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9
Ecology and Ecosystems
INTRODUCTION
Ecological Problems Are Challenging and Complex
As we enter the last decade of the twentieth century, we face greater environ-
mental problems than humans have ever faced. We are confronted with changes
in the distributions and exchanges of elements on broad scales, with the alarming
loss of biotic and habitat diversity, with the consequences of species invasions,
with toxif~cation and contamination of our aquifers and other systems, with the
disposal of hazardous wastes, and with the collapse of resource systems. As
never before, we need to improve our understanding of basic ecological prin-
ciples: of the factors governing the interrelations between organisms and their
environments, of the mechanisms governing the structure and functioning of
ecosystems, and of the patterns of response of ecosystems to stress. Our ability to
deal with environmental problems will depend on learning to manage systems,
which must ultimately be based on advances in basic science.
Ecology occupies a unique position in biology because it relates directly to
issues and concepts that are widely viewed as being in the public domain.
Although most other branches of biology also have great relevance to society, the
concepts they deal with are less a part of everyday experience. The earliest
ecological studies were those by naturalists interested in organisms and their
relations to their environments, and this kind of work remains the core of basic
research in ecology.
Ecologists must be concerned with all levels of biological organization:
cells, organisms, populations, communities, ecosystems, landscapes, and the bio-
sphere. They work with cross-disciplinary approaches and are concerned with
287
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288
OPPORTUNrTIES IN BIOLOGY
phenomena that are inherently complex. Ecologists study the highest level of
biological integration and provide the tools by which humanity is able to manage
the biosphere.
The diversity of organisms and of the interactions among them has made the
study of ecology fascinating but difficult. There is no unequivocally correct way
to reduce that diversity to a set of easily understood and applied rules. Instead,
one must look across a wide range of diverse examples and seek common
unifying principles. In almost every instance, determining the domain of applica-
bility of the results of experiments or comparative surveys is difficult. Neverthe-
less, several important principles exist. Central among these is evolutionary
theory, which describes the ways natural selection, stochastic factors, and histori-
cal constraints have interacted to determine the patterns we see in nature. Other
concepts-such as succession and community~provide organizing principles.
Many ecological problems require a comparative approach for their solution
and are not always amenable to experimentation. Often, for aesthetic and logistic
reasons, ecological experimentation can take place only on small temporal and
spatial scales. However, opportunities for large and long-term experiments do
exist, and they have led to important insights. In the most general sense, ecology
uses the comparative approach extensively in its attempts to order complexity. Its
scientific basis is the description and elucidation of pattern. Many of the most
interesting and relevant problems in the field are so complex that they cannot be
solved by a reductionist approach. In this sense, ecology differs from most of the
rest of biology.
In ecology, the transition from explanation to prediction is a large jump.
Thus, our experience over the past several decades in managing ecosystems and
in dealing with environmental hazards has been punctuated with the accumulation
of unique experiences that seem to fit no pattern. Despite these surprises, we are
developing an increasingly robust predictive theory. The challenge to ecology
generally is to develop further rigorous bases for classification of phenomena and
to construct a framework that can accommodate our past experiences, summarize
our vast but often anecdote knowledge, and serve as a basis for prediction.
IDEAS AND APPROACHES IN ECOLOGY
The Responses of Organisms to Environmental Variation
Environmental Variation Profoundly Influences the Distribution and
Adaptation of Organisms
Ecology is concerned with the interrelations among organisms and their
environments, with the organization of organisms into populations, with the
organization of those populations into communities, and with ecosystems. Aut-
ecology is concerned with how organisms adapt to their environment through
specific biochemical, morphological, and physiological mechanisms.
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ECOLOGY AND ECOSYSIEI~S
289
For plants, examples include changes in leaf shape in different environments
(for example, some desert plants have thorns instead of leaves); the evolution of
an impressive array of complex chemical defenses against herbivory (stinging
nettles, hemlock, poison ivy), and adaptation to water stress (succulents). These
adaptations extend from the biochemical to the physiological and whole-plant
performance levels.
For animals, similar progress has been made in understanding responses to
environmental factors, as well as in understanding the constraints imposed by
morphological, thermoregulatory, and behavioral features. Examples of such
adaptations include the antifreeze proteins found in the blood of Antarctic fishes;
the modified cardiovascular system of seals, which allows them to remain sub-
merged for long periods; the impressive array of chemical defensive (such as
tetrodotoxin) and offensive (snake venom) weapons; and the giant versions of
otherwise small marine species that have been recently found living at deep-sea
hydrothermal vents.
Environmental factors also play a primary role in determining how organisms
are distributed. Although in some cases a single factor seems to correlate well
with the success of the animal or plant, the basis for that success may actually be
complex. The classical studies of ecological races in different plant species
confirmed that adaptation in attitudinal races is complex many genes are in-
volved in determining such features as frost tolerance or time of flowering. An
increased understanding of the mechanistic basis of tolerance can come from
integrating studies of whole organism-integrated responses with studies at cellular
and subcellular levels. Recent advances in subcellular physiology and molecular
biology are providing new tools, and the prospects of establishing physiological
and genetic bases for adaptation are within reach. The practical consequences of
applying improved understanding in this field to crop productivity are obvious,
but the insights will also lead to increased understanding of evolution and ecol
ogy.
Specific genes associated with stress tolerance, such as those coding for
antifreeze proteins, can be identified, cloned, and studied in detail. In turn, the
ways in which the products of these genes interact at the developmental, morpho-
logical, and physiological levels can be determined. The full understanding of
such responses will require the integrated efforts of ecologists, molecular biolo-
gists, and physiologists. Ultimately an understanding of the molecular biology of
adaptive metabolic features will result. Such knowledge can then be applied
directly to evaluate potential new crops as well as to develop kinds of plants and
related agricultural practices that are efficient at using limited water and nutrient
resources.
As discussed in more detail in a later section, anthropogenic environmental
changes also produce stresses and provide opportunities for study, as do natural
catastrophes. The large-scale application of pesticides has caused insect pests to
evolve and has been a most instructive seminatural experiment. The eruption of
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OPPORTUNITIES IN BIOLOGY
Mount Saint Helens has allowed some unique studies of ecological succession.
Studies of the effects of the Glen Canyon Dam on the vegetation along the
Colorado River have also provided ecological insights.
The relations of organisms to their physical environment is only one aspect of
ecology. Their relations to other individuals of their own or other species are also
critical in determining their roles in nature. Combining the two aspects leads to
the study of how populations of plants and animals are regulated and structured.
Structure and Regulation of Populations
Population Ecology Is the Study of How Populations of Organisms Are
Regulated, How They Behave, and How They Evolve
The most basic question in population ecology is how natural populations are
regulated. Why do some species suddenly increase in numbers while others
suddenly decline? Why do some organisms reproduce only once and then die,
whereas others reproduce repeatedly, perhaps dozens of times? Why do some
organisms produce only one offspring at a time and others produce millions at a
time?
