Clements: “3357_c029” — 2007/11/9 — 18:37 — page 611 — #1 Part V Ecosystem Ecotoxicology In amnesiac revery it is also easy to overlook the services that ecosystems provide humanity. They enrich the soil and create the very air we breathe. Without these amenities, the remaining tenure of the human race would be nasty and brief. (E.O. Wilson 1999) Ecosystems represent the highest and final level of biological organization that we will consider in our 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 (Tansley 1935). The critical defining feature of ecosystems that is unique from other levels of biological organization we have considered is the inclusion of abiotic variables. Ecosystem ecotoxicology is necessarily 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 developingbiogeochemical models 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 processes and 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 (Gessner and Chauvet 2002). Furthermore, the unprecedented rate of species extinction occurring at a global scale (Wilson 1999) requires that ecologists and ecotoxicologists develop a better appreciation of the relationship between community patterns and ecosystem processes. Finally, many ecosystem processes 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 anthropogenic stressors affect these critical ecosystem processes and related services. © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 612 — #2 612 Ecotoxicology: A Comprehensive Treatment REFERENCES 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. © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 613 — #3 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. These events are transformations of matter and energy that occur as the ecosystem does its work. Ecosystems are 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 in the nonscientific literature, ecologists and ecotoxicologists still struggle with a precise definition. Recognition that groups of plants formpredictableassociationsacrossbroad geographic regions was a significant breakthrough inthe history of ecology(Clements 1916), and early plant ecologistsdevoted considerable 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 debates regarding 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 of ecosystem 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 of studying biotic and abiotic components of natural systems in isolation. Though the organisms may claim our primary interest, when we are trying to think fundamentally we cannot 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. Ecosystem ecologists are not simply recognizing the influences of the physical environment but are considering organisms 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 along with all components of the abiotic environment within its boundaries.” One can see by this broad definition that while the spatial extent of an ecosystem remains somewhat vague, the emphasis is on including 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 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 614 — #4 614 Ecotoxicology: A Comprehensive Treatment 29.1.1 THE SPATIAL BOUNDARIES OF ECOSYSTEMS Because of the loosely defined spatial and temporal boundaries, some ecologists have argued that ecosystems 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 its component populations and communities. These broad spatial and temporal boundaries of ecosys- tems are necessary because they provide ecologists with the flexibility to match questions with appropriate scales. For example, to quantify the mass balance of nitrogen or phosphorus in a lake ecosystem, 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 in defining the spatial and temporal scale of an ecosystem is necessary, the classic studies of ecosystem dynamics have been conducted in systems with well-defined boundaries such as watersheds and lakes. 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 ques- tion whether the ecosystem is a physical construct, as defined by Tansley, or more like a theoretical concept that serves to organize our thoughts and ideas. Early definitions attempted to place specific boundaries on ecosystems, lakes being the most obvious example. However, we now recognize that ecosystems are connected to and influenced by features outside these traditional borders. Allen and Hoekstra (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 to patterns)? We can readily discuss properties of ecosystems (e.g., trophic structure), but recognize that it may not be possible or prudent to enclose ecosystems in arbitrary boundaries. Arelatively 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 food web ecotoxicology, we described the structure of food webs and how contaminants may influence linkages among trophic levels. Analyses of connectance, trophic linkages, and food chain length provide important insights into community organization and help explain variation in contaminant levels among consumers. In the following sections, we will emphasize factors that affect contaminant transport in ecosystems and the potential effects of contaminants on bioenergetics, nutrient cycling, and other ecosystem processes. 29.1.2 CONTRAST OF ENERGY FLOW AND MATERIALS CYCLING Although the flow of energy and the transport of materials through an ecosystem are generally treated separately in most ecosystem assessments, these processes are so intimately linked that it is often more practical to consider them simultaneously. For example, the flow of energy is closely associated with the transfer of carbon through photosynthesis and respiration. One important dis- tinction between the movement of energy and abiotic materials through ecosystems concerns the second law of thermodynamics, which essentially states that some energy is dissipated as heat with 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 one trophic level to the next (Slobodkin 1961). This inefficiency greatly limits the number of trophic levels 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 energy flow and materials cycling are at least partially responsible for the process of biomagnification in top predators observed for many organic chemicals. Although the amount of energy decreases, © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 615 — #5 Introduction to Ecosystem Ecology and Ecotoxicology 615 Energy Chemicals Zooplankton Planktivores Piscivores Phytoplankton 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, many chemicals, including toxic and bioaccumulative substances, cycle through an ecosystem and may increase with trophic level. (Modified from Stiling (1999).) many abiotic materials, including contaminants, tend to increase in concentration with trophic level. 