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Review Conservation paleobiology: putting the dead to work Gregory P. Dietl1 and Karl W. Flessa2 1 2 Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, NY 14850, USA Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA Geohistorical data and analyses are playing an increasingly important role in conservation biology practice and policy. In this review, we discuss examples of how the near-time and deep-time fossil record can be used to understand the ecological and evolutionary responses of species to changes in their environment. We show that beyond providing crucial baseline data, the conservation paleobiology perspective helps us to identify which species will be most vulnerable and what kinds of responses will be most common. We stress that inclusion of geohistorical data in our decision-making process provides a more scientifically robust basis for conservation policies than those dependent on shortterm observations alone. What is conservation paleobiology? Conservation paleobiology is a relatively new, synthetic field of research that applies the theories and analytical tools of paleontology to the solution of problems concerning the conservation of biodiversity [1]. The primary sources of data are geohistorical in nature: the organic remains, geochemical signals and associated sediments of the fossil record [2]. The conservation paleobiology perspective has the unique advantage of being able to identify phenomena beyond timescales of direct human experience. Such data are crucial for acquiring a long-term perspective on modern systems, which is needed in order to develop more effective conservation policies in the face of an uncertain future. Conservation paleobiology involves two complementary approaches. The first (near-time) approach uses the relatively recent past (the last few million years) as a dynamic context for present-day conditions. The second (deep-time) approach, ‘equally valuable but less systematically pursued’ [2], takes advantage of the entire history of life as a natural ecological and evolutionary laboratory. This latter approach sets the field apart from historical ecology, its sister discipline (Box 1). Here, we review examples illustrating how the neartime and deep-time fossil record can be used to understand ecological and evolutionary dynamics. We build on previous studies where geohistorical data have proven valuable to conservation research. Although the near-time fossil record of ecological dynamics has attracted the most attention to date from conservation paleobiologists, we also discuss how knowledge of the deep-time fossil record of evolutionary dynamics can contribute to the conservation of biodiversity. Our message is that the perspective provided by geohistorical data is essential for the development of successful conservation strategies in the midst of a constantly changing environment. The nature and value of geohistorical data In 2005, the U.S. National Research Council (Committee on the Geologic Record of Biosphere Dynamics) acknowledged the key role that geohistorical data have to play in addressing current biodiversity problems (Figure 1). And yet, conservation-related research rarely employs such data [3]; the majority of studies within the conservation biology literature still focus on very short timescales ranging from years to decades [4]. Limited use of geohistorical data by many conservation biologists might reflect both a lack of knowledge about the availability of geohistorical records, reluctance to use (or trust) data that do not come from well-controlled and replicated experiments [5] and uncertainties about the adequacy of such information. These uncertainties commonly center on precision and accuracy in inferring past environmental conditions and on temporal resolution [2]. However, although the fossil record is incomplete, valuable insights can be gained even from isolated samples that are well positioned before and after significant biotic or abiotic events. For instance, a study on the frequency of insect damage to fossil angiosperm leaves from the Bighorn Basin of Wyoming, dating from before, during and after the Paleocene-Eocene Thermal Maximum (PETM, 55.8 Ma), one of the best deep-time analogs for our current global climate change problems [6], suggests that insect herbivory intensified during that episode of global warming. This finding provides insight into how the ongoing human-induced rise in atmospheric CO2 is likely to affect insect–plant interactions Glossary Conservation paleobiology: the application of theories and analytical tools of paleontology to biodiversity conservation. Ecological legacies: ecological properties attributable to past events. Environmental proxy: indicators based on physical, chemical, or biological features of the geologic record that capture environmental information in indirect form. Geohistorical data: the individual stratigraphic sections, sediment or ice cores, tree ring series, fossil collections or specimens that provide temporal, environmental, and ecological information [2]. Historical baselines: reference conditions against which current changes can be assessed. Natural range of variability: the variation of ecological and evolutionary patterns and processes over time and space. Time-averaging: accumulation of fossil material over a span of time. Corresponding author: Dietl, G.P. ([email protected]). 