Gypsy moths can have sudden outbreaks after being at low population
densities for years. During these outbreaks, millions of acres of forests can be
defoliated in a few weeks. After a year or two, the population density suddenly
declines and gypsy moths are not noticed in that area again for many years.
Although much has been learned about the factors regulating these pests-
predators, food supply, and climate-it is still not possible to predict the out-
breaks more than a year in advance. But the regular inundation of areas of the
eastern United States every 13 or 17 years by immense hordes of cicadas is well
understood. These creatures live underground, feeding on tree roots, for 13 or 17
years. Then, all at once, billions of adults appear; they make streets slippery as
they fall from trees and they produce an almost deafening noise. Their unusual
life-history pattern appears to be a method of escaping predation. When they
emerge in such immense numbers, there are too few predators to seriously dent
their populations even though the predators lucky enough to be in an area of
emergence can eat to satiation. But for the predators it is a one-time feast. They
cannot make a living off this vast banquet because by the time the predators have
produced their offspring, the cicadas will have vanished for another 13 or 17
years.
Not only insect populations fluctuate in this way. Hares, lemmings, lynx, and
many fish species, such as herring, striped bass, bluefish, spot, and tilefish,
fluctuate in numbers over time. In the 1920s, spot became so numerous that they
clogged the cooling-water intakes of New York City's power plants. In a few
cases it is possible to identify environmental changes responsible (for tilefish, a
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ECOLOGY AND ECOSYSTEMS
291
cold snap seems to have devastated the population in the late nineteenth century)
but often the list of possible causative or contributing factors is so long that
understanding each case can require a large research program. Many cases
simply are not understood.
Unraveling the myriad causes of population fluctuations-and population
dynamics in general-and applying that knowledge is an important challenge.
Causal and contributing factors in population ecology are either biological or
nonbiological, and their study involves many disciplines.
Understanding biological factors requires understanding individual life-his-
tory patterns and predator-prey, host-parasite, community, or evolutionary rela-
tions; sometimes all need to be understood at the same time. The complexities
involved have made successes enormously rewarding; even our failures have
been interesting.
Concepts of Population Ecology Are Important in Managing Hunting,
Fishing, and Agriculture
Generally speaking, a large class of applied ecological problems has to do
with production. How much of something can we get, or how much can we limit
something that we don't want? In fisheries, the problem consists of knowing how
large the populations are and then trying to understand the ways in which the
characteristics of population growth affect the size of a suitable catch per unit
time. Estimating the size of populations of fish is difficult, but important concep-
tual advances have been made with the assistance of models.
The lengths of life cycles are likewise important for management practices.
Long-lived, slow-growing species (such as ocean perch, king crab, redwood trees)
require different management or agriculture than do fast-growing, short-lived
species (shrimp and corn). It is all too easy to mistake abundance for high
production, as the story of the passenger pigeon illustrates.
Trees, obviously, lie at one extreme,, but many of the problems with manag-
ing other long-lived organisms such as whales stem from the same features-long
prereproductive spans and low recruitment of juveniles. In whales, these features
are due to low reproductive rates, whereas among trees they are due to low seed
and seedling survival rates in a world dominated by well-rooted adults.
Biological pest control is even more complex, but the application of ecologi-
cal studies has led to great advances. For instance, California red scale, a serious
pest of citrus, has been controlled successfully in many areas as a result of the
thoughtful application of a detailed knowledge of predator-prey relations. Other
successful examples include the control of rabbits in Australia by the myxomato-
sis virus and the control of pnckly-pear cactus in the same continent by a cactus-
feeding moth. Great care is needed in making such introductions, however, lest
the organism that is introduced become a pest itself.
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OPPORTUNITIES IN BIOLOGY
;~:MODELIN:G~AG:RICIJLllJRE:AFTER NATURAL SYSTEMS
Naturallv occurring ecosystems have many traits that: would be desir
astern agricultural systems: :~l~hey tend to~u~se resource~Iight, water
:: :: :nutrients, :and: carbon dioxide effectively And ~ bff:~iently; when u:ndisturbed
:: ~ they tend ~ ~to~:ms:ist :i:nvasions Sibyl computing ~species; and ~ they se:ldom suc-
cu:mb:com~pletely to pest: attics.
: ~Any:~reeIistic view of Agriculture for the future must take into account
three considerations. First the spectacular gains :in agricultural productivity
~ ,
of this century have:~resulted fro:m~increas~: use::of fossil~fuel:d~ernratives,~
: especially nitrogen fertilizers.: Sin - : petroleums: reserves are finite:, oontinu:-
: :ous gain:s::in yie::ld cannot ~ Obtained by increasing our: applications: of:
petroleum:-based:fettilizersindefinit:ely. : :
Second agricultural lands everywhere-including the United States
.
are being degraded bv: imorooe~r~husbandrv. Techniques to:imaintain site:
equally, as well as to restore~the productive capacity :6f ::alr~ady degraded:
:: :land:s: must be deYeIonQd before the degradation beams irreversible
:
go. ~ ~ ~
:
Finally as the:~:world's population surges~past the~five billion mark.
.
,
:: :pQople are being forced onto lands unsuited for agriculture. These :incur
Lesions ~:usually result ~ :in the irreve~rsibie destruction: of :~ natural communities
: ~ :
~ followed by the short-term: ~ag:ricu~ltural:exploitation of the la:nd~ that supports
:: them.::~Bydesign:ingcommu~nitie~s~:pafterned~:diternatural:ecosyste:ms, it may :
Possible to devise land-use: schemes that are more sustainable and
: subsidy free while still maintaining an acceptable level of productivity. Such
~ ~ ,
agroecosystems should improve :human welfare and rsd~:uce the pressure on
natural commundiesthat harbor the earthts legacy of evolution.: :
~: ~: ~::: : Hi: ~: :
Chemical Ecology
Many Ecological Interactions Are Mediated Through Chemistry
All organisms are chemosensitive, and each is the source of substances to
which other organisms respond.- In the course of evolution, this potential for
interactions has been thoroughly exploited, and organisms of the most diverse
kinds have entered into chemical interdependencies, both mutualistic and antago-
nistic, that are central to the fabric of life itself. Chemical ecology focuses on
such interdependencies. It brings the molecular dimension to our understanding
of biological relations those between animal and plant, parasite and host, preda-
tor and prey; between the multicellular and the unicellular, the social and the
nonsocial, the kin and the nonkin. Chemical ecology deals with the chemical
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ECOLOGY AND ECOSYSTEMS
293
messengers of nature, defining their functions and ecological roles, and elucidat-
ing their chemistry. The discipline is thriving on many fronts, with biologists and
chemists joined in an exciting venture of exploration and discovery.
Progress in chemical ecology is being accelerated by recent technical innova-
tions in analytical chemistry. Vastly improved procedures have been developed
for separating complex mixtures into their individual components, as well as for
quantitating and chemically characterizing naturally occurring compounds. New
methods of structure determination both for small organic molecules and for
biological macromolecules have been developed, and the amount of sample
needed for analysis is constantly decreasing. Since most chemical substances of
signal value are produced and "broadcast" by organisms at low-sometimes
vanishingly low-concentrations, these refinements in analytical sensitivity and
efficiency have proved invaluable.