29.1.3 COMMUNITY STRUCTURE,ECOSYSTEM FUNCTION AND STABILITY The precise characterization of ecosystem properties has important implications for how we define ecosystem 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 relatively stable. 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 had low stability. The important point is that populations and communities may appear to behave quite differently 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 stability and diversity described in previous sections. Attempts to define the stability of ecosystem processes based 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 relatively new in the history of ecology. There is also considerable variation among ecologists in their precise descriptions 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 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 616 — #6 616 Ecotoxicology: A Comprehensive Treatment (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 of materials 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 spatiotemporal issues in ecosystem ecology remains a challenge (O’Neill et al. 1986), and how we describe an ecosystem is often influenced by personal bias or point of reference. 29.2.1 EARLY DEVELOPMENT OF THE ECOSYSTEM CONCEPT 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 ecosystem was an attempt to resolve the conceptual disagreements among plant ecologists concerning the hierarchical versus organismicnatureofacommunity. In 1942, theecosystem concept was formalized by Raymond Lindeman into the “trophic dynamic aspect,” widely recognized as one of the most significant contributions in the early history of ecology (Lindeman 1942). The most striking aspect of this original work was Lindeman’s attempts to quantify seasonal dynamics of vegetation and animal production 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 feeding habits or trophic level (e.g., browsers, plankton predators, benthic predators). More importantly, he highlighted the interactions between biotic and abiotic components of the ecosystem. Important concepts such as the substitution of units of energy (calories) for biomass, estimates of production based on turnover, and calculation of ecological efficiencies anticipated questions that would figure prominently in contemporary ecosystem research. However, the most significant contribution of the work was therecognition that energy, or morespecifically calories, was the mostappropriate 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 published after 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 development and 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 concerned primarily 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 thermodynamics appealed to many ecologists. This work also initiated a series of disputes among ecologists regarding the usefulness of mathematical models for quantifying ecosystem dynamics. Ecosystem ecologists were criticized for reducing the complexity of ecosystems to fewer and fewer components, and for simplifying interactions among these components using strictly deterministic models. Golley (1993) notes that much of the ecosystem research conducted during this period was little more than “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 earlyhistory of ecosystemresearch.Although there issome dispute that the complex 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 ecological research (Ulanowicz 1997), is likely to improve the ability of ecosystem ecologists to address © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 617 — #7 Introduction to Ecosystem Ecology and Ecotoxicology 617 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 on the environment, this book stands out as one of the first to emphasize the importance of including anthropogenic activities in any assessment of ecosystem structure and function. 29.2.2 QUANTIFICATION OF ENERGY FLOW 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 relative inefficiency 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 from plants 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 the sunlight striking the field is of the wavelength that can be used by plants, and only about 1% of this is converted to Net Primary Production (NPP). Fisher and Likens (1973) quantified all organic material 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 demonstrated that the movement of energy through an ecosystem could be quantified; however, the food chains in these initial studies were relatively simple. Quantifying energetics of more complex systems proved to be a daunting task. One significant event duringthis period facilitatedthe development of new tech- niques to quantify energy and materials flow in ecosystems. Funding provided by theAtomic Energy Commission (AEC) allowed researchers to study the distribution of radioactive materials in biotic Incident sunlight 47.1 × 10 8 46.5 × 10 8 GPP = 58.3 × 10 6 NPP = 49.5 × 10 6 8.8 × 10 6 R 49.3 × 10 6 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 R 20 Population increase 117 Plants Mice Weasels Unused portions 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).) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 618 — #8 618 Ecotoxicology: A Comprehensive Treatment 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, includ- ing Oak Ridge National Laboratory and Savannah River Ecology Laboratory, were associated with nuclear 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 chains was a significant discovery that linked basic and applied ecological research. A readily available source of funding from the AEC certainly facilitated this association (Golley 1993). Experimental techniques such as the addition of radioactive tracers improved the ability of ecologists to quantify the movement of energy and materials through an ecosystem. By labeling primary producers with a radioactive isotope, most commonly phosphorus-32 ( 32 P), ecologists can trace the movement of energy through a foodweb. Whittaker (1961) pioneered this technique in aquatic ecology and used microcosms to measure movement of 32 P through an aquatic food web. Similar experiments were conducted by Ball and Hooper (1963) in a Michigan trout stream. Tracer experiments became a mainstay for the emerging field of radioecology that allowed ecosystem ecologists to estimate the rate 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 development proved to be especially significant to the study of ecosystem ecotoxicology because many of the transport and fate models used to quantify the movement of radioactive materials were eventually adopted and modified for the study of contaminants. Quantifying the movement of energy and materials through an ecosystem generally required a mass budget approach in which inputs and outputs were measured. Thus, lakes and streams became appropriate modelsfor thestudy ofecosystems because, unliketerrestrial ecosystems, theboundaries were 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 (Ulanowicz 1997). In fact, despite precedence for the term ecosystem being attributed to Tansley, earlier writings of Stephen Forbes, an American limnologist, also highlighted the role of abiotic processes and interactions within communities and recognized the importance of studying ecosystem function in addition to structure (Forbes 1887). Using extensive data collected from Silver Springs, FL, H.T. Odum was the first to quantify the inputs and outputs of materials through 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, have been the workhorse of ecosystem research. 29.2.3 THE INTERNATIONAL BIOLOGICAL PROGRAM AND THE MATURATION OF ECOSYSTEM SCIENCE 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 period devoted 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 a somewhat 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, funding opportunities for large-scale and long-term ecosystem-level studies. The pioneering investigations into biogeochemical cycling at Hubbard Brook Experimental Forest, New Hampshire demonstrated that 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 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 619 — #9 Introduction to Ecosystem Ecology and Ecotoxicology 619 surrounding upland areas and pioneered the field of watershed research. From an applied ecotoxic- ological perspective, the creation of a watershed budget for Hubbard Brook also provided some of the first concrete evidence of the effects of acid rain on ecosystems in the United States (Likens et al. 1996). 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 fascination for detail of natural history. (Holling and Allen 2002) Ecologists who believed whole ecosystems were the most appropriate scale of their investigations soon realized they faced several significant challenges. An ecosystem perspective would require estimates of biomass and production of all resident species—clearly an impossible task. If ecologists were 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 assuming that related species performed similar functions. The second alternative, the approach used by most contemporary ecologists, was to assign species to functional groups and characterize energy and materials 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, categorizing organisms into functional feeding groups ignored seasonal and ontogenetic variation. Suter (1993) lists several additional impediments to ecosystem-level assessments, including greater costs, lack of standardization, lack of consensus over relevant endpoints, ecosystem complexity, high variation, and relative insensitivity. Because much of ecosystem ecology remains purely descriptive, there has been criticism that the hypothetico-deductive approach advocated by many philosophers of science (Popper 1972) has been neglected. These practical and conceptual impediments partially explain why 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 ignore and still have an adequate representation of overall ecosystem function. However, downplaying the importance of species in favor of characterizing ecosystems based entirely on processes has received harsh criticism, particularly in the field of conservation biology (Goldstein 1999). Finally, our ability to understand effects of contaminants on ecosystems is both facilitated and impeded by their self-organizing and cybernetic characteristics (O’Neill et al. 1986). The perspective that ecosystems are controlled by stabilizing negative feedback relationships is relatively widespread in ecology. Indeed, the ability of some ecosystems to quickly return to predisturbance conditions following perturbation implies some degree of organization and homeostasis. This is encouraging and suggests that, despite inherent complexity, ecosystems are legitimate objects of study and that patterns and processes are tractable. However, the resilience and resistance of ecosystem processes to disturbance may hamper our ability to quantify these responses. 