30 0169-5347/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2010.09.010 Trends in Ecology and Evolution, January 2011, Vol. 26, No. 1 Review [()TD$FIG] Trends in Ecology and Evolution Box 1. Relationship between conservation paleobiology and historical ecology Conservation paleobiology shares its philosophical foundations with historical ecology, which Swetnam et al. [85] defined as ‘the use of historical knowledge in the management of ecosystems’. Data sources in historical ecology are derived from written records, long-term ecological monitoring, archaeology and paleoecology. Balée [86] later offered a more narrowly focused definition of historical ecology as a ‘research program concerned with comprehending temporal and spatial dimensions in the relationships of human societies to local environments and the cumulative global effects of these relationships’(for a similar view, see [87]). Time scales in historical ecology studies are typically limited to relatively recent time intervals, geologically speaking (i.e., the Pleistocene). In addition, historical ecology concentrates on ecological dynamics (e.g., changes in species distribution and abundance). Two perspectives that distinguish between the fields are: (1) the temporal scope of conservation paleobiology also extends to the prePleistocene record; and (2) conservation paleobiology, in addition to ecological dynamics, concentrates on evolutionary dynamics (e.g., adaptive responses of species to changing climates or ecological interactions). in the long run, which is difficult to predict from short-term studies that have shown highly species-specific responses [7]. In addition, time-averaged information, as typically captured by the geological record, has been shown to be extremely useful in disentangling natural changes from those induced by human activities. For instance, the frequent mismatch between the composition of time-averaged death assemblages of mollusks and local living communities has been shown repeatedly to be reliable evidence for human-induced ecological change in marine habitats worldwide [8,9]. A growing list of indicators, based on physical, chemical or biological features of the geological record (Figure 2), also captures environmental information in indirect or proxy form. For example, the shells of mollusks typically carry several kinds of proxy information: taxonomic identity, body size, growth patterns (e.g., growth rings), ecological interactions (e.g., predation trace fossils), geochronological estimates and geochemical signals (e.g., stable isotopes of oxygen and carbon; see examples in [10]). Multiproxy methods are increasingly used, thus exploiting the complementary strengths of each proxy [11]. The absolute age dating of geologic materials and time series has also improved greatly over the last 25 years [12]. In particular, the high-resolution data available in the most recent 100,000 years have proven to be an excellent source of historical baseline information on species and ecosystems [13]. High-resolution radiocarbon and amino acid racemization dates are now routinely available at modest costs while tree-rings, sclerochronology and varved sediments often provide annual or sub-annual resolution ([14–16]). Baselines and natural range of variability Baselines, or reference conditions that show no discernable human influence against which current changes can be compared, are a fundamental concept in conservation paleobiology. How deep (in other words, how far back in time) a baseline reference needs to be depends on the question. For instance, van Leeuwen et al. applied geohistorical data to identify invasive alien species [17]. The fossil pollen and January 2011, Vol. 26, No. 1 Modeling Information integration Geohistorical data Experimental/ observational data TRENDS in Ecology & Evolution Figure 1. Tripartite data integration. Approaches to conservation based on geohistorical data are certainly not a substitute for modern-day observational and experimental data or modeling. Instead, each source of information benefits from the other’s strengths. Therefore, the ideal relationship is one of reciprocal interaction (arrows) to increase our ability to predict (or develop scenarios of) possible biotic responses to global environmental changes in an uncertain future. The position of the inner circle within the ternary diagram reflects the relative dominance of each approach. The current relationship is heavily biased toward models being parameterized from modern-day observational and experimental data (i.e., the inner circle is positioned closer to the side of the ternary diagram opposite the geohistorical data apex). For instance, attempts to forecast the impact of global climate change on species have often relied on bioclimate envelope models, in which modern-day observed patterns of species distribution are combined with environmental variables (typically climatic and other physical variables) to predict distributions of species under future climate scenarios [90]. The critical assumptions in these models is that climate exerts a dominant role in governing species distributions and that the occurrence–climate association (ecological niche) of individual species does not change. The validity of these assumptions is currently hotly debated (e.g., see [90–92]). A more integrated approach (i.e., a centrally positioned inner circle) would also consider past distributions of species across a broad range of