Although biologists have long recognized the ubiquity and fundamental
importance of chemical interactions, they have tended to underestimate the sub-
tlety of roles mediated by chemical ecological factors. Virtually every primary
activity of an organism, be it related to growth and development, food acquisition
and defense, or sex and reproduction, may be subject to regulation by chemical
factors produced either by the organism itself or by other living sources. Phero-
mones are the best known of these factors. Defined as intraspecific chemical
messengers, they have been most thoroughly studied in insects, in which they
regulate courtship; in social species they also regulate many of the basics of
communal life (foraging, kin and nest recognition, and caste determination).
Pheromones have proven useful in applied control programs, both for trapping of
pests and for monitoring their densities, and such use is likely to expand as our
knowledge of these substances increases.
Pheromones also play important roles in higher animals, including mammals.
In mice, for example, information on sex, state of male dominance, and degree of
genetic relatedness may all be conveyed by pheromonal cues. A male mouse may
even, through its sheer chemical presence, prevent implantation in a female of
eggs fertilized earlier during a mating with another male. Chemical induction of
infanticide by a male who by killing the offspring of another opens increased
reproductive possibilities for itself.
Biologists are only beginning to envision the full scope of functional possi-
bilities of pheromones. Courtship in insects, for example, as in animals generally,
involves more than the mere recognition of and attraction to the opposite sex. At
close range, males and females may subject one another to a process of appraisal,
in which specific fitness criteria are quantitatively assessed. In certain butterflies
and moths, the males transmit certain alkaloids to the females, which the males
initially sequester from plants. Receiving these toxic molecules, the females
transmit them to the eggs they lay and thus protect these eggs from their predators.
Prior to mating, the male provides the female with a measure of his intended
nuptial gift by releasing a pheromone that is biochemically derived, in quantita
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OPPORTUNITIES IN BIOLOGY
:~::
I::: :: :~:: : ::
H HUMAN PEE PHONES: ~ :: ::
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OWN ~"~orms~of~l~ife~,~:h~ad~always~met~with~co~nsid~erdblQ~s~kepticTism.~:~We~are~
~:~pri:mari:lly~visu~al~ ::and~ acoustic: ~in~ou~r~co:nvent~ns~,~:~the:~:argu~medt~ went, and
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~:~:~:~hiu:m~ans:~:~6f~::~extrao~in:a~:~::~olfadtory~::capabi~lit~les~::~and~:::~:of~ inte:r~tions~:~:~that~:~ar~:~::::~
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::~:mothe~rs:~ ~:~Discrim~i~natio~n~ t~e~st~s~w~ith~worn~::cl~ot~hi~ng~sho~wed~ Am oth~ers~to~ be~abl~e~:~:~:
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~:~:to fire - ~niz:e: t heir :c hi Worsen ~ By odor anc ~ :~aa~u t:s ::to c ltreren~t:'ate~: aetwe:en::t ~e:::: I:
Sexes And to :recog no Beth girl ~in~::iY:idu~bl~sex Al pa Hers.: ~ ~:Bat~h~sex~es~ can
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~:~:~identify~:~mal~es~an~d~fe~m~al~es~on~:~t~h~e basis of breath and ~pplm~:~odor.: ~:~
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~:~::~arm~secretio~ns.~:~ It~:~h~ad~long ~bee~n~known::~for::exam~pl~ thdt~women~::::who~wo~dc~:~::
~ :: our ~ live ~ together: tend: to ~sy:~n~c~h~ron~ize:~the~i:r:~ ~m:e~nst~ru~al:~cycles:.~:~The~:~:eff0t is::::
:~ ~:~:~chem~ical:~:~::~Ext:racts~::~f~rom~:~::the~:~:unde~:rarm~s::~:~:of::~::wom~en ~a,oplied~::to t:h:e:~pper:::~l~ip~ of :~:::~:~
:: : : :: : : ~ :: : ::
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: ~ : ~ : ~ : ~ : ~ ~ : M : a l ~ Q ~ ~ ~ ~ ~ ~ ~ u n d e ~ : r a r m ~ : : : e x t r a c t : ~ : : ~ m ~ ~ a y ~ ~ ~ ~ i n d ~ : u c e t h e ~ : : ~ : e f f e ~ t : : ~ ~ : ~ ~ ~ ~ ~ ~ ~ ~ A p p l : i c a t ; o : n o f t h : e : ~ : : ~ t r a c t : t o : : ~ ~ ~ ~ ~ ~ ~ ~ ~ : : ~ :
::: ::: :~: ::: I: :: :~: ::::::: ~ ~ ::: ~ it: ::: I: : ::::::: ~ ::: .~ I: ~ :~ ~ :: ~ ::::::: ~ I: :::~: :: ~ ~ ~::~:: :~ I: ~ I:: ~ I: ~ : ::: ~ :: ~ ~ :: ~:~ In: : ~ I: I:
th:e~upper: up of::wo:m:en with: abnormal cycl:es~:~and no::~:curre:nt: sexu:al:~:~:relat~on-~:~::::~:::
~: :~ ship normalized ~the~:~rhyt~h~m.~ Neither ~ the ::chem~ist:~of ~:~:~t:h:ese:~ph~eromon~al~:~ ~:~:~
::::::::: factors:: nor ~t:he;:r:~mod~e~:: of :~action~h~:at :~is,~::::wh~ether~;they~:~are~ ::~i:nhaled ~or: :topi-:
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live proportion, from the alkaloid. Males of high pheromonal titer are selectively
favored by the females.
Plants likewise use the chemicals they produce for a variety of ecological
purposes. For example, allelopathic phenomena, involving growth inhibition of
plants by chemicals released into the soil by nonspecific or heterospecific neigh
bors, have long been of interest to ecologists and chemists alike. Most of the
"unusual" molecules that plants produce, however, are used to deter herbivores or
disease-causing agents.
Much remains to be learned also about chemical interactions in aquatic
organisms. A vast array of natural products has been isolated from marine
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ECOLOGY AND ECOSYSTEMS
295
organisms, but the function of most of these compounds, aside from those that
play a defensive role as toxins or feeding deterrents, remains unknown. Here, too,
subtle actions are bound to be uncovered. Some rotifers, for example, grow large
protruding spines only when certain of their predators, other rotifers, are present
in their ponds at high densities. The spines are defensive, and their outgrowth is
prompted, as if by cross-specif~c embryonic induction, by chemicals emanating
from the predators themselves.
The prospect of eventual characterization of human pheromones is especially
intriguing. Recent studies have shown pheromonal factors to be at play in a
variety of human interactive behaviors, but in no case have mediating chemicals
been isolated or identified. The characterization of human pheromones is likely
to pose special problems since the substances tend to occur as complex mixtures,
subject to both individual variability and variability over time.
