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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change 3 Research Opportunities Six major research initiatives for the environmental sciences have been identified in recent NRC reports (NRC, 2001c, 2004), the first four of which cannot be met without using the geologic record of ecological dynamics: Biological diversity Biogeochemistry Ecological impacts of climate variability Habitat alteration Invasive species Infectious diseases The current state of the biosphere is a consequence of both present day processes and prior conditions. A full understanding of current patterns and processes, therefore, requires geohistorical analysis to supplement knowledge based on shorter-term observations, experiments, and modeling. The geological record provides the rich source of information that is essential for developing an understanding of the origin and controls of biological diversity, the controls and dynamics of biogeochemical cycling, the ecological impacts of climate change and variability, and the extent and consequences of habitat alteration. It also has the potential to provide important insights into the dynamics of invasive species and the environmental context of infectious diseases. There are three general areas of environmental science research in which addressing the historical antecedents of present day patterns and processes will be especially important. Each takes advantage of the rich
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change and detailed geological record of how past ecosystems responded to environmental change at a variety of timescales. Together they provide the basic knowledge needed to predict and manage the response of the biosphere to likely change in the future. 1. The Geologic Record as an Ecological Laboratory. The geological record comprises an immense array of natural laboratories for studying how ecological systems operate under diverse conditions and at broad timescales. Using these laboratories to answer fundamental questions about biological diversity and biogeochemical processes is both possible and urgently needed. Ecological and evolutionary processes at timescales beyond direct human observation have influenced biodiversity and biogeochemistry at all scales (see examples described below). Currently, most ecological theory is based on short-term observations and mechanics, which are then extrapolated to longer timescales. Ecological studies using geohistorical records are needed to characterize ecological processes that occur over longer timescales; identify patterns and mechanisms that are masked by the short timespans of direct observation; and recognize those aspects of modern ecological systems that are contingent on past events. The geologic record also contains a series of “alternative worlds” suitable for testing the universality of ecological theory. 2. Ecological Responses to Past Climate Change. The geologic record contains information on how ecological systems—from individual species to biomes—have responded to a wide array of climate changes in the past. Just as paleoclimatological studies have revealed sensitivities and vulnerabilities in the global climate system that could not have been identified from analysis of modern systems alone (NRC, 2002a), so too are paleoecological studies revealing ecological responses to past climate changes that could not have been predicted solely from modern ecological investigations and theory (see examples described below). Studies that link paleobiological and paleoclimatic records are urgently needed to assess the ecological consequences of ongoing and future climate changes. The past two centuries have experienced only a fraction of the potential variability within the global climate system. Therefore, direct observations of biotic responses to climate variability and change provide only a limited view of the full range of possible changes and responses. Parallel to the call for “extending the record of [climate] observations” using the geological record (NRC, 2001c), a concerted effort is needed to use geohistorical records to gather critical information on how ecosystems will respond to future change. Specific time intervals can also serve as model systems for understanding effects of climate changes of particular magnitude, rate, extent, and duration.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change 3. Ecological Legacies of Societal Activities. The geologic record contains a rich store of information on ecological changes occurring during periods of past societal activity. The relative timing and nature of these correlations provide abundant material for evaluating direct and indirect causation—such records are important for gauging the extent to which our modern world has already been altered by human activities (see examples described below). They are also needed to predict effects of future societal modification of habitats and biological systems themselves. Determining whether particular ecological phenomena are induced by ongoing societal activity, comprise legacies of past human activities, or would have occurred in the absence of societal activity is a vital first step toward appropriate management. Determining how societal activities have shaped modern ecological systems at local, regional, and global scales is essential for understanding the world we have inherited, for assessing ecological theory developed within altered ecosystems, and for predicting how ecosystems will change in the face of ongoing and future societal activities. It is also necessary for determining baselines of natural ecological variability against which human activities and management decisions can be evaluated. THE GEOLOGIC RECORD AS AN ECOLOGICAL LABORATORY Ecological studies have tended to focus on patterns and processes that are observable on the timescales of direct human experience—weeks to decades, and occasionally centuries. Although ecological succession, one of the core concepts of ecology, is concerned with changes occurring over timescales