environmental conditions outside modern experience. When this type of integration has been attempted, the latter assumption that ecological niche characteristics are conserved has had mixed results. For instance, Martı́nez-Meyer and Peterson in a study of 23 extant North American mammals and their Pleistocene fossil records, showed that for nine species the model was able to predict the Pleistocene species distributions from the modern-day data [93]. The distributions of the remaining species, however, were not predicted accurately by the model. These results highlight aspects of a species’ relationship with its environment that are not apparent from modern observations alone, underscoring the value of an integrated approach in conservation. plant record of the Galápagos Islands shows that at least six putative non-native or doubtfully native species were in fact present before the first arrival of humans. This baseline information is crucial because a current conservation priority in the Galápagos Islands is the removal of invasive species [17]. Van Leeuwen et al.’s results thus highlight the importance of baseline data as well as the risk involved in applying management practices that use inappropriate baselines. The idea of using baselines is also the cornerstone of the field of restoration ecology (and historical ecology; Box 1), with baselines typically defined as conditions found before European colonization or before industrial humans [4]. It is often not appreciated, however, that even a few hundred years ago the human footprint on the planet was widespread [18,19]. In other words, we often base our conservation efforts on data derived from already degraded ecosystems. As Jackson reminds us, we need to be cautious 31 ()TD$FIG][ Review Trends in Ecology and Evolution January 2011, Vol. 26, No. 1 [34] and Avila-Serrano et al. [35] were able to compare present-day benthic productivity conditions in the Colorado River estuary with the range of variation in the system over the past 1000 years. Their geohistorical analysis demonstrated a dramatic decrease in benthic productivity (as estimated by the abundance of bivalve mollusks) in the estuarine ecosystem after the diversion of the Colorado River for human uses (Box 2). Carrasco et al. also showed that shortly after humans arrived in North America, mammalian diversity dropped drastically relative to the normal diversity baseline that had existed for millions of years [36]. These examples (and many others like them) highlight how the use of geohistorical data can distinguish human impact from natural variation (or noise) in the system. Figure 2. Examples of proxy indicators. Environmental proxies may be based on physical (lithological), chemical (geochemical) and individual fossil and fossil assemblage (biological) features of the geologic record. Some proxies have utility throughout much of the geological past (e.g., oxygen isotopes), while others are limited to relatively recent time intervals and conditions of exceptional preservation, such as ancient DNA (see Table 2.1 in [2] for a more complete list of proxies and their interpretations). ‘when we make strong statements based on ecological calculations and insights derived from degraded, remnant ecosystems. . .’ [20]. The most striking example concerns the extinctions of the Pleistocene vertebrate megafauna (animals greater than 44 kg (100 pounds) [21,22]) in North America and other regions of the world; an event that coincides with the first appearance of humans across the landscape. While a human cause for extinction remains debatable [21], undoubtedly the loss of these large, strongly interactive species has had lasting ecological and evolutionary consequences [23,24]. There is also overwhelming evidence that today many top consumers, such as whales [25,26] and sharks [27], have either been eliminated or depressed in their numbers to the point where these animals no longer fulfill their earlier ecological roles in structuring ecosystems [28,29]. Predicting ecological consequences of these declines in abundance remains a challenge [30], which is made even more difficult because ‘many ecologists fail to appreciate the prior existence of strong, stabilizing interactions’ in the systems that they study [31]. Insights gained from geohistorical baselines provide a needed but largely untapped context of what is possible in setting conservation or restoration goals for these ecologically important species. It is typically acknowledged that the reference to baselines should take into account the dynamic nature of ecological systems. Thus, another important concept in conservation paleobiology is the natural range of variability [32,33], which is often estimated so as to better understand the bounds or envelope around temporally dynamic patterns and processes. For instance, Kowalewski et al. 32 The geologic record as a natural ecological and evolutionary laboratory Another fundamentally important role of geohistorical data is to provide access to a wider range of past environmental conditions (alternative worlds of every imaginable circumstance) to address the ecological and evolutionary dynamics of species. In other words, the geohistorical record is a natural laboratory from which we can address the responses of species to environmental changes, helping us to understand which species will be most sensitive and what kinds of responses will be most common. There are three main ways in