ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 29 potx

We will also see that because of the focus on movement of energy and abiotic materials e.g., C, N, P, ecosystem ecology integrates the fields of chemistry,physics, and biology and is, th

Part V

Ecosystem Ecotoxicology

In amnesiac revery it is also easy to overlook the services that ecosystems provide humanity They enrichthe soil and create the very air we breathe Without these amenities, the remaining tenure of the humanrace would be nasty and brief

(E.O Wilson 1999)

Ecosystems represent the highest and final level of biological organization that we will consider inour treatment of ecotoxicology It is appropriate that we conclude with a discussion of ecosystems,which have been considered by some ecologists to be the fundamental units of nature (Tansley1935) The critical defining feature of ecosystems that is unique from other levels of biologicalorganization we have considered is the inclusion of abiotic variables Ecosystem ecotoxicology isnecessarily a multidisciplinary science, and the ecosystem processes that respond to contaminants

go beyond those of populations and communities Because these processes are often scale dependent(Carpenter and Turner 1998), effects of contaminants on ecosystem function also vary across spati-otemporal scales Ecosystem ecologists have made tremendous progress developing biogeochemicalmodels of nutrient dynamics, and these models can be readily adapted to predict contaminant move-ment within and between ecosystems Quantifying effects of contaminants on ecosystem processesand demonstrating causal relationships between stressors and responses is challenging As a con-sequence, ecosystem responses are not routinely measured in ecological risk assessments However,characterization of ecological integrity based exclusively on structural measurements has provided

a somewhat incomplete picture of how ecosystems respond to anthropogenic perturbations (Gessnerand Chauvet 2002) Furthermore, the unprecedented rate of species extinction occurring at a globalscale (Wilson 1999) requires that ecologists and ecotoxicologists develop a better appreciation ofthe relationship between community patterns and ecosystem processes Finally, many ecosystemprocesses are intimately connected to ecosystem goods and services that are essential for the welfare

of humanity The goal of this section is to demonstrate how contaminants and other anthropogenicstressors affect these critical ecosystem processes and related services

Carpenter, S.R and Turner, M.G., At last a journal devoted to ecosystem science, Ecosystems, 1, 1–5, 1998 Gessner, M.O and Chauvet, E., A case for using litter breakdown to assess functional stream integrity, Ecol Appl., 12, 498–510, 2002.

Tansley, A.G., The use and abuse of vegetational concepts and terms, Ecology, 16, 284–307, 1935.

Wilson, E.O., The Diversity of Life, W.W Norton & Company, New York, 1999.

29 Introduction to

Ecosystem Ecology

and Ecotoxicology

29.1 BACKGROUND AND DEFINITIONS

Ecosystems behave in ways that are very different from the systems described by other sciences

(Ulanowicz 1997)

Ecosystems can be seen more powerfully as sequences of events rather than as things in a place Theseevents are transformations of matter and energy that occur as the ecosystem does its work Ecosystemsare process-oriented and more easily seen as temporally rather than spatially ordered

(Allen and Hoekstra 1992)

Although the term ecosystem is broadly recognized by the general public and appears frequently inthe nonscientific literature, ecologists and ecotoxicologists still struggle with a precise definition.Recognition that groups of plants form predictable associations across broad geographic regions was asignificant breakthrough in the history of ecology (Clements 1916), and early plant ecologists devotedconsiderable effort to understanding the mechanisms responsible for these patterns Perhaps because

of the tremendous influence of Frederic Clements on the field of ecology, the contentious debatesregarding holistic and reductionist interpretations of natural systems continued well into the 1930s.These debates figured prominently in the establishment and maturation of the emerging field ofecosystem ecology Rejecting the Clementsian superorganism perspective that growth, development,

and senescence of a community was analogous to that of individual organisms, the term ecosystem

was first introduced by Arthur Tansley in 1935 when he appropriately recognized the difficulty ofstudying biotic and abiotic components of natural systems in isolation

Though the organisms may claim our primary interest, when we are trying to think fundamentally wecannot separate them from their special environment, with which they form one physical system