of decades to millennia (Cowles, 1899, 1901; Clements, 1916; Glenn-Lewin et al., 1992), successional studies have only rarely taken advantage of geohistorical records of actual change at individual sites. Instead, ecologists have relied on space-for-time substitution (Pickett, 1989) or direct observations of successional change at shorter timescales. Chronosequence studies have pitfalls, however (S.T. Jackson et al., 1988; Davis, 1989; Fastie, 1996), and the oldest “permanent plots” where long-term changes can be monitored systematically date only to the mid-19th century (Pickrell, 2001). Most of the remaining conceptual and empirical core of ecology, whether at the organismal, population, community, ecosystem, or global level, tends to be focused on the “here and now,” with only nominal acknowledgment of longer-term patterns and dynamics. Thus, we are building our understanding of ecology on a very small sample, comprising an ultra-thin and perhaps unrepresentative slice of the history of the biosphere. Ecologists now recognize that many important ecological processes operate at timescales far beyond human life spans and that in many cases
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change we cannot simply scale up in a linear fashion from studies spanning a few years to phenomena spanning thousands to millions of years (see Box 3.1). Many recent trends in ecology are concerned with the search for general rules that govern ecological systems, for example, in the areas of community assembly (Weiher and Keddy, 1999), biodiversity dynamics (Rosenzweig, 1995; Hubbell, 2001), macroecology (Brown, 1995; Maurer, 1999), scaling (Brown and West, 2000; Enquist et al., 2003; Smith et al., 2004), biodiversity/productivity relationships (Loreau et al., 2001; Tilman et al., 2001), global biogeochemical cycles (Schlesinger, 1997), trophic interactions (Williams and Martinez, 2000; Brose et al., 2004), and ecological stoichiometry (Sterner and Elser, 2002). Rigorous and definitive testing of general theory of this kind in ecology can be extraordinarily difficult but the past, as represented in the fossil record, provides a potentially powerful means for assessing and refining such ecological hypotheses. So far, the considerable potential of the geologic record has not been fully exploited by ecologists. Furthermore, although ecological rules identified from modern observation and theory are generally treated as universal, they may be contingent on the modern environment and biota. They may have relict, non-equilibrium features from both natural events and human activities of the past. Subjecting these hypothesized rules to rigorous tests involving both “natural,” pre-human conditions as well as various “alternative worlds” of the recent and deep past will determine the extent to which ecological laws are analogous to physical laws, or whether they evolve as the biosphere evolves. Observations in deep time can help determine whether deep structure and principles exist in ecological systems. Opportunities for Scientific Advance Major opportunities exist for increased interactions between ecology and the geosciences, particularly paleoecology and paleobiology. Future research efforts should be particularly focused on studies and syntheses that have the capacity for: testing fundamental ecological theory and principles at timescales greater than the past two centuries; identifying important ecological patterns and processes that emerge only at timescales beyond those of direct human observation; and determining whether the basic laws and principles identified in ecology today are universal and thus applicable throughout geologic time, or are contingent on the modern biosphere, and thus evolve through time as environments and biota change. If the latter, we need to know whether such evolution proceeds in a predictable and systematic fashion, and which major ecological concepts are universal and which are subject to change.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change BOX 3.1 Fast and Slow Processes in Ecology Ecological processes occur over a vast array of temporal scales, comprising at least 16 orders of magnitude. For example, a phosphate ion released into solution in the photic zone of the ocean may be absorbed by phytoplankton in less than a second (10−8 year), while an organically bound phosphate incorporated into ocean sediments may be locked up for 108 years or more and then released only after subduction, metamorphism, uplift, and ultimate release by weathering (Ruttenberg, 2004). The mean generation time of a single-celled prokaryote may be less than a day, while that of many tree species is more than a century. With a constant environment, interspecific competition may take days, years, or centuries to reach equilibrium, respectively, in communities of microbes, intertidal mollusks, or forest trees. Ecologists are increasingly recognizing that ecological systems are influenced by both “fast” and “slow” processes, and that many ecological patterns are dictated by interactions between processes operating at very different rates (Carpenter and Turner, 2001). Developing theoretical, observational, and experimental frameworks for understanding these interactions is a major challenge for ecology and related sciences. In practice, ecologists have dealt with this problem by focusing on particular timescales (usually short), treating slow processes as constants (parameters), and calculating equilibrium values for the faster variables (Carpenter and Turner, 2001). However, this approach has limitations. Because of disturbances, climate variability, and other factors, ecosystems are in a state of continual flux. Although ecologists have addressed this problem by incorporating random fluctuations or disturbances into their models, such variations may not be simple random variation about a constant mean. For example, climate variability is non-stationary at timescales from interannual to multimillennial and beyond. Disturbances (e.g., fire, windstorms, hurricanes, flood and wave events) are often