Natural products, the very substances that are the subject of chemical eco-
logical studies, have proven invaluable to humankind. They constitute the treas-
ure trove from which most compounds of technical or medicinal use have been
derived, yet the treasury has only begun to be explored. Relatively few kinds of
organisms-certainly fewer than 1 in 25-have been examined chemically at all,
and thousands of kinds of new compounds, some of them completely unexpected,
await discovery. To find the full array of chemicals that exist in nature, however,
chemical exploration must be greatly accelerated owing to the quickening pace of
extinction throughout the world. The practical potentialities of this area likewise
provide another reason to emphasize conservation both in nature and in stock
centers.
Behavioral Ecology
Behavioral Ecology Is a Growing Field
Behavior is the study of how animals sense, react to, and manipulate their
social and ecological environments. The modern science of behavior arose from
the marriage of comparative psychology and ethology, with some admixture of
sensory and motor physiology and endocrinology. In the past two decades, the
focus has shifted to research on the adaptive basis of complex individual and
social behavior and has led to the growth of sociobiology, which seeks to under-
stand the evolution of behavior in its social and environmental context.
Over the past decade, the major topics of research in behavioral ecology have
included (1) studies of communication, with an increasing emphasis on chemical
communication; (2) foraging behavior, focusing on habitat selection, movement
patterns, and prey choices in a patchy resource environment; (3) sexual behavior,
especially the evolution of behavioral traits under sexual selection; (4) the roles of
the sexes in an ecological and evolutionary context; (5) the ways in which
conflicts of interest between organisms are resolved ontogenetically and phylo
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OPPORTUNITIES IN BIOLOGY
genetically; (6) kinship studies, focusing on recognition of and behavioral differ-
ences toward relatives and nonrelatives; and (7) learning, studied as an adaptation
in an ecological context. Other important topics have included parental care,
predator-prey interactions and strategies, the ecological and social context of
aggressive behavior, and the behavior of social insects.
Significant Progress Has Been Made in Understanding the Neural Basis of
Simple Motor Patterns and Reflexes in a Number of Animals
These include insect walking and flight behavior, attack behavior in the
octopus, the swimming behavior of leeches and sea slugs, and optomotor re-
sponses in horseshoe crabs. The search for neural mechanisms to account for
more complex behavior has been slow; consequently, many neurobiologists have
turned to more tractable research questions, such as those in molecular and
developmental neurobiology. Of considerable interest is the interface between
sensory physiology and the study of animal communication; in this area, new
technical means are now open for understanding the structure and action of
chemical communication signals, such as pheromones. Collaborations are also
developing between physiologists interested in metabolism, energy regulation,
and water use and behaviorists studying behavioral energetics such as behavioral
thermal and water regulation and the energetic costs of reproduction. There will
also be new research to tie the physiology of digestion, nutrition, and detoxifica-
tion to foraging behavior in relation to dietary requirements and secondary plant
chemistry. Very little is known about how animals satisfy their nutritional needs
while minimizing intake of the toxic substances that are so abundant in many of
their natural food sources.
The Adaptive Basis of Behavior in Habitat Selection by Animals Is a Growing
Area of Research
Life history and population growth vary with habitat, so the behavioral basis
of habitat selection can have a profound effect on population processes. At one
level, for example, behavioral physiologists have known for years that animals
prefer particular temperatures and will seek out these temperatures on thermal
gradients in the laboratory. In the field, behavioral thermoregulation has been
demonstrated many times. For example, the body temperatures of day-active
desert ground squirrels actively cycle. The squirrels forage above ground to cool
down. Behavioral physiologists have rarely considered the longer term fitness
consequences of habitat selection, however. It has not been demonstrated that
animals can optimize their thermal environments, given those available, in the
sense of choosing those which maximize growth, survival, and reproduction.
Indeed, research has been lacking on almost all aspects of behavioral habitat
selection, particularly those involving complex biotic factors rather than simple
abiotic ones.
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Even though habitat selection is presumed to be adaptive, the behavioral
decisions involved are often complex and indirect; many challenging questions
remain. For example, an animal may not tee able to meet all of its requirements, or
at least achieve its optimal performance, in one habitat at one time. When
demands on time and energy reserves conflict, behavioral priorities must be set to
aid in habitat selection.
Theoretical Studies Have Led to Insights into Foraging Behavior
Until the 1970s, the behavior of animals foraging for food was poorly
understood; foraging patterns seemed complex and unpredictable. Then a series
of theoretical insights brought order to chaos. It turns out that foraging rules are
governed by simple principles, one of the most important of which is that of
maximizing food harvest per unit time. However, food is not uniformly distrib-
uted in the habitat, animals usually do not have prior knowledge of where it will
be found, and they have to search for it. Models have been developed that predict
how animals should search, given their sensory capabilities and the distribution of
food in the environment, to maximize their food intake per unit time. These
predictions are testable, and the best models are remarkably accurate.
Diet choice is another part of behavior predicted by foraging theory models,
and it is the aspect of such models that has been tested most thoroughly. The
application of foraging theory to a diverse set of organisms has made possible the
emergence of a general theory of foraging, which applies to organisms as differ-
ent as bumblebees and moose and to processes as diverse as growth patterns in
plants "foraging" for light and the sexual behavior of males "foraging" for mates.
Recent work on parental house wrens foraging for food for their nestlings
under risk of predation illustrates the promise of behavioral hierarchy studies in
the context of habitat selection and life-history research. In the absence of
predators, parental birds forage until a large prey item (insect) above a critical size
is found before returning with it to the nest. Preferred large insects are rarer than
small insects, take longer to find, and are generally farther from the nest and in
different habitats, so foraging trips and time away from the nest are relatively
long. When a natural potential predator of nestlings such as a snake is experimen-
tally placed in a visible location near the nest box, however, the birds make much
shorter foraging trips, return frequently to the nest, and spend considerable time
watching or attacking the snake. Parental birds have successfully driven off snake
predators on several occasions. When parents make short foraging trips in the
presence of the snake, the average prey size returned to the nest is smaller and the
total amount of nestling food collected per unit time is lower. Nestlings have
high, constant demands for food, so a reduced feeding rate is a real threat to their
growth and survival. If the parents devote all of their time to fighting predators,
the young will be deprived of their essential food. But if they ignore the predators
and continue foraging for large insects, the predators might eat the nestlings. The
parental behavior is a compromise between these two undesirable results.
/
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OPPORTUNITIES IN BIOLOGY
animals, but also in organisms as varied as threatened species of primates and
commercially important tropical timber trees. In plants, seed banks and tissue
culture centers are promising aids to preservation, and analogous techniques are
available for animals also.
Research in Ecosystem Restoration and Regeneration Will Yield Many
Observations Useful in the Development of Ecological Understanding
Over the next several decades, conservation biology will focus increasing
attention on the restoration of degraded or destroyed ecosystems. This effort is
important for two reasons. First, restored ecosystems are likely to be superior to
zoos and arboretums for the maintenance of viable populations of endangered
species. Second, regenerated ecosystems will reduce the pressures to exploit
nature reserves when other wildlands have disappeared. One of the highest
priorities for conservation biology, for example, is reforestation in tropical coun-
tries with serious deforestation problems. It is difficult to protect, let alone to
justify, nature reserves in third-world tropical countries when people have neither
timber nor firewood. However, reforestation in the tropics is not a simple matter
either technically or socioeconomically.