(Tansley 1935)

Thus, one distinguishing feature of ecosystem ecology, which was recognized early in its history,was the necessity of considering integrated physical, chemical, and biological processes Ecosystemecologists are not simply recognizing the influences of the physical environment but are consideringorganisms and the abiotic environment as part of a single system This holistic perspective is funda-mentally different than how lower levels of organization have been treated in ecology Likens (1992)defined an ecosystem as a “spatially explicit unit of the earth that includes all of the organisms alongwith all components of the abiotic environment within its boundaries.” One can see by this broaddefinition that while the spatial extent of an ecosystem remains somewhat vague, the emphasis is onincluding organisms and the environment We will also see that because of the focus on movement

of energy and abiotic materials (e.g., C, N, P), ecosystem ecology integrates the fields of chemistry,physics, and biology and is, therefore, necessarily a multidisciplinary science

613

29.1.1 THESPATIALBOUNDARIES OFECOSYSTEMS

Because of the loosely defined spatial and temporal boundaries, some ecologists have argued thatecosystems lack the logical interconnectedness typical of other levels of biological organization(Reiners 1986) Clearly, the spatial boundaries of an ecosystem often extend beyond those of itscomponent populations and communities These broad spatial and temporal boundaries of ecosys-tems are necessary because they provide ecologists with the flexibility to match questions withappropriate scales For example, to quantify the mass balance of nitrogen or phosphorus in a lakeecosystem, it is necessary to include materials contributed from the surrounding watershed Sim-ilarly, to quantify the transport of organochlorines or other persistent organic pollutants through

an aquatic food web, assessment of atmospheric sources may be required Although flexibility indefining the spatial and temporal scale of an ecosystem is necessary, the classic studies of ecosystemdynamics have been conducted in systems with well-defined boundaries such as watersheds andlakes Thus, ecologists recognize the necessity of including inputs of materials from outside sources,but in practice ecosystem boundaries are more precisely defined

While Tansley considered ecosystems “the basic units of nature on the face of the earth,” there

remains some debate in the literature over whether ecosystems actually exist or are simply an artifact

of our inability to adequately describe nature (Goldstein 1999) Contemporary ecologists still tion whether the ecosystem is a physical construct, as defined by Tansley, or more like a theoreticalconcept that serves to organize our thoughts and ideas Early definitions attempted to place specificboundaries on ecosystems, lakes being the most obvious example However, we now recognize thatecosystems are connected to and influenced by features outside these traditional borders Allen andHoekstra (1992) note that it is unworkable to consider an ecosystem simply as a place on a land-scape Thus the question becomes, is ecosystem science simply the study of processes (as opposed

ques-to patterns)? We can readily discuss properties of ecosystems (e.g., trophic structure), but recognizethat it may not be possible or prudent to enclose ecosystems in arbitrary boundaries

A relatively broad delineation of ecosystem boundaries will also influence the scope and coverage

of ecosystem ecotoxicology considered in the following sections In our previous discussion of foodweb ecotoxicology, we described the structure of food webs and how contaminants may influencelinkages among trophic levels Analyses of connectance, trophic linkages, and food chain lengthprovide important insights into community organization and help explain variation in contaminantlevels among consumers In the following sections, we will emphasize factors that affect contaminanttransport in ecosystems and the potential effects of contaminants on bioenergetics, nutrient cycling,and other ecosystem processes

29.1.2 CONTRAST OFENERGYFLOW ANDMATERIALSCYCLING

Although the flow of energy and the transport of materials through an ecosystem are generallytreated separately in most ecosystem assessments, these processes are so intimately linked that it isoften more practical to consider them simultaneously For example, the flow of energy is closelyassociated with the transfer of carbon through photosynthesis and respiration One important dis-tinction between the movement of energy and abiotic materials through ecosystems concerns thesecond law of thermodynamics, which essentially states that some energy is dissipated as heatwith each energy transformation It is well established that energy flow through biological systems

is a highly inefficient, one-way process, with approximately 10% of energy transferred from onetrophic level to the next (Slobodkin 1961) This inefficiency greatly limits the number of trophiclevels in an ecosystem and accounts for the rarity of large predators (Colinvaux 1978) In con-trast, abiotic materials such as nutrients and carbon are cycled through ecosystems, and the amount

of these materials increases with trophic level (Figure 29.1) These differences between energyflow and materials cycling are at least partially responsible for the process of biomagnification intop predators observed for many organic chemicals Although the amount of energy decreases,