tied to climate, and consequently disturbance frequencies and amplitudes cannot necessarily be modeled as random variables, even at routine ecological timescales of 101-103 years. Some events may be so infrequent (e.g., impacts, volcanic eruptions, methane releases from the seafloor) or slow-acting (e.g., sea-level change) that observation and experimentation are essentially out of the question. Many ecological processes, particularly biogeochemical, biogeographic, and evolutionary processes, occur over even longer timescales, ranging from 103-108 years (i.e., at geologic timescales). Such slow processes can be accelerated, decelerated, reversed, shunted to alternative pathways, or (in the case of phyletic evolution) terminated, as a consequence of climatic, tectonic, macroevolutionary, and other events. The controls on slow processes and the long-term effects of rare events can be understood only by using the geologic record.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change These issues cut across a wide range of timescales, ranging from recent millennia to the entire record of life on Earth. They are vitally important to appropriate application of ecological knowledge and theory to conservation biology, biosphere maintenance, and global change biology. As we enter a period of potentially rapid environmental change, with extensive biological invasions and impoverishment of biodiversity, we need to know which ecological rules will apply and which are likely to be altered. A critical goal will be to improve dialogue between paleobiologists and ecologists, particularly in the realm of theory and concepts. But this must be accompanied by integrated studies aimed at addressing the central questions of ecology across a range of timescales. For example, use of the geologic record is vital to addressing national ecological research priorities in the areas of biodiversity and biogeochemical cycles. It can also provide important perspectives on infectious diseases and invasive species. One major goal is to integrate paleobiological studies with ecological experimentation and modeling. Both communities will benefit from increased interactions. A danger in the traditional ecological focus on the “here and now” of experiments on existing systems and environments is that the role of historical contingency and long-term processes can be overlooked. On the other hand, paleoecologists may discount the implications of short-term ecological experiments simply because short-term events are often difficult to resolve in the geologic record. Greater collaboration and coordination between these disciplines can help determine where observable short-term processes can scale up to explain long-term patterns, suggest experimental tests of paleoecologically generated hypotheses, and identify the ecological consequences of past environmental events. Biodiversity While understanding patterns of extant biological diversity is one of the central themes of ecology, the factors and processes that govern it are still inadequately understood. Consequences of biodiversity loss are not fully known but may be substantial, ranging from elimination of potentially useful species to loss of ecosystem services and even collapse of ecosystem function. The geologic record provides information on speciation and extinction rates as well as biogeographic changes that regulate biodiversity. Resolving one of the Grand Challenges in Environmental Sciences (NRC, 2001c; p. 26)—“Produce a quantitative, process-based theory of biological diversity at the largest possible variety of spatial and temporal scales”—requires use of the geologic record.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change The Evolutionary Dynamics of Biodiversity. Biodiversity is shaped by biogeographic and evolutionary dynamics that occur over thousands to millions of years. Although the formation, invasion, migration, substitution, and even extinction of species can potentially be inferred from phylogenetic structure, the fossil record is a direct and rich source of data needed to discover the biological principles underlying the origin, maintenance, collapse, recovery, and extinction of species. Paleobiological time series leading to the present day, together with time series embedded entirely in the geologic record provide opportunities to identify and test fundamental ecological principles underlying biodiversity (see Box 3.2). Isolated but well-situated snapshots and brief time exposures in the geologic record provide additional opportunities. Virtually all the key variables in conventional “macroecological” analysis of species assemblages in the present day—namely, species composition, richness, relative abundance, geographic range, body size and many life history factors, growth or metabolic rates, environmental tolerance, trophic or functional group, speciation and extinction rate—are measurable in the fossil record (see Chapter 2). Moreover, some kinds of information can only be determined using paleobiological data. For example, although net diversification rates can be inferred from species richness in clades and communities without a long historic record, the more useful raw speciation and extinction rates can only be determined from the fossil record. Only data on raw rates can reveal whether the striking variation in richness observed among ecological communities reflects differences in the production of species, in rates of species loss, or some combination of the two. These are qualitatively different dynamics, and call for completely different management strategies. These issues can be addressed by studies in deep time (e.g., Valentine, 1990; Sepkoski, 1998; Jablonski and Roy, 2003) as well as in the late Quaternary (e.g., Colinvaux, 1997; Hooghiemstra, 1997; Willis and Niklas, 2004). Ecological theory and experimentation has led to the recognition that certain species are “community keystones,” whose extinction might have profound and long-lasting effects (Paine, 1966; Chase and Leibold, 2003). However, other studies—including a number of paleoecological studies—suggest that communities can retain fundamental structure and function despite continual turnover in species composition driven by routine geographic range shifts and extinctions (Valentine and Jablonski, 1993; Holland and Patzkowsky, 2004; S.T. Jackson and Overpeck, 2000; Webb et al., 2004). This suggests that there can be considerable interchangeability among species within ecological and functional categories. Are these apparently conflicting perspectives an artifact of scale? Paleoecological studies can take advantage of natural experiments in which community