Solving these problems will require a major increase in research on ecosys-
tem restoration, particularly in ecosystems of critical worldwide importance. In
the case of tropical reforestation, for example, there must be comprehensive
investigations of the properties of various trees that might be used for reforesta-
tion and the factors that control the establishment of these species in degraded
lands. Research in ecosystem restoration will yield a great many observations
useful in the development of ecological theory as a whole.
Species Invasions can Result From Environmental Change and Can Alter
Ecosystems and Even Exterminate Species
The consequences of species invasion are of current interest because of their
relevance to biological control and because of the analogies that have been made
between this phenomenon and the fundamentally different one concerning the
deliberate release of genetically altered organisms. New disease-causing organ-
isms are being introduced more widely than ever before-some deliberately (as
for biological control~and understanding the properties of such introductions is
becoming increasingly important. Where ecological information has been lack-
ing or has received inadequate attention, even deliberate introductions have
sometimes led to major ecological problems. A major challenge to ecology,
therefore, is the understanding of the characteristics of species and environments
contributing to invasiveness, and to likely consequences.
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313
Global Climate Change
Cumulative Environmental Pollution Can Have Surprising Effects and Can
Lead to Global Change
Equally important problems are associated with the cumulative effects of
environmental pollution. Not only are environmental perturbations often re-
peated, but also their combined effects may be more substantial than those of the
individual events taken separately; sometimes they are qualitatively different.
Consequently, traditional methods of assessing the significance of particular
actions on a project-by-project basis may fail to predict and, therefore, fail to help
us manage these cumulative effect~comprehensive efforts may be necessary.
As we burn fossil fuel, manufacture plastics and other synthetic products,
generate electricity, or package consumer goods, we perturb the environment both
physically and chemically. The effects of these pollutants are cumulative and
usually cross jurisdictional boundaries. The burning of fossil fuel in Ohio can
affect lakes in Quebec; the building of dams in Idaho and in Egypt can affect
fisheries in Oregon and in the Mediterranean; the use of fertilizers in Maryland
and Virginia can affect fisheries as far distant as Nova Scotia
The problems caused by pollutants released into water and air, or widely into
the terrestrial environment, are political as well as scientific. Although it may be
economically advantageous for one country not to limit the emission of sulfur
dioxide from industrial plants, it may be highly disadvantageous to nations that lie
in the path of the pollution. A river may be a convenient dumping ground for
chemical wastes, removing them from the area where they are produced; but those
wastes may cause considerable economic loss downstream. At a scientific level,
we do not yet understand ecosystems' capacities for recovery, for detoxification,
or for resisting various kinds of pollution, and the interactions of environmental
pollutants are not fully understood, either. Many problems associated with
chemical and physical stresses have not yet even been clearly defined. Nor do we
yet understand how best to manage environmental problems that cut across
jurisdictional boundaries, although the international agreement on protecting
ozone in the stratosphere, developed under the auspices of the United Nations
Environmental Program and signed in 1987, appears to offer a good model.
The concentration of carbon dioxide in the atmosphere, for example, changes
globally as a function of time scale. Annual cycles are controlled by seasonal
changes in net carbon uptake and release by biota, whereas a sustained historical
increase is caused by the cumulative effects of increasing fossil fuel combustion
and deforestation. Even though most fossil fuel is burned in the northern hemi-
sphere, the increase in atmospheric carbon dioxide is global. Large variations are
associated with glacial advances and deglaciation. These well-documented changes
will undoubtedly stimulate additional efforts to study their effects on climate and
on biological processes.
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OPPORTUNITIES IN BIOLOGY
Altered concentrations of carbon dioxide are the best known but far from the
only major anthropogenic change in global geochemistry. Other trace gases such
as nitrous oxide and methane are increasing globally, acid precipitation has
increased regionally, and upper stratospheric ozone levels have decreased even
while ozone is increasing in the troposphere locally. Together, spectrally active
("greenhouse") gases other than carbon dioxide are now thought to have an
impact equal to that of carbon dioxide on the earth's heat budget. However,
natural climatic variability makes it difficult to know just how large the actual
climatic effects are.
Ozone and acid deposition are affecting forests and aquatic ecosystems over
much of North America and Europe; such changes may become global as the
human population increases and more industrial development occurs in tropical
regions. Once again, however, it is difficult in many (but not all) cases to separate
the effects of natural environmental fluctuations from those of ozone and acid
deposition.
TECHNOLOGICAL AND METHODOLOGICAL ADVANCES
Remote Sensing
Evaluation of Global Change Requires the Ability to Document It and to Study
Patterns at Fine Scales by Remote Techniques
Remote sensing is not a new technique in ecology interpretation of aerial
photographs has been a valuable part of ecological studies for at least 50 years.
The scope and quality of remote sensing has changed dramatically in the last 10
years, however, and we have every reason to believe that its ecological applica-
tions are far from reaching their potential.
Many techniques are now available or under development. The best-known
of these include the coastal-zone color scanner, which has revolutionized the
study of marine primary production because it permits measurements of chloro-
phyll concentrations over wide regions; and the advanced very high resolution
radiometer, used to measure light absorption by leaves on a daily, seasonal, or
annual basis worldwide. The radiometer "sees" a pixel 1 kilometer or 6 km
square; it is therefore ideal for continental-scale studies. Other sensors, such as
the U.S. thematic mapper or the European SPOT, sample much smaller pixels (30
m square for the mapper, 15 m square for SPOT); they are more useful for
regional studies.
Other remote-sensing techniques are less well developed but perhaps poten-
tially even more useful to ecologists. Synthetic-aperture radar can "see" both the
top of a forest canopy and (under some conditions) below the soil surface to
bedrock as well as the ground surface itself, and it can do so at night or through
clouds. Laser profilers can measure canopy height and the presence of treefall
gaps in intact rain forests; fluorescence measurements may further provide infor
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ECOLOGY AND ECOSYSIE:MS
315
mation on chemical composition or physiological status. Airborne sensors with
very high spatial and spectral resolution may be especially useful to ecologists;
the airborne imaging spectrometer, with 10-nanometer spectral resolution and 10
m square pixel size, is now being used in geobotanical and ecosystem studies that
require more determinations of the chemistry of forest canopies. Many of these
technologies are still being developed; when fully deployed, they should greatly
enhance our capabilities in these critical areas.
Analytic Chemistry
The widespread use of high-pressure liquid chromatography and gas chroma-
tography-mass spectrometry in environmental laboratories has made possible the
measurement of chemical substances at concentrations much lower than could be
measured a decade ago. Ecologists concerned with air chemistry or trace pollut-
ants can reasonably expect to detect parts per trillion. Improvements in automa-
tion and the capacity to analyze multiple samples have helped fulfill the need for
adequate replication of samples.