Zooplankton Planktiv

ores Pisciv ores

Ph ytoplankton

FIGURE 29.1 Hypothetical changes in energy and chemicals in an aquatic food web Because energy is

dissipated as heat as it is transferred through a food chain, it decreases with trophic level In contrast, manychemicals, including toxic and bioaccumulative substances, cycle through an ecosystem and may increase withtrophic level (Modified from Stiling (1999).)

many abiotic materials, including contaminants, tend to increase in concentration with trophiclevel

29.1.3 COMMUNITYSTRUCTURE, ECOSYSTEMFUNCTION

ANDSTABILITY

The precise characterization of ecosystem properties has important implications for how we defineecosystem resistance and resilience Previous studies have reported that structural characteristics,such as abundance or the number of species, are generally more sensitive than ecosystem processes,such as energy flow or nutrient cycling (Schindler 1987) Consider the example of acidified lakes,which have been studied extensively in ecosystem ecology If we define resistance based on alter-ations in primary productivity of an acidified lake, we may conclude the ecosystem was relativelystable However, if we assessed stability based on loss of species or changes in community com-position, responses known to be considerably more sensitive, we may conclude that the system hadlow stability The important point is that populations and communities may appear to behave quitedifferently when they are considered in isolation from ecosystems The simplification of ecosystems

to component parts has also contributed to the controversy over the relationship between stabilityand diversity described in previous sections Attempts to define the stability of ecosystem processesbased on the diversity of its components (e.g., number of species) have met with mixed success

29.2 ECOSYSTEM ECOLOGY AND ECOTOXICOLOGY:

A HISTORICAL CONTEXT

Compared to the study of population and community ecology, an ecosystem perspective is relativelynew in the history of ecology There is also considerable variation among ecologists in their precisedescriptions of ecosystems, which have been compared to individual organisms and precisely engin-eered (though relatively inefficient) machines Ecosystems have been described as static or dynamic,

as open or closed, and as predictable or stochastic collections of unrelated, noninteracting species

(Ulanowicz 1997) For some contemporary ecologists, the field of ecology is predominantly a study

of the movement of energy and materials through ecosystems Others consider the movement ofmaterials to be an outcome of the interactions among organisms and with the abiotic environment.These different characterizations reflect some uncertainty in the literature with respect to the ecosys-tem as an object of study or simply a concept Over the past 50 years, the predominant perspective of

an ecosystem has evolved from the idea of spatiotemporal constancy to coupled dynamics in space

in time Despite this evolution, developing a comprehensive framework to address spatiotemporalissues in ecosystem ecology remains a challenge (O’Neill et al 1986), and how we describe anecosystem is often influenced by personal bias or point of reference

29.2.1 EARLYDEVELOPMENT OF THEECOSYSTEMCONCEPT

As noted above, Tansley (1871–1955) coined the term ecosystem and was the first to publish the

concept in a technical paper In the History of the Ecosystem Concept in Ecology, Golley (1993)

argued that Tansley’s inclusion of biotic and abiotic processes in the definition of an ecosystemwas an attempt to resolve the conceptual disagreements among plant ecologists concerning thehierarchical versus organismic nature of a community In 1942, the ecosystem concept was formalized

by Raymond Lindeman into the “trophic dynamic aspect,” widely recognized as one of the mostsignificant contributions in the early history of ecology (Lindeman 1942) The most striking aspect ofthis original work was Lindeman’s attempts to quantify seasonal dynamics of vegetation and animalproduction in a small lake (Cedar Bog Lake, Minnesota) and to characterize an ecosystem based

on energy flow He also organized different groups of species into categories based on their feedinghabits or trophic level (e.g., browsers, plankton predators, benthic predators) More importantly,

he highlighted the interactions between biotic and abiotic components of the ecosystem Importantconcepts such as the substitution of units of energy (calories) for biomass, estimates of productionbased on turnover, and calculation of ecological efficiencies anticipated questions that would figureprominently in contemporary ecosystem research However, the most significant contribution of thework was the recognition that energy, or more specifically calories, was the most appropriate currency

by which to characterize ecosystems Ironically, Lindeman’s original manuscript was rejected by

Ecology, primarily because of its overly theoretical nature The paper was accepted only after strong

appeal from Lindeman’s Ph.D advisor, the famous Yale limnologist G.E Hutchinson, and publishedafter Lindeman’s death in 1942

While Lindeman’s classic paper introduced the trophic dynamic concept and formalized the study

of ecosystem ecology, it was the publication of Eugene P Odum’s (1953) classic text Fundamentals

of Ecology a decade later that placed ecosystem studies in the mainstream of ecological research.

This textbook greatly influenced a generation of ecologists during a critical period of developmentand allowed the ecosystem concept to finally emerge as a legitimate topic of ecological research.Ecology was gradually attempting to move from a predominantly descriptive science concernedprimarily with natural history to a more mechanistic-based science that sought to achieve the status

of chemistry and physics Interpretation of ecological processes using laws of thermodynamicsappealed to many ecologists This work also initiated a series of disputes among ecologists regardingthe usefulness of mathematical models for quantifying ecosystem dynamics Ecosystem ecologistswere criticized for reducing the complexity of ecosystems to fewer and fewer components, andfor simplifying interactions among these components using strictly deterministic models Golley(1993) notes that much of the ecosystem research conducted during this period was little morethan “machine theory applied to nature.” The ecosystem as a machine concept and the application

of large-scale ecosystem models, referred to as “brute force reductionism” (Allen and Starr 1982)figured prominently in the early history of ecosystem research Although there is some dispute that thecomplex box-and-arrow models of system ecologists represent testable hypotheses (Golley 1993),they at least provided ecologists with mechanistic explanations for patterns observed in nature.Providing insight into mechanisms, which has long been considered the holy grail of ecologicalresearch (Ulanowicz 1997), is likely to improve the ability of ecosystem ecologists to address

applied issues Although Odum’s textbook preceded the environmental movement by over a decade,

it appealed to a growing number of ecologists concerned with human impacts on natural systems

At a time when humans were only beginning to understand the potential effects of their actions onthe environment, this book stands out as one of the first to emphasize the importance of includinganthropogenic activities in any assessment of ecosystem structure and function

29.2.2 QUANTIFICATION OFENERGYFLOW THROUGH

ECOSYSTEMS

The flow of energy described in the conceptual diagrams of Elton and Lindeman was quantified

in the early 1960s These initial analyses confirmed theoretical predictions showing the relativeinefficiency of energy transfers from primary producers to herbivores and predators Golley’s (1960)classic study of energy dynamics conducted in an old field with a relatively simple food chain fromplants to herbivores (mice) and predators (weasels) (kcal/ha/year) showed that only a small fraction

of the energy in primary producers results in predator production (Figure 29.2) About 50% of thesunlight striking the field is of the wavelength that can be used by plants, and only about 1% ofthis is converted to Net Primary Production (NPP) Fisher and Likens (1973) quantified all organicmaterial input and output to develop an energy budget for Bear Brook, a small second-order stream

in the northeastern United States Over 99% of the energy input to the stream was allochthonous,indicating that Bear Brook was a strongly heterotrophic system