which species respond to changes in their environment: (1) they can move, tracking environmental changes; (2) they can stay put and adapt to the changing environment; and (3) they can fail to track habitats or to adapt, thus becoming extinct. In this section, we review select examples of how geohistorical information (derived from a variety of proxies; Figure 2) can inform current trends and predict the responses of species to a changing global environment in an uncertain future. These examples are meant to be illustrative (but not exhaustive) of the potential applications of conservation paleobiology (for additional examples, see [1]). Geographic range shifts Shifts in species ranges are a predicted and inevitable consequence of global climate change [37,38]. The underlying assumption is that climate change causes organisms to migrate to (or track) habitats to which they are already adapted. Predicting changes in species distributions under different scenarios of global climate change is thus a major agenda for many conservation organizations. We need to know whether and where species will move in response to future climate change [3]. Our understanding of what happens to species distributions as climate changes is greatly informed by geohistorical data [39,40]. Only geohistorical data can indicate where species occurred in the past outside their present range limits. There are now several examples of geographic range shift in response to the contraction and expansion of glaciers during the Pleistocene. For instance, Greenstein and Pandolfi detailed how reef-building coral species from Western Australia have responded to climate change since the Late Pleistocene [41]. They found that many coral species (particularly acroporids) contracted their ranges Review Trends in Ecology and Evolution January 2011, Vol. 26, No. 1 Box 2. Historical baseline for the Colorado River Delta The Colorado River no longer reaches the sea. Since 1960, its water has been diverted to cities and farms in the U.S.A. and Mexico. There is no scientific survey of the area before human impact. The upstream dams and aqueducts that divert the river from the wetlands, floodplains and estuary the river once supported were completed well before the implementation of laws requiring consideration of environmental impact or endangered species. Old photographs, maps and travelers’ accounts (e.g., [88]) provide a qualitative measure of the natural ecosystems that have been lost, but the shells and fish otoliths in natural accumulations, middens and museum collections provide a quantitative record of the delta’s decreased productivity, altered composition, changes in trophic structure and depressed growth rates. High-precision radiocarbon and amino acid dating, analysis of growth increments in shells and otoliths, and stable isotope analysis of biogenic carbonates enable a remarkably detailed look at the Colorado River’s estuarine ecosystem in the era before upstream dams. Studies summarized by Calderon and Flessa [89] document the ecological impacts of upstream water diversions. Shell accumulations (Figure I) dating from before the upstream diversions were made, show the following. Population densities of shelly mollusks were up to ten times greater than those of today. A now rare, endemic mollusk once flourished in the brackish waters of the estuary and was an important trophic resource for snails and crabs. In addition, growth banding and stable isotopes in otoliths of a once commercially important but now endangered fish indicate that first-year growth was faster when fresh water reached the river’s estuary, leading to larger populations in the past and that species recovery depends on river management as well as fisheries management. north towards the tropics in response to a drop in sea surface temperatures of at least 2 8C since the last interglacial (about 120 ka). Species are not always able to track suitable habitats, however. For instance, Dalén et al. used ancient DNA to show that the arctic fox (Alopex lagopus) in Europe was not able to track shifting climates as its range contracted, eventually becoming extinct at the end of the Pleistocene[42]. However, the species persisted in refugia in northeastern Siberia. Geohistorical records are thus critical in identifying where species survived periods of changing climates (i.e., refugia [43]). For instance, during the glacial intervals of the Pleistocene in Europe, the Balkans, Iberia and Italy served as refugia for many temperate plants, insects and vertebrates [43]. Such information can significantly enhance our ability to understand and predict future effects of global climate change on species distributions, because most ecological models are parameterized from modern-day observations alone (Figure 1) [44]; and so might fail to accurately predict range shifts in response to changing climates in the future. Geohistorical analysis can help locate and assess the persistence of source regions by documenting the patterns of spread of species. Source regions are important conservation priorities because populations from these areas provide recruits for other populations and adaptations that characterize the species as a whole largely originate within them [45]. For example, geohistorical data indicate that the northeastern Atlantic has served as a source region since the Pliocene for many cool-temperate marine species that are found on both sides of the Atlantic [46]. Given the ecological similarities between range shifts and introductions of species [47], it is insightful to look to The environmental impacts of water diversions on the Colorado River’s estuary can no longer be ignored simply because they are undocumented. Abundant, high-resolution geohistorical data have provided crucial documentation and the next step is restoration. Analysis of oxygen isotopes in shells and otoliths suggests that dedicating 5–10% of the river’s annual flow, managed to include an annual pulse flow, could restore the core of this vanished ecosystem. [()TD$FIG] Figure I. Beaches along the Baja California shore of the Colorado River Delta are estimated to contain two trillion shells, evidence of the estuary’s high productivity in the past. (Photo credit: Karl Flessa) the geohistorical record of invasion (basically a history of range expansion) for insights into the likely effects of global climate change on species distributions. For instance, there is growing concern that warming climates will increase the rate of introduction and establishment (whether through human-assisted introduction or natural range shifts) of invasive marine species, particularly predatory species, into the Antarctic [48]. The endemic, shallowwater bottom fauna of Antarctica is functionally unique in the world’s oceans [48,49]. In particular, in terms of structure and function, the Antarctic bottom fauna: (1) lacks fast-moving, shell-crushing predators, such as sharks and crabs; and (2) soft-sediment habitats are dominated by organisms that live on the surface and feed on the plankton [48,49]. This unique food-web structure was established about 41 million years ago in the Eocene as temperatures started to cool and shell-crushing predators were excluded from Antarctic waters [48]. As global climate change continues, the re-invasion of shell-crushing predators, which has already begun [50], will have drastic effects, modernizing the shallow-water bottom fauna and profoundly altering the character of marine life in Antarctica [48]. This deep-time perspective is beyond the reach of modern ecology. Analysis of geohistorical records also suggests that present-day global climate change might lead to processes of large-scale biotic interchange, the spread of many species from one geographic area to another [51]. This process has been a common form of invasion in the history of life and is almost always a highly asymmetrical process, with one of the geographic areas typically serving as the donor biota and the other serving as the recipient biota [52]. Well studied examples include the Great American interchange, 33 Review the spread of species (especially mammals) between North and South America [53] and the Trans-Arctic interchange (the spread of species between the North Pacific and North Atlantic Oceans [54]). Most episodes of biotic interchange coincide with warm climates [51]. Thus, as the world warms, we might enter a new era of biotic interchange, especially at high latitudes [55]. Insights gained from geohistorical analysis of past episodes of biotic interchange provide predictive power for the long-term effects of invasion [56]. Adaptive phenotypic responses Many species today are failing to track global climate changes by shifting their geographic ranges [38,57]. For these species, their potential for adaptive response to new environmental conditions is crucial to their long-term persistence. Those species that are tracking local changes in their environment might also undergo adaptive changes [57–59]. This realization suggests that there is an urgent need to develop practical approaches to managing adaptation (i.e., eco-evolutionary dynamics or contemporary evolution [60–65]). Unfortunately, whereas geographic range shifts in response to global climate change are increasingly well documented [38], adaptive responses are far less well understood. Our understanding of how local adaptive responses influence species’ persistence is hindered by a lack of basic data on which ecologically important traits are most likely to evolve and keep species in the ecological game [66] and whether species will be able to adapt fast enough to keep up with their changing environment [67,68]. Geohistorical records have the unique advantage of providing evidence for how species adapt under a wide range of environmental conditions, unlike those of the present day. For instance, Bruzgul et al. used the late Holocene fossil record of the tiger salamander (Ambystoma tigrinum) from Lamar Cave in Yellowstone National Park, Wyoming, to assess responses in morphology and life history to changes in climate [69]. This species is able to exploit alternative life histories in response to environmental changes by either metamorphosing into a terrestrial adult, or remaining aquatic and retaining a larval (i.e., paedomorphic) morphology [69]. Bruzgul et al. found that A. tigrinum responded to the largest climatic shift in the Yellowstone region over the last 3000 years, the Medieval Warm Period (1150 – 650 ybp), which is characterized by a warm and dry climate, by increasing body size. There was also a consistent ratio of paedomorphic to metamorphic individuals over the duration of the time interval studied, demonstrating that climate changes did not have an effect on all life history characteristics. This study provides timely information for managers on how this sensitive indicator amphibian species will respond ecologically and evolutionarily to predicted future climatic warming in the Yellowstone region. This example also highlights the utility of geohistorical records in helping to outline scenarios [70] of possible complex and uncertain futures. Extinction selectivity The need for long-term geohistorical perspectives on species is especially critical in understanding extinction risks. 