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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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290 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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292 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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294 OPPORTUNITIES IN BIOLOGY :~:: I::: :: :~:: : :: H HUMAN PEE PHONES: ~ :: :: ~ ,, ~ . ~ :: ~ _ ~ ~ ~ ~ . ~ ~ ~:~:;~:~:::~ :~A~n~i~m:a:l ph~eromon~e<:s :s:u::rely but:~:~:~human:~phero:mones .7 ~:~::~:::~1~ne:~not~lo:n~ that:::: ~ : ~ :~:: ~ ~ ~:~ I: ::: ~ ~ ~ ~ ~ ~ I: ~ : ~ ~ ~ ~ : ~ ~ :: ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ I:: : ~ ~ ~ ~ ~ ~ ~ ~ ~ We Might :com~:m~unicate~ch:e~mically~ :like~i~n~s~,~:::~:microorg~an~s~ms~,:~:and other::::: : 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 ::~:~u:~nl:ikely~::~to:~:respond~ch~em:ically~:~to:~:~:~on~e:~;another ~:i~n:::~:~any:~m~:aJor~:~beh~av~ral ~:~:~ ::: :~ ~0 ntext. ::: ~:~:~This~ ~:~view I: ~ visit ~;c~h~ang i no: Known ~:~ with The ~ ~ unexp ecte:d~:~:discovery Bin: : ~:~:~:~hiu:m~ans:~:~6f~::~extrao~in:a~:~::~olfadtory~::capabi~lit~les~::~and~:::~:of~ inte:r~tions~:~:~that~:~ar~:~::::~ :clearly~pkeromon~e~-m~ediat~ed.~Nu~rsi~n:g~i:~aTnts~,~f6~r~exa~mple,~show~olfactory~:~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~reCognitio~n~:~;of~the~:~mothers~.~:~:~ ~ ,~py:::~:~mpy:~wa Evans ~ow:~:suc No ~m~ot~o~ns~:~:::~ ~ ~ ~ ~ ~ . ~ . ~ ~ . :: ~ ~ ~ ~:~:~ few 1~en~:~p~rese:~nten::~:wit ,:~:~:t ~e~:mot 1~:ef:~s~:: breast Spat ~ ~ ,ut~lgnore~pac so Omit Her ::~: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~:~:~: ~ ~ ~ ~ . ~ ~ ~ ~ . ~ ~ ~ :, ~ ~ ~ #, i, ~ ~ ~ ~:~: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 ~ ~ ~ I: ~ ~ : :: ~ ~ T: :: ~ ~ :: ~ ~ : ~ ~:~:~identify~:~mal~es~an~d~fe~m~al~es~on~:~t~h~e basis of breath and ~pplm~:~odor.: ~:~ : ::: ::~: ::: : :~ :: ~ :: ~ ~ : :~: :: ~: ~ ~ Most~re~mark~le~are~the~ effects~elicit~ed ~by~che:mical~ e~rac~ts~of~u:nd~e~r-~ ~ i:: ~:~::~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 :~:::~:~ :: : : :: : : ~ :: : :: ~:~:~:~others~:caused: :th~e~ cycl~es~:~of~ the::: recip:'ents~to shift~k~va:rd~: sy~nch~ro~ny~wit~:h~th:at~::~::~:~ :::::: of:::~the~d:onors. Aninters~e:xual:effect::~has::also:be:e:n~d~emon~rated.~:eg:ular ~ : ~ :::: :: ::: : : : ~ ~ ~ ~::~:: ~ I: ~ ~ ~ ~ ~ ~ I: ~ : :~ ~:~:sexual activity:~wit:h~:~a~m~an~m~:ay:~no:rm:alize~:::th:e~:~menstr:u:al~cy~le:~of~:t:h:e~woman~.~: ~::~:: : ~ : ~ : ~ : ~ : ~ ~ : 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-: ~ ~ . ~ ~ ~ ~ :~:~callv:::absorbed='s known.~::~:::lnterest::~in the::~substance involved is~::con:sider-~: : :: ~ ~: ~ ~ ~ . ~ ~ ~ . ~ ~ , ~ ~ able~:~since:: pheromones~:~:co~Jd: obviously :be:::::use~ :'n te~rt:' fly ~stuc 'es. :: :: ~:: ::: 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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296 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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ECOLOGY AND ECOSYSTEMS 297 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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312 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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ECOLOGY AND ECOSYSTEMS 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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314 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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316 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 i] 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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320 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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322 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.