Ecosystem-level studies by Golley (1960), Fisher and Likens (1973), and others demonstratedthat the movement of energy through an ecosystem could be quantified; however, the food chains inthese initial studies were relatively simple Quantifying energetics of more complex systems proved

to be a daunting task One significant event during this period facilitated the development of new niques to quantify energy and materials flow in ecosystems Funding provided by the Atomic EnergyCommission (AEC) allowed researchers to study the distribution of radioactive materials in biotic

tech-Incident sunlight 47.1 × 10 8

Available to mice 15.8 × 10 6

Consumption 25.0 × 10 4 17.0 × 10 4

R 7.4 × 10 4

Production 5.17 × 10 3

12.0 × 10 3

Consumption 5.82 × 10 3 5.43 × 10 3

260

Production 130

Import: 13.5 × 10 3

Population increase 1.57 × 10 3

FIGURE 29.2 Energy flow in an old field ecosystem showing the relative amounts of energy (kcal/ha/year)

from incident sunlight to top predators (Modified from Golley (1960).)

and abiotic compartments following intentional releases associated with tests of explosive devices.

It is no coincidence that several prominent centers for ecosystem studies in the United States, ing Oak Ridge National Laboratory and Savannah River Ecology Laboratory, were associated withnuclear testing facilities and involved the emerging area of radiation ecology Recognition that radio-active materials moved between biotic and abiotic compartments and accumulated in food chainswas a significant discovery that linked basic and applied ecological research A readily availablesource of funding from the AEC certainly facilitated this association (Golley 1993) Experimentaltechniques such as the addition of radioactive tracers improved the ability of ecologists to quantifythe movement of energy and materials through an ecosystem By labeling primary producers with

includ-a rinclud-adioinclud-active isotope, most commonly phosphorus-32 (32P), ecologists can trace the movement ofenergy through a foodweb Whittaker (1961) pioneered this technique in aquatic ecology and usedmicrocosms to measure movement of32P through an aquatic food web Similar experiments wereconducted by Ball and Hooper (1963) in a Michigan trout stream Tracer experiments became amainstay for the emerging field of radioecology that allowed ecosystem ecologists to estimate therate of movement of materials and energy through the system The large number of studies that fol-lowed reflected the growing perspective that energy is the universal currency in ecosystems and that

an understanding of energy flow was critical to the study of ecosystem ecology This developmentproved to be especially significant to the study of ecosystem ecotoxicology because many of thetransport and fate models used to quantify the movement of radioactive materials were eventuallyadopted and modified for the study of contaminants

Quantifying the movement of energy and materials through an ecosystem generally required amass budget approach in which inputs and outputs were measured Thus, lakes and streams becameappropriate models for the study of ecosystems because, unlike terrestrial ecosystems, the boundarieswere well defined As we will see, recognizing the exchanges that take place between ecosystems,such as the movement of materials from terrestrial to aquatic habitats, has become an important area

of ecosystem research (Ulanowicz1997) In fact, despite precedence for the term ecosystem beingattributed to Tansley, earlier writings of Stephen Forbes, an American limnologist, also highlightedthe role of abiotic processes and interactions within communities and recognized the importance ofstudying ecosystem function in addition to structure (Forbes 1887) Using extensive data collectedfrom Silver Springs, FL, H.T Odum was the first to quantify the inputs and outputs of materialsthrough an ecosystem, thus calculating a mass budget and providing an estimate of metabolism(Odum 1957) This approach proved especially insightful because estimates of mass budgets, either

of natural materials such as nutrients or of synthetic organic compounds such as pesticides, havebeen the workhorse of ecosystem research

29.2.3 THEINTERNATIONALBIOLOGICALPROGRAM AND

THEMATURATION OFECOSYSTEMSCIENCE

The early history of ecosystem science focused on three general areas of research: characterization

of the structure and function of whole ecosystems, quantification of energy flow, and estimation

of ecosystem productivity (Golley 1993) There was relatively little effort during this initial perioddevoted to the study of nutrient cycling and the flow of abiotic materials through ecosystems Per-haps more importantly, relatively little funding was available to pursue what was considered to be asomewhat intractable research topic This changed in the early 1960s when the International Biolo-gical Program (IBP) provided a unique focus on ecosystem research and, more importantly, fundingopportunities for large-scale and long-term ecosystem-level studies The pioneering investigationsinto biogeochemical cycling at Hubbard Brook Experimental Forest, New Hampshire demonstratedthat ecosystem-level questions were both manageable and could address critical applied issues (Bor-mann and Likens 1967) By quantifying inputs and outputs of various nutrients, cations, and anions,these researchers demonstrated that materials budgets for an entire ecosystem could be developed.More importantly, they expanded the traditional boundaries of stream ecosystems to include the

surrounding upland areas and pioneered the field of watershed research From an applied ological perspective, the creation of a watershed budget for Hubbard Brook also provided some ofthe first concrete evidence of the effects of acid rain on ecosystems in the United States (Likens et al.1996).