29.3.1 TEMPORAL SCALE In addition to questions regarding the appropriate boundaries and spatial scale, the relevant time scale required to adequately quantify ecosystem-level processes requires careful consideration. According to hierarchy theory, responses at higher levels of organization occur slower than those at lower levels (Figure 29.3). For example, bioaccumulation of contaminants and resulting physiological and behavioral alterations in organisms may occur rapidly following the discharge of toxic materials to an ecosystem. However, it may require considerably longer time before we observe discernible effects on ecosystem processes. The temporal scale of ecosystem responses is an important consideration © 2008 by Taylor & Francis Group, LLC Clements: “3357_c029” — 2007/11/9 — 18:37 — page 620 — #10 620 Ecotoxicology: A Comprehensive Treatment Pollutant Input Behavioral Response Biochemical response Response Morphological Response Population Impact Functional Changes Time scale (h) 10 1 10 2 10 5 0 10 3 10 4 Bioaccumulation Structural Changes Altered Performance Pollutant input Behavioral response Physiological response Morphological response Population impact Functional changes 0 Bioaccumulation Structural changes Altered performance 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).) Response Time (decades) Response 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 ecological investigations, 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 time periods (Figure 29.4). A sampling regime that is too short will not capture the patterns occurring over longer time periods. It is therefore critical that the time scale of ecosystem responses and the methodological approaches and sampling frequency designed to assess these responses match the expected 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 be adequate to characterize some ecosystem processes, these short-term ecosystem studies will likely miss novel events, such as droughts or other natural disturbances, which are important features of ecosystems. © 2008 by Taylor & Francis Group, LLC [...]... “3357_c 029 — 2007/11/9 — 18:37 — page 629 — #19 Ecotoxicology: A Comprehensive Treatment 630 each level of organization is represented on each layer of the cake and comparisons among levels of the hierarchy can be made within a single spatial scale or between different spatial scales This representation is intuitively appealing because it also allows us to consider how properties at one level of organization... Hoekstra (1992) provide a very different conceptual arrangement of the hierarchy of biological organization They suggest that separating an ecosystem to its component parts does not reveal populations and communities, and use the analogy of a layer cake to illustrate connections among levels of biological organization across different spatial scales (Figure 29. 8) In their depiction, © 2008 by Taylor & Francis... O’Neill et al (1986).) Large scale P I Intermediate scale C E Pop Ind Comm Small scale Eco Population Individual Community Ecosystem FIGURE 29. 8 An alternative depiction of individual, population, community, and ecosystem responses across different spatial scales Each level of organization is represented on each layer Comparisons among levels of the hierarchy can be made within a single spatial scale or... ecologists and ecotoxicologists to study ecosystem processes are similar to those used by population and community ecologists (Figure 29. 5) A combination of modeling, comparative, long-term monitoring, and experimental approaches have been used to address a variety of basic and applied research questions (Pace and Groffman 1998) Experimental approaches often involve pulsing a system and measuring the associated... energy and materials exchanges can serve as sensitive indicators of anthropogenic perturbation Although ecosystems are widely regarded as fundamental units of ecology, effects of contaminants on ecosystem processes have not received significant attention in the ecotoxicological literature and are rarely considered within a regulatory framework Despite the stated goal of maintaining both structural and... inputs and outputs were measured, lakes and streams became appropriate models because the boundaries were well defined • Our ability to understand effects of contaminants on ecosystems is both facilitated and impeded by the fact that ecosystem processes are controlled by negative feedback relationships • A better understanding of fate and transformation of contaminants is considered a major accomplishment... functional integrity of ecological systems, biological assessments in support of the U.S Clean Water Act emphasize population and community-level analyses Other federal programs within the United States, such as the Department of Interior Natural Resource Damage Assessment program, rely almost exclusively on assessing responses at lower levels of organization (e.g., individuals and populations) Sheehan... ecosystems and that any analysis of the structure and function of an ecosystem must account for anthropogenic inputs Even relatively remote areas located within protected habitats or wilderness areas receive TABLE 29. 2 Significant Accomplishments in Basic and Applied Ecosystem Science • • • • • • • Understanding the movement of energy and materials through freshwater and marine ecosystems Assessment of feedbacks... different spatial scales This representation allows investigators to consider how properties at one level of organization (e.g., ecosystem processes) and at one spatial scale level are influenced by other levels of organization at different scales (Modified from Allen and Hoekstra (1992).) prey populations and places more emphasis on the rate and regulation of energy inputs and capture to the system Allen and... detritivores Consumers Rate regulation Grazers Leaves and detritus Energy inputs Periphyton and diatoms Primary producers Energy capture FIGURE 29. 7 Contrasting structural and functional emphasis in an aquatic food chain The structural hierarchy emphasizes population and community interactions whereas the functional hierarchy emphasizes ecosystem processes and exchanges with the abiotic environment (Modified . radio- active materials moved between biotic and abiotic compartments and accumulated in food chains was a significant discovery that linked basic and applied ecological research. A readily available source. striking aspect of this original work was Lindeman’s attempts to quantify seasonal dynamics of vegetation and animal production in a small lake (Cedar Bog Lake, Minnesota) and to characterize an ecosystem. includ- ing Oak Ridge National Laboratory and Savannah River Ecology Laboratory, were associated with nuclear testing facilities and involved the emerging area of radiation ecology. Recognition that radio- active