34 Trends in Ecology and Evolution January 2011, Vol. 26, No. 1 Applying geohistorical data to modern extinction problems is desirable because habitat loss was also the cause of many species extinctions in the geological past [71]. By using the geological record to examine the filtering effects of extinction, we can better understand what types of species have more favorable odds of surviving under future environmental conditions. As with species invasions and despite the great difference in the scale of observation, geohistorical and ecological evidence largely agree on which traits increase extinction risk [72], such as slow growth rate and restricted geographic distribution. For instance, analysis of bird extinctions from islands across the Pacific Ocean over the past 3500 years shows that complexes of interacting traits, which characterize species ecology (e.g., endemism, body size, diet), are the best predictors of extinction risk through time [73]. It is also increasingly evident that past extinctions act as important filters on current vulnerability [74]. For instance, fossil evidence suggests that birds of the Hawaiian Islands suffered a large-scale extinction event around the time of Polynesian arrival [75]. In a comparison of the ecological characteristics of bird species before and after this extinction interval, Boyer showed a strong extinction bias against larger bodied and flightless, ground-nesting species [76]. This pattern supported a role for direct human hunting in the extinction event. By the time of first European contact in the 18th century (which also led to a second wave of extinction with its own selectivity pattern), most large-bodied species had already disappeared; those that survived had traits that helped them survive both extinction waves. A similar story can be told for other places and species. For instance, rapid and intense Pleistocene climatic change led to few extinctions of western European insect species [77]. Instead, species shifted their ranges, tracking acceptable habitats. As Coope described it: ‘Those species that could not make such an adjustment would have become extinct at the beginning of glacier/interglacial cycles... The species that cleared the first fence of the climatic hurdle race could similarly clear all the rest.’ [77]. This idea of extinction filters suggests that we should expect some species to respond to current climatic changes in the same way as they did to past environmental changes. Geohistorical data thus tell us that we have inherited a world in which there is considerable variation in the vulnerability of species to extinction due to historical contingencies (e.g., ecological legacies from past selective agents [78]). Moving from promise to application Mitigating environmental damage requires that we know what has been damaged; adapting to environmental change requires that we know what to expect. In both cases, geohistorical data are essential to formulating both a general policy and a specific response. But changes in conservation policy rarely occur because of the inclusion of a new set of data. Expecting geohistorical data to make such a difference is thus unreasonable. Instead, moving from promise to application requires the data of conservation paleobiology and the means to translate those data Review into a useful form. In other words, there is an increasing need, shared with other environmental sciences, to demonstrate knowledge transfer between science and policy [79,80]. This task is easier said than done. A widely acknowledged impediment to conservation action is the inability to translate research results into effective policies and management due, in large part, to differences in scales, methodology, treatment of uncertainty and goals and/or values between different communities [81–83]. However, progress can be made. Conservation biologists have learned to work with both agencies and environmental non-governmental organizations in order for their data to have an impact (although evaluating the success of conservation efforts and identifying the most effective approaches remain important challenges [84]). Conservation paleobiologists need to do the same. Genuine outreach, putting the dead to work, requires real effort. Conservation paleobiologists first need to learn from agencies and non-governmental organizations what new data are needed and then need to translate their results to address real-world problems in conservation. Concluding statement Mounting evidence indicates that when conditions are right and paleobiologists are clever, data and insights useful in the practice of conservation biology will result. We have argued, through selected examples, that beyond baselines and the range of natural variability, understanding of the range of ecological and evolutionary responses of species to past environmental changes can be greatly informed by geohistorical data. A crucial challenge for the immediate future is making conservation paleobiology results policy-relevant. 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