ecotoxic-29.3 CHALLENGES TO THE STUDY OF

WHOLE SYSTEMS

The answer for ecosystems lies neither in the elegant simplicity of classical physics nor in the fascinationfor detail of natural history

(Holling and Allen 2002)

Ecologists who believed whole ecosystems were the most appropriate scale of their investigationssoon realized they faced several significant challenges An ecosystem perspective would requireestimates of biomass and production of all resident species—clearly an impossible task If ecologistswere to study ecosystems in their entirety, a system-level approach was necessary Various solutions

to this dilemma were offered, including limiting analyses to the few dominant species and assumingthat related species performed similar functions The second alternative, the approach used by mostcontemporary ecologists, was to assign species to functional groups and characterize energy andmaterials flow through these groups This approach required numerous simplifying assumptions,and many ecologists were critical of the loss of information that occurred when aggregating feed-ing habits of different species In addition to minimizing species-specific differences, categorizingorganisms into functional feeding groups ignored seasonal and ontogenetic variation Suter (1993)lists several additional impediments to ecosystem-level assessments, including greater costs, lack ofstandardization, lack of consensus over relevant endpoints, ecosystem complexity, high variation,and relative insensitivity Because much of ecosystem ecology remains purely descriptive, there hasbeen criticism that the hypothetico-deductive approach advocated by many philosophers of science(Popper 1972) has been neglected These practical and conceptual impediments partially explainwhy ecotoxicologists have not pursued a more rigorous program of research in ecosystem assess-ments Clearly, the relevant question for many of these issues is how much detail can we ignoreand still have an adequate representation of overall ecosystem function However, downplaying theimportance of species in favor of characterizing ecosystems based entirely on processes has receivedharsh criticism, particularly in the field of conservation biology (Goldstein 1999)

Finally, our ability to understand effects of contaminants on ecosystems is both facilitated andimpeded by their self-organizing and cybernetic characteristics (O’Neill et al 1986) The perspectivethat ecosystems are controlled by stabilizing negative feedback relationships is relatively widespread

in ecology Indeed, the ability of some ecosystems to quickly return to predisturbance conditionsfollowing perturbation implies some degree of organization and homeostasis This is encouragingand suggests that, despite inherent complexity, ecosystems are legitimate objects of study and thatpatterns and processes are tractable However, the resilience and resistance of ecosystem processes

to disturbance may hamper our ability to quantify these responses

on ecosystem processes The temporal scale of ecosystem responses is an important consideration

Pollutant Input

Behavioral Response Biochemical response

Response

Morphological Response

Pollutant input

Behavioral response

Physiological response

Morphological response

FIGURE 29.3 Time scale for responses to chemical pollutants at different levels of biological organization.

Effects of chemicals on physiological and biochemical endpoints are expected to occur within hours to days,whereas community- and ecosystem-level responses may require months to years (Modified from Sheehan(1984).)

Time (decades)

Time (months)

FIGURE 29.4 Importance of temporal scale in ecosystem assessments Response trajectories to ecosystem

perturbations measured over very short time scales (e.g., months), the typical duration of many ecologicalinvestigations, will likely be quite different from those measured over longer time scales (e.g., decades)

in assessing impacts of anthropogenic perturbations Ecosystem responses to global climate change

at one temporal scale (e.g., hundreds of years) may show very different responses over shorter timeperiods (Figure 29.4) A sampling regime that is too short will not capture the patterns occurringover longer time periods It is therefore critical that the time scale of ecosystem responses and themethodological approaches and sampling frequency designed to assess these responses match theexpected time scale of the perturbation In a review of over 800 experimental and field studies,Tilman (1989) reported that over 75% were limited to 1–2 years Although a 2-year duration may beadequate to characterize some ecosystem processes, these short-term ecosystem studies will likelymiss novel events, such as droughts or other natural disturbances, which are important features ofecosystems