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change BOX 3.2 Changing Community Structure in Permian Brachiopods Paleobiological data can be used to provide tests of fundamental ecological theory. Permian rocks in west Texas, deposited 275-265 Ma ago, contain well-preserved and well-studied assemblages of brachiopods. More than 850,000 specimens are now housed at the National Museum of Natural History in Washington, D.C. Olszewski and Erwin (2004) tallied these specimens in order to determine the species abundance distributions and how these distributions changed through the 10-million year interval. Maximum likelihood fits to the species abundance curves (shown below) were closer to the zero-sum multinomial distribution predicted by Hubbell’s (2001) neutral model of ecological communities than to the classic log-normal distribution. The shapes of the distributions of the lower two intervals differ significantly from those of the upper two intervals, and these differences are attributed to restrictions in population size, decreased isolation, and decreased chances of immigration resulting from lower sea levels in the upper two intervals. Accordingly, the geologic record of Permian fossils in this area preserves the results of a natural experiment on how environmental change affects ecological community structure over long time periods. composition has changed with range shift or extirpation to assess this question. Similarly, little is known about the timescales and patterns of survival in remnant populations and the consequences for community structure. Is richness conserved? Do new species substitute for lost ones? Are functional groups thinned proportionally? Understanding these dynamics in terms of general principles, and at the timescales at which
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change Species abundance distributions of brachiopods from Permian rocks of west Texas. Youngest time interval (Upper Word) on top, oldest interval (Cathedral Mountain) on bottom; S = number of species. The red line is maximum likelihood fit to log-normal distribution; blue line is maximum likelihood fit to zero-sum multinomial distribution. SOURCE: Olszewski and Erwin (2004); used with permission. species migrations and community turnover actually occur, will contribute significantly to our understanding of individual species behavior and the net outcomes for biodiversity and community structure in habitat fragments—an understanding that has obvious practical implications for management of natural systems in the face of human activities and impacts.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change One reason why migration has emerged as such an important species response to environmental change is that species often exhibit evolutionary stasis, and are insufficiently malleable in an evolutionary sense to adapt to radically altered conditions. The fossil record clearly indicates pervasive evolutionary stasis in diverse groups in the face of substantial environmental change (Huntley et al., 1989; Coope 1995; J.B.C. Jackson and Cheetham, 1999; J.B.C. Jackson and Johnson, 2000). However, evolutionary responses have been documented in several cases (e.g., Smith et al., 1995; Rousseau, 1997; Benton and Pearson, 2001). What permits or drives some species to depart from stasis and thus adjust to changing physical and biological environments? Do the evolutionary responses observed represent expansion into new niche space or redistribution of existing variation (Huntley, 1999; S.T. Jackson and Overpeck, 2000)? These questions are particularly pressing in the tropics, where many species are linked in close ecological partnerships. What intrinsic biological or environmental factors promote the coevolution of “mutualists”1 or of producers and consumers? Paleoecological records suggest that coevolutionary adjustments may have been very rapid, given the rapid pace of Quaternary climatic change and community response documented in the tropics, but it is also possible that the coevolutionary associations observed today comprise a limited subset—those where the partners happened to migrate together over repeated glacial/interglacial cycles. These alternative explanations suggest radically different potentials for the regeneration capacity of coevolutionary partnerships that have been disrupted. This can only be tested—using the fossil record—by determining the antiquity of such partnerships. Although molecular data can provide information on the age of taxa, their mutual readjustments can only be documented using morphological and paleobiogeographic data from geohistorical records (e.g., Labandeira, 2002). Rules of Extinction. When the rate or magnitude of the environmental change exceeds the ability of a species to adapt or migrate, the result is local, regional, or global extinction. Because global extinction is irreversible and because even local extinction can remove key genetic resources and severely perturb communities, one fundamental aim of conservation is to minimize extinction or at least to minimize its effects. Consequently, a high priority for managing the present day biota is a set 1 Mutualists are organisms or species that are involved in “mutualistic” interspecific relationships in which both species benefit. Examples are plants and pollinators (plants feed insects in exchange for pollen transport), and mycorrhizae (fungi provide hard-to-get dissolved minerals in exchange for sugars from plant root).