Nondestructive analyses can allow measurements to be made that reflect the
spatial organization of chemical constituents and their concentrations under con-
ditions in which they are active physiologically. Nuclear magnetic resonance is
useful to ecologists studying ecophysiology and water chemistry; electron micro-
probes have provided detailed information on soil chemistry in the vicinity of
plant roots; and microelectrodes measure dissolved oxygen and other chemicals
within living cells or in soil.
Remote chemical measurements represent a special case of nondestructive
analysis. Tunable diode lasers can be used to measure the distribution of ozone
and aerosols at a range of kilometers or in the neighborhood of an individual leaf.
Laser fluorescence can be used to measure the photosynthetic potential of individ-
ual leaves. These and other laser-based technologies will be applied more widely
to ecological studies in the future, with consequent gains in insight about natural
phenomena
Tools for Studying Paleoecology
For systems at equilibrium, the time dimension is relatively unimportant;
consequently ecologists sometimes ignore history in deciphering patterns that can
be seen in contemporary biotic systems. Paleoecologists have brought a time
dimension to the study of ecology; as the interest in equilibria! systems wanes,
ecologists are becoming more receptive to including time among the factors that
they routinely take into account. The development of adequate quantitative
techniques has allowed critical comparisons to be made between living and fossil
communities, providing increased insight into both areas of study. In addition,
the ability to make absolute determinations of past time by means of radioisotope
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OPPORTUNTTlES IN BIOLOGY
determinations has been invaluable. The accurate reconstruction of paleoclimates
millions of years into the past has become possible, and the first results are
biologically exciting.
Stable Isotopes
The ratios of stable isotopes of particular chemical elements are now deter-
mined readily by the use of mass spectrometers. Already such studies have
advanced our understanding of physiological processes and fluxes through eco-
logical systems. Studies of plant and animal physiological ecology, food webs,
historical ecology, marine ecology, and biogeochemical cycling have all bene-
fited from this approach.
Virtually all elements have at least two stable isotopes, with one of them
being far more common than the others. For example, the two stable isotopes of
hydrogen-OH and 2H (deuterium) occur at abundances of 99.9844 and 0.0156
percent, respectively. Similarly, there are two stable forms of carbon ~2C (9X.89
percent) and ]3C (1.1 1 percent). Because of differences in physical properties and
in enzyme-based discrimination for or against one of the alternative forms in these
systems, natural differences in the stable isotopic composition of biotic and
abiotic compounds occur in ecologically relevant processes, including metabolic
activities and transfer rates between organisms at different trophic levels.
In plant physiological ecology, the use of stable isotopes provides a reliable
means of scaling up from instantaneous metabolic rates to longer term estimates
of physiological activity. Thus carbon isotope ratios provide information on
water-use efficiencies, hydrogen isotope ratios on water sources, and nitrogen
isotope ratios on nitrogen-fixation rates.
Carbon-isotope ratios in plants can be used to distinguish among different
photosynthetic pathways. These studies have allowed an extensive evaluation of
how plants function in different ecological situations. The ratio of carbon iso-
topes in a particular plant reflects both enzymatic and diffusion considerations.
The initial photosynthetic reaction by the enzyme ribulose 1,5 big-phosphate
carboxylase discriminates against i3C, and 13C diffusion is slower though stomata,
the specialized openings on leaves. As the stomata open, thus allowing greater
diffusion of carbon dioxide into the leaf for photosynthesis, water loss (transpira-
tion) increases. Consequently, water-use efficiency (the ratio of photosynthesis to
transpiration) is strongly correlated with the carbon isotope ratio in tissues.
Such studies are likewise important in animal ecology. Free-ranging animals
from rodents to penguins can be injected with doubly labeled water (water
enriched both with 2H and DO), and then released to resume normal activities in
the field. The deuterium (2H) leaves the animal only when the animal loses water,
but the oxygen is lost both in the water and through respiration as carbon dioxide;
the oxygen in carbon dioxide and water comes into equilibrium through the
enzyme carbonic anhydrase. The rate of loss of the two labeled isotopes thus
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ECOLOGY A1VD ECOSYSTEMS
317
constitutes an integrated measure both of water loss and of metabolic rates for that
animal important features in understanding how an animal functions.
The old adage "you are what you eat" holds for stable isotopes, which are
therefore valuable for studies of food webs. Many single and multiple mixtures of
stable isotopes are used to study plant-herbivore interactions as well as relations
between animals and other organisms further up the food chain. In a recent study,
for example, stable isotopes of carbon, nitrogen, and sulfur were used to trace the
flow of organic material within a salt marsh ecosystem and to identify the origins
of the detrital substances that were accumulated by He mussels downstream.
Organisms carry some of their history in the isotopic composition of the
structures that they form over time (tree rings in pants, scales in fishes, and shell
layers in mollusks). The study of these structures can often yield accurate
information about past environmental conditions and about the diets of particular
kinds of animals in the past.
In whales, for example, the baleen plates (planlcton-f~tering structures) are
formed continuously and reflect the different isotopic compositions of plankton
communities in the areas visited by the whales during the course of their migra-
tion. Investigators have used this information to trace whale migrations, an
ingenious application of isotope ratios to an ecological problem. By adding ~4C
dating techniques, these investigators were even able to determine the length of
time that the whales spent in different areas.
Biotechnology
Modem molecular techniques promise to revolutionize the study of microbial
ecology by making it possible to follow the fate of particular genetically engi-
neered bacteria and other microorganisms in the environment. These techniques
will likewise enable us to determine the rate of recombination in populations of
bacteria much more accurately than we have been able to do previously and will
enhance studies of ecological genetics, including those of symbiotic interactions.
Our ability to produce precisely engineered bacteria will make it possible to study
the adaptive significance of single-gene mutations in nature and under experimen-
tal conditions. In principle, such modifications are likewise possible in eukar-
yotic organisms, and they will eventually allow ecological experiments to be
carried out with greater precision in such organisms also.
Models in Ecology
Ecological phenomena consist of processes that take place at different rates.
Understanding and predicting such phenomena are facilitated by mathematical
methods, which illuminate the relative importance and quantitative characteristics
of these processes.
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318
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OPPORTUNITIES IN BIOLOGY
Two philosophies of modeling stand in opposition. In the first, one seeks
highly detailed descriptive models, intended for implementation on large comput-
ers. These models contain much of the fine structure of ecosystems, but present a
number of difficulties: Their specificity hinders their portability to other systems;
their large number of parameters presents statistical difficulties in estimation; and
their reliability as predictive tools is questionable because of the many ways
errors can arise and propagate.
At the other extreme, overly simplified models may be general and portable,
but submerge much of what is relevant to the mechanisms underlying their
dynamics. They tend to be phenomenological and holistic and to ignore many
important factors. Both types of models can be useful in guiding research, but
dangers lie in believing the output of either type of model without adequate field
testing.
For understanding and managing the environment, a compromise is neces-
sary. No single model suffices, and one needs a combination of models at
different levels of detail, much as one might use a nested set of maps to drive to a
new location: The broad-scale map, ignoring much detail, is necessary for getting
one's bearings and reaching the vicinity of the destination; a more detailed map,
limited in scale and objectives, allows one to find one's way past the bridges and
old barns that dot the landscape and to reach the final goal.