29.4 THE ROLE OF ECOSYSTEM THEORY

29.4.1 SUCCESSIONTHEORY AND THESTRATEGY OFECOSYSTEM

DEVELOPMENT

Ecologists have long recognized that ecosystems change over time and that these changes are oftenorderly and predictable Characterizing long-term temporal changes in ecosystem structure andfunction is problematic, and therefore researchers studying succession have relied heavily on spacefor time substitutions For example, by comparing vegetation patterns along newly formed sanddunes of Lake Michigan, ecologists could visualize the transition from pioneer to climax speciesalong a defined spatial gradient (Cowles 1899) Because ecological succession is often initiatedfollowing natural or anthropogenic disturbance, it is appropriate to consider temporal changes withinthe context of ecosystem perturbation and recovery

Odum’s (1969) classic paper “The strategy of ecosystem development” recognized the parallelsbetween developmental biology of organisms and succession of ecosystems He stated that the

“strategy” of succession as a short-term process is basically the same as the “strategy” of long-term evolutionary development of the biosphere The overall strategy of ecosystem development according

to Odum was to achieve as large and diverse an organic structure as possible within the constraints ofavailable energy and materials Due to the misgivings of many ecologists concerning the organismal

or superorganismal properties of ecosystems, Odum’s use of the word “strategy” is somewhat tunate However, this paper is especially significant because it was one of the first to relate ecosystemprocesses such as succession and stability to anthropogenic disturbance Ecosystem developmentalchanges, such as shifts in the ratio of primary production (P) to respiration (R) and changes inspecies diversity, are considered functional indices of ecosystem maturity Because of the solid the-oretical underpinnings and the generality of these responses, they may represent useful measures ofecosystem responses to contaminants (Table 29.1)

unfor-Ecologists generally employ two very different methodologies to study ecosystem processes Thefirst approach employs the traditional hypothetical-deductive method and relies on a combination ofinduction, observation, and experimentation This approach tends to be more site specific, and theresults often pertain to a specific set of questions in a particular ecosystem Much of applied ecosystemecology, in which researchers attempt to identify the causes of specific alterations in ecologicalprocesses, uses this form of inquiry In the second approach, ecologists develop theoretical principlesand mathematical models to draw inferences about processes that can be generalized across differentecosystems Sagoff (2003) discusses several conceptual obstacles faced by researchers using thesetheoretical or “top-down” approaches Because these obstacles pertain to how we define ecosystemsand how we isolate cause and effect relationships, a brief discussion is warranted here

Perhaps the most serious challenge to theoretical ecosystem ecology is defining the class ofobjects that constitute an ecosystem Sagoff (2003) argues that most definitions are either over-

or under-inclusive The broad definition cited above would include such diverse systems as thebacterial assemblages living in a cow’s intestines as well as all of Lake Superior A second challenge

is the remarkably diverse ways in which ecosystem ecologists have attempted to explain processesoccurring in nature Ecologists have borrowed heavily from information systems, chaos theory,statistical mechanics, thermodynamics, cybernetic systems, and hierarchy theory (to name a few)

to develop ecosystem models (Sagoff 2003) Finally, the use of mathematical models to addressapplied issues in environmental biology may represent the greatest challenge to theoretical ecosystemecology Although some researchers are pessimistic about our ability to integrate ecosystem models inapplied ecology (Sarkar 1996), it is likely that the following decades will see numerous opportunities

to test model predictions in altered ecosystems Deviation in the behavior of these systems from modelpredictions may be used as a measure of the level of perturbation

We recognize that it will not be possible or practical to study the response of all ecosystem ponents to anthropogenic stressors To measure effects of soil acidification on decomposition rate,