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change of predictive rules for species extinction and survival—both the overall rates (what proportion lost?) as well as patterns of selectivity (which species or kinds of species, which habitats, which precursor states?). In a given situation, these rules probably vary according to the magnitude and kind of disturbance, the intrinsic characteristics of the species involved (e.g., reproductive rates, geographic range sizes, niche parameters), or the original richness, spatial extent, and productivity of the community (i.e., the ecological context). The fossil record affords an opportunity to test these factors across a range of extinction intensities and drivers. Some commonalities are beginning to emerge among taxa and across time intervals (e.g., the effect of geographic range, body size, relative abundance; see Jablonski, 1995; McKinney, 1997; Purvis et al., 2000; Manne and Pimm, 2001; Harcourt et al., 2002), but more comparative work is needed to expand both taxonomic and ecological coverage, and to test whether survivorship patterns change qualitatively with disturbance intensity or type. This is an area where modeling efforts and simulations, through collaborations between paleontologists and ecologists could be particularly productive. Such models could act as a spur and guide for paleontological field work to iteratively refine the models. Determining the characteristics of demonstrably resilient systems or groups, particularly in relation to biotic or environmental crises, will be critical for effective management of diversity as a whole. Resilient groups may not have characteristics considered desirable from a human perspective. Community Structure: Unity, Anarchy, or Both? Geologic records of different ecosystems and time periods often yield contrasting views of community unity and integrity. Paleobiological records indicate that terrestrial and temperate marine communities did not migrate as cohesive, integrated units in response to Quaternary environmental changes. Instead, species shifted their geographic ranges individualistically, producing species associations that do not occur today (Box 3.3). This fluid pattern of community assembly and disassembly was unexpected by ecologists, and speaks to fundamental questions of the inertia of community structure, the strength and particularity of biotic interactions among species, and the likely consequences of species extinctions and invasions for community resilience. Some systems, however, such as tropical coral reefs, show greater stability of community composition during the Quaternary (J.B.C. Jackson, 1995; Pandolfi, 1999; Pandolfi and Jackson, 2001), and many pre-Quaternary paleobiological studies suggest long intervals of community stability (Brett et al., 1996; Ivany, 1996; Schopf and Ivany, 1998). Assessment of the origin of this variation—whether it stems from the nature of the physical environment (S.T. Jackson, 2000; S.T. Jackson and Overpeck, 2000), differential resilience of communities to
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change Populations, communities, ecosystems, and the biosphere all have “memory;” they carry vestiges of past events and trends, owing to delayed response to forcings or perturbations, persistence of physical entities, and the contingent nature of many ecological phenomena on previous states or events. This “ecological memory” is encapsulated in the concept of ecological legacies, defined as current properties of an ecological system that can be explained only by events or conditions that are not present in the system today (see Box 3.11). Thus, many features of modern systems, from the absence of large size classes in coastal fish populations to the high fire risk in many western forests to the relatively high pre-industrial methane concentrations in the earth’s atmosphere, may all be regarded as legacies of past societal activities (respectively, fishing, fire prevention, and rice cultivation) (J.B.C. Jackson et al., 2001, Swetnam et al., 1999; Ruddiman and Thomson, 2001). We can neither understand nor manage the ecological systems of our planet adequately without determining which features are ascribable to natural processes and patterns and which are ecological legacies of societal origin. The geologic record can and must play a critical role in assessing such legacies. In nearly all cases, geohistorical records provide the only source of information on ecological history that extends to pre-human (i.e., true baseline) conditions. Opportunities for Scientific Advance Geohistorical records can contribute to understanding of human impacts and legacies in innumerable ways, from individual lakes, wet-lands, and forest stands to regional, continental, and global scales. Particular demands will arise at the local level as resource managers and restoration ecologists come to appreciate the need for historical information and context (Egan and Howell, 2001). Greatly improved understanding and application can come from studies that integrate ecological, paleoecological, and archaeological approaches to: discriminate effects of human activities from those of climate variability; assess the magnitude and extent of impacts on specific ecological systems under different societies, cultures, and technologies; determine how local effects of human land use and habitat alteration scale up to aggregated effects across heterogeneous regions, including entire catchment basins; exploit opportunities to use past or ongoing human activities (e.g., top predator removal, landscape alteration, nutrient enrichment) to evaluate fundamental ecological theory;
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change BOX 3.11 Ecological Legacies The legacy concept in ecology originated from the observation that ecosystem recovery following large natural disturbances (e.g., the 1980 Mt. St. Helens eruption and the 1988 Yellowstone fires) was influenced by organisms that survived the disturbance as well as physical and biotic structures (windthrow mounds, dead trees, soil carbon, coarse woody debris) that persisted from the previous community (Franklin and Halpern, 1989; Foster et al., 1998). The concept has since been expanded to incorporate any property of an ecosystem that is attributable to a past event (e.g., a wildfire) or system state (e.g., a previous community at the site). For example, age structure of a forest or woodland stand may be linked to a disturbance event centuries ago (Swetnam and Betancourt, 1998; Frelich, 2002), soil structure of a forest stand may be attributable to occupancy of the site by prairie vegetation several thousand years ago, and nitrogen reservoirs in Antarctic lakes may be attributable to high lake levels of the Pleistocene (Moorhead et al., 1999). Human activities, including management practices, also impose legacies across a wide range of spatial scales (Wallin et al., 1994; Swetnam et al., 1999; Foster et al., 2003). Ecological systems carry these historical signatures for a variety of