A case in point is the use of fate and transport models to evaluate the
distribution of chemicals in the environment. These models come in two forms:
generic and site-specific. Generic models incorporate the basic mechanisms of
diffusion, advection, and reaction; parameters are assigned phenomenologically
and over broad scales. For near-field effects (for air pollutants near smokestacks
or for chemicals in particular estuaries), detailed descriptions of local geometries
and topographies become important. In such cases, one must turn to the computer
for implementation. Models of both forms are essential for a comprehensive
appreciation of the phenomena involved.
The roots of mathematical ecology can be traced to the demographic studies
of Graunt and others as early as the seventeenth century. The well-known
arguments of Malthus, which indicated that a 'population growing without bound
would soon outstrip the capacity of the environment to support it, were based on
detailed analyses of births and deaths in human populations. In turn, these led to
the first important mathematical efforts in ecology: namely, those of Verhulst and
others to describe the dependence of population growth rate on population size
and to infer the consequences of such relations. The most important extensions of
these single-species models in the classical literature were to systems of interact-
ing populations, especially the famous differential equation derived independ-
ently by Lotka and Volterra. These equations predict the outcomes of situations
in which two or more species compete for the same limited resources.
The classical tradition has been carried forth to the present, especially regard-
ng the development of an elegant theory of interspecific interactions and evolu
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ECOLOGY AND ECOSYSTEMS
319
tionary relationships. Recent work has extended the theory to an examination of
the structure of ecological food webs and the factors governing their organization
and has taken the subject into diverse mathematical disciplines such as graph
theory and dynamical systems theory.
Increasingly, however, mathematical approaches have been recognized as
being valuable not only for theoretical investigations, but also for finding solu-
tions to some of the applied problems that society must confront. Thus, for
example, mathematical descriptions of population dispersal, which are among the
oldest models in theoretical ecology, are now being used to quantify the move-
ments of agricultural pest species and the rates of advance of exotic species
invading new habitats. Further, the theory of island biogeography, which predicts
the relation between area and number of species, and more extensive mathemati-
cal models of spatially distributed populations are being applied to the design of
reserves, in the planning of parks, and in regional landscape ecology.
Optimization and control theory, which have undergone substantial mathe-
matical development in recent years, are being applied as integral parts of pro-
grams for the management of renewable resources, especially in fisheries, for-
estry, and agriculture. Such approaches combine biology and economics through
mathematical models; this combination will become an increasing imperative for
us as we face energy shortages and resource depletion. Inherent limitations to
predictability are apparent in any rigorous mathematical analysis and have made
essential the development of adaptive management strategies, which couple short-
term prediction with continuously adjusted management rules.
Epidemiology has had a few mathematical basis since the turn of the century.
The impact of epidemiological models has been limited, although the problems
that we face in combating the spread of diseases in humans and other animals and
in plants are of overwhelming importance. Recent years have seen a dramatic
increase in the mathematical modeling of epidemics and an increasing recognition
of the need to view such problems in their proper ecological context as host-
parasite interactions. It seems likely that epidemiology will be a most important
area of growth in mathematical ecology over the next quarter-cen~ry. Current
work uses mathematical models to help to understand the factors underlying
disease outbreaks and to develop methods for control, such as vaccination strate-
g~es.
Finally, the need for environmental protection in the face of threats from such
competing stresses as toxic substances, acid precipitation, and the generation of
power has led to the development of increasingly sophisticated models that
address the stress-related responses of community and ecosystem characteristics;
for example, succession, productivity, and nutrient cycling. Such models owe
much to their classical origins, but typically differ substantially in form. They
recognize the importance of explicitly considering environmental factors, the
physical characteristics of the environment, and nonbiotic system components,
and they focus attention on holistic measures of system response. Examples
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OPPORTUNITI~ IN BIOLOGY
~ ~ ~ : :::NONEQI~JILIB~RIAL::~SYSTEMS~: IN~ ~ECOLOGY ~: ~ ~ ~
The ~tradition of ~:~co~nsider:i~n~g ~eq~uilibri:al ~ ~stem~s~ in: theo~retical~ ~ology::: i:s ~:
E~ ~ ~ ~ ~ :: : ~ ~
::changing.~:: :arly mathemat~cal :~models:~ of: ~:~:ecological:~:~systems ~:~dealt wit:h~ ~::~:
~: :~ ~ ~ ~ ~:~ ~ ~ : ~ : ~ ::: ~ ::~: ~ ~: : :~ ~ . ~ ~ :. :~: : ~ . : : :::: ~ ~: : ~: :~ :~ :~ ~: ::: ~ :: ~: :~
~ syst~ems~:~at::~o;r near~equi:librium.~::Thus, the IOglStlG ~equatio n relates::popula-:::~:
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ T~
t lon~:growth ~to:: ''ecolog:lcal:~resistance~n: or the ~population's~:size~ and:::~en~lro~n-~ ~::
:::~ ~mental ~carryin~g ca~pac~ity. ~It~a~llow~s th~e calcul~atio~n :~the~ numbers ~of organ-~
:~:::~::~:~is~ms an;~enYironm:ent: can~ s:u~pport~:at :equilibr~um ~ or:~th:~e ~g:ro~h~ rate ~of~:th~e :~ ~:
:,
. ~ .