reasons. First, ecological processes occur across a wide range of timescales, so one component of a system may change rapidly in response to disturbance or environmental change while another may change very slowly. For instance, a crown fire kills most trees and burns most fine fuels in a forest stand within minutes, while coarse woody debris and soil carbon can persist for decades after the fire (Foster et al., 1998). Plant species composition can change in response to climate change within decades, while soil carbon reservoirs may require centuries or millennia to respond. Second, ecological systems can leave a strong imprint on the environment that may persist long after the originating system has disappeared or been replaced by another. For example, vegetation influences the physical and chemical structure of soils, and many species, ranging from prokaryotes to trees and mammals, have bioengineering properties, creating landforms that can persist for decades to millennia (Jones et al., 1994; Crooks, 2002). Third, many properties of biological systems are contingent on history. The genetic structure of populations contain overprints of population history (Hewitt, 2000; Petit et al., 2003), age structure of populations is often governed by previous mortality, recruitment, and/or dispersal events (Swetnam and Betancourt, 1998), and soil nutrient reservoirs are influenced by the climatic and vegetation history of the site. Human land use imposes legacies in soil and vegetation that can persist for hundreds to thousands of years (Motzkin et al., 1996; Dupouey et al., 2002; Foster and Aber, 2004). The concept of ecological legacies provides a vital link between ecology and the historical sciences.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change identify the differential sensitivity of ecosystems to persistent legacies of human activities; and address human legacies that affect species, habitats, and ecosystems of particular management concern and societal value. The kinds of systems and timescales at which geohistorical records can be applied to these problems are legion. Humans affect ecosystems in a variety of ways, ranging from inadvertent introduction of diseases and invasive organisms to direct utilization of native species to appropriation of entire habitats for conversion to agricultural or urban landscapes. Hunter/fisher/gatherer societies have different impacts from pastoral/agricultural societies, which differ again from industrial/urban ones. There is considerable diversity among cultural groups within each of these broad categories. And impacts of any given culture should be contingent on the local ecosystems and the prevailing environment, including climate and climate variability. All ecological studies and management efforts should be preceded or accompanied by serious investigation of the extent to which system properties represent legacies of past human activities (Hamburg and Sanford, 1986). Well-focused, integrative studies of the kind described above will put us in position to use the past to manage for the future. Natural Variability and Shifting Baselines Environments and biotas vary naturally around some ever-shifting mean value. The magnitude of this variation depends on characteristics of the environment as well as the frequency, duration, and temporal resolution of observations. Documenting this natural variability is important because the demonstration of human impact requires that we separate the “signal” of human action from the “noise” of non-anthropogenic variation. Assessing the natural range of variability requires geohistorical perspectives because observational environmental and ecological records do not nearly encompass the full range of natural variability over the past few centuries, and there is abundant evidence for human impact on the environment throughout the entire observational record. Similar problems plague all ecological investigations because no living systems can be considered pristine. Completely “natural” systems (in the sense that they are unaffected by the consequences of human activity) are only available in the geologic record. Fortunately, as described earlier, the quality of environmental and biotic information in the record often allows the reconstruction of environmental variation with seasonal and annual resolution. A common problem in measuring human impact is what fisheries biologist Daniel Pauly (1995) called the “Shifting Baseline Syndrome.”
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change Scientists and the general public alike tend to define a “natural” or “pristine” baseline as the way things were either when they were first observed, or from descriptions of the very recent past. Human impacts are then measured from this baseline of personal, short-term experience, despite the likelihood of prior human-caused change. For example, in fisheries, each new generation of fishers tends to resist limits to their catch because they consider the present day catch as the norm, rather than the enormous catches of the previous generations. The now-classic example concerns codfish from the Gulf of Maine—for the 5,000 years leading up to the 20th century, the average size of fish caught and eaten was about 1 m, whereas the average size steadily decreased in the 20th century (J.B.C. Jackson et al., 2001; see Figure 3.2). Ignorance of this change allowed continued fishing when the average size had already dropped by two thirds in the 1970s and 1980s, leading to the subsequent total collapse of the fishery. We can still unambiguously define an environment and its associated biota as pristine based on the way things were at that place—plus or minus the natural range of variation—immediately before the first arrival of modern Homo sapiens. This admittedly discounts the environmental effects FIGURE 3.2 Time series of mean body length of Atlantic cod from kelp forests in the coastal Gulf of Maine. The earlier five data points are derived from archaeological records, whereas the last three points are from fisheries data. SOURCE: J.B.C. Jackson et al. (2001); used with permission.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change of pre-modern humans in Africa before 150 Ka, but for our purposes here we can probably dismiss these effects as relatively minor. All other definitions of pristine depend on more arbitrary assessments of a lack of “significant” human impact, despite the presence of human populations, and are subject to the problem of shifting baselines noted above. The value of recognizing the pristine condition of an environment and biota is that it provides a basic frame of reference to assess all subsequent human and natural environmental change. Most interestingly, such true baselines allow us to ask the question: “What was the world like just before the arrival of modern humans?” Of course, the first arrival of people around the world was not synchronous, and required almost the entirety of human existence from first origins in Africa approximately 150-100 Ka. Very approximately, modern humans arrived in Europe and Asia between about 100-50 Ka, in Melanesia and Australia about 60-40 Ka, in the Americas about 20-12 Ka, in Polynesia 2 Ka, and less than 1,000 years ago in New Zealand and Madagascar. This delayed arrival of people in progressively remote locations from Africa provides a powerful tool to distinguish between changes in climate and human impacts as causes for biotic change. Assessing Human Impact A reliable frame of reference needs to be established for evaluating human impacts over many timescales throughout the Holocene. Human land use and habitat alteration have left signatures on the landscape and in geohistorical records dating in many cases to the first human colonizations. Sedimentary charcoal indicates increased incidence and extent of wildfires upon colonization of Australia by aboriginal hunter-gatherers ~40,000 years ago. Hunter-gatherer societies throughout the world have used fire as a tool to manipulate habitat, with effects on vegetation composition and structure detectable in geohistorical records at local to regional scales (e.g., McGlone, 1989; Burney, 1999). Agricultural societies have had even greater impacts, converting natural vegetation to agricultural lands. Such conversions in the prehistoric record have ranged from transient and local (e.g., Iversen, 1973; McAndrews, 1988; McAndrews and Boyko-Diakonow, 1989) to persistent and regional (Berglund, 1992; Binford et al., 1987; Brenner et al., 2002). Societies with large populations and advanced technologies, both agricultural and industrial, have tended to have even greater and more persistent effects. Because aquatic systems accumulate sediments and are influenced by land use in surrounding catchment basins, they are particularly effective recorders of human impacts. Lakes accumulate sediments continuously and thus are especially effective recorders of human activities in the sur-
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change rounding watershed (e.g., Smol, 2002; Alin et al., 1999, 2002; Cohen, 2003) as well as distant regions. For example, paleontological and geochemical studies of lake sediments in the Adirondacks and other regions provided critical confirmation that regional ecosystem acidification was a result of mineral acids from distant industrial sources (Charles et al., 1990, 1994; Battarbee and Charles, 1994). Signatures of human activities also show up in fluvial, wetland, and estuarine sediments, even though deposition may be irregular or discontinuous. Studies of estuarine sediments have revealed the extent to which coastal ecosystems have been altered by human agricultural, industrial, and fisheries activities (J.B.C. Jackson et al., 2001; Curtin et al., 2001; Cooper and Brush, 1991; Brush, 1997). Sediments of lakes and peat bogs have been used to document onset of metal mining and smelting (e.g., Shotyk et al., 1998; Martinez-Cortizas et al., 1999), providing biogeochemical baselines. Human impacts on Holocene environments and biotas are generally much more closely related to cultural, technological, and economic attributes of societies than to the simple presence of humans (see Box 3.12). This is often most clearly evident from comparisons of human activities and their consequences during the rapid economic transitions associated with colonial occupation (e.g., the Americas before and after European conquest and colonization, Australia before and after British colonization). Other critical transitions include the advent of intensive agriculture, industrialization and resource exploitation based on fossil fuels, and transitions from local to regional to global markets. Documentation of human-caused changes in the distribution, abundance, and size of species and the distribution and composition of communities offers the potential for reconstructing the structure and function of natural communities. This is fundamental for understanding the adaptations and limits of species that now inhabit strikingly different habitats than those lived in previously. Some possibly general patterns of the consequences of human activities have been identified, but require considerable further examination in a wide variety of settings: the loss or ecological extinction of megafauna; shortening of food webs; distortion of biogeochemical cycles; shifts from heterogeneous to homogeneous biogeographic distributions due to invasive species; general loss of biodiversity; and replacement of eukaryotes in the oceans by bacteria and archaea. As pointed out by Margalef (1968), all these processes may act to reverse ecological succession while at the same time increasing produc-
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change BOX 3.12 Prehistoric (and Historic) Impacts of Humans on Polynesian Faunas Islands of the central tropical Pacific—Polynesia—have been invaded by two successive waves of human culture. The prehistoric Polynesian (or Lapita) culture swept westward across the region beginning about 4,000 years ago, reaching the most remote islands (Hawaii, Easter Island, New Zealand) in the past 1,000-2,000 years. The Polynesians cultivated imported crops, tended domesticated livestock (pigs, chickens), and foraged for wild plant and animal foods. They also imported dogs and rats. The late 18th and 19th centuries saw secondary colonization and conquest of the Pacific islands by European and Euro-American cultures. On the larger islands (e.g., Hawaiian islands, New Zealand), these latter cultures implemented extensive and intensive grazing (cattle, sheep) and cultivation (sugar cane) in the 19th century, as well as imported a wide variety of non-native plants and animal species. Paleontological studies provide rich records of the impacts of these activities on native fauna and flora. Easter Island saw particularly devastating impacts of Polynesians, with deforestation leading to extinction of all the native trees (Dransfield et al., 1984; Flenley et al., 1991). Native bird communities throughout Polynesia underwent rapid species extinctions as the Polynesians arrived; as many as 2,000 species of birds, comprising some 20 percent of global avian diversity, may have disappeared during the Polynesian invasions (Steadman, 1995). A recent integrated paleontological excavation on the island of Kauai (Hawaii) illustrates the severity of the losses associated with Polynesian activities, as well as the impacts of the second, European, colonization wave (Burney et al., 2001). Native species of crabs, marine mollusks, land snails, and birds that survived the Polynesian invasion were exterminated during the 19th century (see figures below). Pollen and macrobotanical data indicate that plant species that are now rare and restricted to remote montane sites were widespread in the coastal lowlands before human colonization. Few of the native vertebrate and invertebrate animals recorded in the pre-Polynesian sediments remain today. As Burney et al. (2001) observed, the paleontological record at their site “documents the purloined riches of a truly lost world.”