~ :: ::popu abo~n:~as~ It~approac ~es t 1:a t eq:uilib:riu:m::. ::Simi:l:a~rly:: mode:l:s~:::of ~com~m:u:-~: ~:~ ::
~ ~ ~ ~ ·
~: ~:nity~st:ruc~ure:~h~ave~tradition::ally atte~mpted~ to :~esti~mate:theln:~`mbe~r of ~species ~:1~:
that~:could~:~occ~u~py~::an ~environ:merit~l~at some :~sq~u~ili:brial :divers:Ry.:~ :1 :Alt~hough~: ~1
~th~es~e: and~other ~equilibrial~mo.deis have been useful for~g~ui~ding thinkin~g an~d
::::: focus~i~ng: re~se~arck-indeed~,~:~ma:ny~ of ~the~m:~w~e~re~d~esigned~ for~that purpose~ ~ :~
~ ~ . . ~ ~ ~ ~ ~ ~ ~ :: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~: ~
:: ~t leY laYe:::too o te n O:e:e n Int:eroretec as d:e:scrintions~:::of~:~the n at:ural world~.~::~:~:~:~::::::: ~
: ~ :
:
: ::
~:~:~ ~i: ~ :But bOth~:th:e biological: and th~e~:::~n:onbiological world: are constantl:y :~ang~-::~::~::
: ~ : i n g ~ : . : ~ : : ~ S ~ ~ e s s ~ i o a : : : o c ~ u r s ~ i ~ n ~ ~ : c o m m u ~ n ~ t : i e s ~ ; ~ p o p ~ u l a t i o n s o f p l a n t s , : a n i m a l s , : ~ ~ a n d ~ ~ : ~ : ~ ~ ~ ~ : ~ :
~: ~ ~microorga~n~ism~s~levo~lve, ~and~ QMl~9iC8l~ ~6ffi~ciencies~of~organisms~ ~and oom
~:~: ~mu~nitie~s :change. ~:~:~: ~:~: ~: :~ ~ ~ ~ ~ ~ :~ ~i~::~ ~ ~:~ ~ : ~: ~: ~ ~:::: : ~ ~ ~ ::: ::: ~: : ~ ~ ~::~: ::~: ~ ~ :~:: ~::::~ : ~: ~ ~:
: ::
:: ::
Non~biological changes~ occ:ur:::~ :i:n bot:h~:: space ~ and:~ :ti:me.: I ~:Experim~ents~: ~::: i~
:~:haYe: shown : lO:W ~the~: :spatlal com~p exlty~: o~::::t ,~e 8:nviro:nment Tor~ :examp :e~:
~ ,
:~:p~re:sence~or::~abs:ence:~:of~refuges~;~or:~of d~iffere::nt: h~abitats) can:::a~fect:po:puiation::~
gro~rth:ort~he~coe~x:iste:nce~:of:`o~or~mote species.: And~the~d~gree~:to~:~which::
t 1e ~ envir~nme~nt varie~s~=er time~ i:s: sti oel~ng ;c Iscover~a~.: ~: ~is~exciting;~
process :or alscovery is alded::on: the on::e::::h~and by::satellites :and~ comp:uters
~:t ~at can::c ::et~ect~su at ::e~:p ~yslca :e ,~anges ove:r s ~ort ~peno{ s an:c :on t ~e ot 1:er:
nana Dy ~a:nalysls o~ nlst:orlcal recoras, ootn:pnysleal ~su:cn as Ice :cores' an~:a
wrffle:n:l~(such~::~:as:~:::the historica I :position::::::of g:lacieirs, ~weather~:::reco~s~: :and~
histo~rical records of agriculture3. These~::methods:::are revealing ihe~ complex~
ana cnang~ng patterns ot var~ao~'ty~ ~n atm:ospheric, terrestrial, ano ~oceano-
: grapn~c cl~mate, ~wh~ch ~n turn or've cnang:es: ~n o~o~og~cat systems.~: ::: :::
:
I~ . · , ~ ~ , ,
~ ~ n a~a~t;~on to tnese: onYs~ca' cnannes are tne: new'~ a~soovereo cnanges:
: that:: sometimes: ::occur i:n It:he ::mathe;~m:aties of ~ population biOIogye :~About ~ O ~:::
~:: ~ years~:~ago, biologists were~:~m~ad:e aware t;hat some~:~s:eeming~ly: s:im;ple~ eq:ua-:
:: ~:: tions: :that :describe:: :populatio:n ::::g:rowth start to behave chaot:ically for: so:me: ~
::: values of their: principal :paramet:e:rs. The: study of the:~:chaotic be:havior of~: ~::::
: ~fami:liar eq~uatio:ns has~ ::become a~::~m:inor :growth~:: :industry~ in m:ath:ematical ~: ~ ~
scien:ces, but: :it:::~:has roots~ in ~ph~ysical: reality:::: ~I~:n::::~::a variety: of ~discipli:nes~,:: ~:
incl~'di:ng: bio!ogy:,: real sy:ste:ms exh:ibR: :chaoti:c: ~:be:havior. :: :: For: all~:these
: ~ ~reasons, increas:ing :attention::~:::i~s ~ being~ ::paid~ to~ ~ nonequilibri:a I systems ~in
:ecology.: Althoughha:rdertotreat~theoreticaily,they::~appearto~::be:::common
: : : ::
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~ OLOGY AND EiCOSYSIEMS
321
include large-scale models being developed to examine the likely effects of power
plants on estuarine communities or of air pollution on regional patterns of forest
productivity.
A major difference between such models and those discussed earlier is that
they are in general too large to be analyzed fully; one must rely heavily on com-
puters to simulate possible outcomes. In such applications, however, mathemati-
cal analyses are as essential as ever since one must find ways to simplify, to guide
simulations, and to derive understanding. Clearly, our applied needs will con-
tinue to increase and to represent new and vital challenges for mathematical
ecology.
In ecological modeling, the observed degree of variability changes as a
function of the spatial and temporal scales of observation as one moves to finer
and finer scales. Thus the concept of equilibrium is inseparable from that of scale.
The insights from any investigation are therefore contingent on the choice of
scale, and there is no single correct scale of observation.
In many efforts to model particular ecological situations, irrelevant details
are introduced on the mistaken premise that somehow more detail assures greater
truth. In fact, there can be no one "correct" level of aggregation for a given study.
If the taxonomic species, for example, is used as the unit of classification, the
differences among the individuals within it are automatically ignored. In ecology,
functional systems of classification are often preferable to taxonomic ones, and a
failure to recognize this relationship has led to difficulties.
CONCLUSION
Research in Ecology Is Brought into Focus by Practical Applications and Needs
These are exciting times for ecological science. The accumulation and
organization of experiences from well-crafted experiments and from accidents of
nature are providing opportunities to derive basic principles and to formulate
concepts. The analytical tools of the science have also seen rapid advances, and
powerful computers provide us with limitless new opportunities. Finally, the
blurring of the distinction between what is pure and what is applied, necessitated
to some extent by environmental crises, will enrich and inspire basic research.
Perhaps the greatest challenge facing us will be in understanding how "physi-
cal, chemical, and biological processes that regulate the total Earth system, the
unique environment it provides for life, the changes that are occurring in that
system, and the manner by which these changes are influenced by human ac-
tions." These words are taken from the objectives of the International Geosphere-
Biosphere Program, which has undertaken a long-term study of these problems.
Atmospheric processes affect and are affected by biological processes in the
earth's ecosystems, and we must improve our understanding of these interrela-
tions. To do so, we must find ways to study the dynamics of ecosystems
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OPPORTUNITIES IN BIOLOGY
simultaneously on different scales-including landscapes, regions, and conti-
nents-and analogous large-scale phenomena in the oceans.
The crisis in biodiversity also necessitates holistic ecological approaches to
proWem-solving on broad scales. That the conservation of species cannot be
separated from the preservation of their habitats has given birth to new ap-
proaches to ecosystem restoration and rehabilitation. Tropical forests are among
the most severely affected, and they have deservedly received tremendous atten-
tion. But the problems are generic ones Hat affect all our ecosystems.
Biotechnology has been the source of a new set of challenges for ecologists,
who have had to view it in terms of both its tremendous potential and its risks.
Genetic engineering holds the promise of increasing crop yields, of providing
nonpolluting alternatives to chemical fertilizers and pesticides, and of breaking
down pollutants that already exist in the environment. Because of a basic lack of
information about microbial ecology, however, and a lack of familiarity win the
methods now being used to alter the properties of organisms, ecologists have
moved cautiously in capitalizing on these opportunities despite their obvious
value to society. The coming years should see the resolution of these problems
and the widespread application of new techniques for scientific advance and
human benefit.
Representative terms from entire chapter:
habitat selection