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change Abundance of fossil snails in sediments of the Mahaulepu Sinkhole, Kauai. Note apparent extinctions of at least five native species during the Polynesian period, extinction of six species since European and Euro-American colonization, survival of three native species, and introduction of one exotic species. SOURCE: Burney et al. (2001); used with permission.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change Occurrence of vertebrate bones in sediments of the Mahaulepu Sinkhole, Kauai. Note the severe loss of native bird species (11 species portrayed are extinct on the island, as are other species not on this diagram), Polynesian introductions of rats (Rattus), fowl (Gallus), pigs (Sus), and dogs (Canis), and European introductions of rats and toads (Bufo). SOURCE: Burney et al. (2001); used with permission.
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change tivity. Although this increases food supplies on land in the case of agriculture, the process greatly decreases food supplies in the ocean because microbes and jellyfish come to dominate the resulting communities. Simple food-web models can be employed to estimate the standing crops of species at different trophic levels using historical data for estimated abundances of apex predators that have greatly declined before ecological observations began (Christensen and Pauly, 1993). Historical data describing the composition and structure of communities offer powerful but under utilized tools for estimating the ecosystem consequences of human impacts. Historical Ecology and Restoration Ecology Truly pristine pre-human conditions may not provide realistic targets for resource management or ecosystem restoration. Such pristine conditions have not occurred more recently than centuries to thousands of years ago, depending on the ecosystem and its location, and the world is already in a different climate regime. Nevertheless, they are an essential frame of reference for evaluating the natural geographic ranges and habitat distributions of species, their characteristic size frequency distributions, and a host of other parameters that are the essentials of basic ecological research. Furthermore, such fundamental baselines provide goals for ecological restoration, to the extent that prevailing climatic conditions allow. And by allowing us to differentiate natural phenomena from legacies of human activities, and to determine the specific human activities to which particular legacies are attributable, they provide a sounder basis for determining realistic and appropriate management and restoration targets. Knowing whether a particular vegetation pattern or disturbance regime is attributable to natural conditions, pre-European Native American land use, or Euro-American impacts, for example, is an essential first step in management decisions. Decisions ultimately rest on a number of social, scientific, economic, and political considerations, but historical knowledge can provide more informed judgments. As an example, pinyon pines (Pinus edulis) do not grow at Chaco Canyon National Monument today. Yet paleontological studies indicate that they occurred there naturally from ~6,000 years ago until the populations were exterminated by Anasazi fuel harvesting 1,000-800 years ago (Betancourt and van Devender, 1981). This information complicates, but also enriches, decisions. One of the greatest values of historical data describing ecological conditions before substantial human impacts is the formulation of goals for ecological restoration. This is particularly important in the case of habitats and ecosystems that were not described or monitored before human disturbance. Although much restoration can be guided by conditions in
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The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change nearby, relatively undisturbed settings, there is an increasing appreciation for the value of historical data from both documentary evidence and natural biotic and environmental archives (i.e., the proxy data described in Chapter 2) (e.g., Brenner et al., 1993; Brush, 1997). Indeed, for long-lived species such as trees, the long temporal framework provided by tree ring analysis is essential for understanding such important phenomena as fire frequency and migration rates (Swetnam et al., 1999). This historical approach to restoration has now matured to the point where techniques for reconstructing reference ecosystems and their historic range of variation are well established for a variety of terrestrial and coastal ecosystems (see Egan and Howell, 2001). The application of historical ecology to issues of ecosystem restoration and management has enormous potential for both basic and applied research. In basic research, the integration and cross-calibration of observational, documentary, and proxy ecological data provides both a near-continuous and long-term ecological record as well as the opportunity to develop confidence limits for proxy indicators farther back in the geologic record. In applied research, the approaches of historical ecology demand close collaboration among biologists, geologists, and archaeologists engaged in ecosystem management and restoration—practical issues that are an increasingly important focus of the U.S. Geological Survey.
Representative terms from entire chapter: