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- Volume 39, 2008
Annual Review of Ecology, Evolution, and Systematics - Volume 39, 2008
Volume 39, 2008
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Top Predators as Conservation Tools: Ecological Rationale, Assumptions, and Efficacy
Vol. 39 (2008), pp. 1–19More LessWe review the ecological rationale behind the potential compatibility between top predators and biodiversity conservation, and examine their effectiveness as surrogate species. Evidence suggests that top predators promote species richness or are spatio-temporally associated with it for six causative or noncausative reasons: resource facilitation, trophic cascades, dependence on ecosystem productivity, sensitivity to dysfunctions, selection of heterogeneous sites and links to multiple ecosystem components. Therefore, predator-centered conservation may deliver certain biodiversity goals. To this aim, predators have been employed in conservation as keystone, umbrella, sentinel, flagship, and indicator species. However, quantitative tests of their surrogate-efficacy have been astonishingly few. Evidence suggests they may function as structuring agents and biodiversity indicators in some ecosystems but not others, and that they perform poorly as umbrella species; more consensus exists for their efficacy as sentinel and flagship species. Conservation biologists need to use apex predators more cautiously, as part of wider, context-dependent mixed strategies.
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Revisiting the Impact of Inversions in Evolution: From Population Genetic Markers to Drivers of Adaptive Shifts and Speciation?
Vol. 39 (2008), pp. 21–42More LessThere is a growing appreciation that chromosome inversions affect rates of adaptation, speciation, and the evolution of sex chromosomes. Comparative genomic studies have identified many new paracentric inversion polymorphisms. Population models suggest that inversions can spread by reducing recombination between alleles that independently increase fitness, without epistasis or coadaptation. Areas of linkage disequilibrium extend across large inversions but may be interspersed by areas with little disequilibrium. Genes located within inversions are associated with a variety of traits including those involved in climatic adaptation. Inversion polymorphisms may contribute to speciation by generating underdominance owing to inviable gametes, but an alternative view gaining support is that inversions facilitate speciation by reducing recombination, protecting genomic regions from introgression. Likewise, inversions may facilitate the evolution of sex chromosomes by reducing recombination between sex determining alleles and alleles with sex-specific effects. However, few genes within inversions responsible for fitness effects or speciation have been identified.
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Radial Symmetry, the Anterior/Posterior Axis, and Echinoderm Hox Genes
Rich Mooi, and Bruno DavidVol. 39 (2008), pp. 43–62More LessThe strangeness of echinoderm pentaradiality results from superposition of radial symmetry onto ancestral deuterostome bilaterality. The Extraxial-Axial Theory shows that echinoderms also have an anterior/posterior (A/P) axis developed independently and ontogenetically before radiality. The A/P axis is first established via coelomic stacking in the extraxial region, with ensuing development of the pentamerous hydrocoel in the axial region. This is strongly correlated with a variety of gene expression patterns. The echinoid Hox cluster is disordered into two different sets of genes. During embryogenesis, members of the posterior class demonstrate temporal, spatial, and genetic colinearity within the extraxial region. We suggest that displacement of genes from the more anterior Hox classes toward the 5′ end of the chromosome leads to control of the later-developing, radially symmetric axial region. Genetic disorder is therefore another way of using colinearity to build the unique echinoderm symmetry.
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The Great American Schism: Divergence of Marine Organisms After the Rise of the Central American Isthmus
Vol. 39 (2008), pp. 63–91More LessAfter a 12-million-year (My) process, the Central American Isthmus was completed 2.8 My ago. Its emergence affected current flow, salinity, temperature, and primary productivity of the Pacific and the Atlantic and launched marine organisms of the two oceans into independent evolutionary trajectories. Those that did not go extinct have diverged. As no vicariant event is better dated than the isthmus, molecular divergence between species pairs on its two coasts is of interest. A total of 38 regions of DNA have been sequenced in 9 clades of echinoids, 38 of crustaceans, 42 of fishes, and 26 of molluscs with amphi-isthmian subclades. Of these, 34 are likely to have been separated at the final stages of Isthmus completion, 73 split earlier and 8 maintained post-closure genetic contact. Reproductive isolation has developed between several isolates, but is complete in only the sea urchin Diadema. Adaptive divergence can be seen in life history parameters. Lower primary productivity in the Caribbean has led to the evolution of higher levels of maternal provisioning in marine invertebrates.
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The Ecological Performance of Protected Areas
Vol. 39 (2008), pp. 93–113More LessProtected areas are a cornerstone of local, regional, and global strategies for the conservation of biodiversity. However, the ecological performance of these areas, both in terms of the representation and the maintenance of key biodiversity features, remains poorly understood. A large and rapidly expanding literature bears on these issues, but it is highly fragmented, principally comprises particular case studies, and employs a diverse array of approaches. Here we provide a synthetic review of this work, discriminating between issues of performance of inventory and condition at the scale of individual protected areas, portfolios, and networks of protected areas. We emphasize the insights that follow and the links between the different issues, as well as highlight the major problems that remain unresolved.
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Morphological Integration and Developmental Modularity
Vol. 39 (2008), pp. 115–132More LessBiological systems, from molecular complexes to whole organisms and ecological interactions, tend to have a modular organization. Modules are sets of traits that are internally integrated by interactions among traits, but are relatively independent from other modules. The interactions within modules rely on different mechanisms, depending on the context of a study. For morphological traits, modularity occurs in developmental, genetic, functional, and evolutionary contexts. A range of methods for quantifying integration and modularity in morphological data is available, and a number of comparative and experimental designs can be used to compare the different contexts. How development produces covariation between traits can have substantial implications for understanding genetic variation and the potential for evolutionary change, but research in this area has only begun and many questions remain unanswered.
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Herbivory from Individuals to Ecosystems
Vol. 39 (2008), pp. 133–152More LessHerbivores not only consume resources, but they are resources for other consumers. Consequently, they have much potential to mediate effects that cascade up and down trophic chains in ecosystems. The way those effects are mediated depends on individual-scale properties of herbivores including constraints determining resource limitation, herbivore feeding mode, the adaptive trade-off to balance nutrient intake and predation risk avoidance, and the need to maintain homeostatic balance of elemental chemistry in the face of widely varying elemental composition of plant resources. These factors determine the rates of ecosystem functions such as production, decomposition and nutrient cycling. This review integrates those factors to build a conceptual framework for looking at herbivore-mediated effects in ecosystems. The framework systematically resolves how herbivores and carnivores directly and indirectly interact with plants to shape ecosystem functions. It can be used to motivate new field experimentation aimed at elucidating mechanisms of trophic control of ecosystem function.
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Stoichiometry and Nutrition of Plant Growth in Natural Communities
Vol. 39 (2008), pp. 153–170More LessStoichiometric relations in plants, with emphasis on C:N:P, are reviewed. Both theoretically and empirically it is found for whole plants as well as for different tissues that nitrogen concentrations increase slower than phosphorus concentrations. A lack of data prevents the establishment of relations between nitrogen and other elements. Optimal element ratios where elements are simultaneously limiting growth can be established. There is a considerable variability around these optimal ratios in observed values. Conclusions about the ecological significance of stoichiometric relations based on these observations may therefore be biased. The significance of this variability remains to be established.
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Plague Minnow or Mosquito Fish? A Review of the Biology and Impacts of Introduced Gambusia Species
Vol. 39 (2008), pp. 171–191More LessGambusia affinis and G. holbrooki, introduced worldwide from eastern North America, are collectively the most abundant, widespread freshwater fish in the world, which is not surprising because they tolerate, and sometimes thrive under, an exceptional range of environmental conditions and have high reproductive potential. Some know them as mosquitofish because of a legendary ability to control mosquitoes, and diseases they carry, while others doubt this ability or argue that indigenous fish are equally or more effective. However, rigorous evidence to support these views remains scant, so the legend persists. Some know them as plague minnow because of negative impacts on many native animal species, and abundant evidence exists to support this view. Despite such polarized attitudes toward them, their high abundance and wide distribution, and a large scientific literature devoted to them, many important aspects of their biology remain poorly known.
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The Impact of Natural Selection on the Genome: Emerging Patterns in Drosophila and Arabidopsis
Vol. 39 (2008), pp. 193–213More LessSpecies in the Drosophila and Arabidopsis species groups share the feature of having relatively small, streamlined genomes. In Drosophila, evidence for pervasive negative and positive selection is overturning long-held views about the functional significance of noncoding DNA, the frequency of positive selection, and the extent to which coding and noncoding polymorphism and divergence between species is neutral. However, despite sharing some similarities with Drosophila, Arabidopsis shows quite distinct patterns of selective constraint and positive selection. Two conspicuous differences between these species groups are their effective population sizes and population structure, which may explain lower levels of selective constraint in coding and noncoding DNA of Arabidopsis, more evidence for balancing selection and less evidence for canonical signature of positive selection than in Drosophila species. As more comparative genomic data accumulate in the Arabidopsis group, the combination of polymorphism and divergence data allow these initial contrasts to be quantified on a genomic scale.
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Sanctions, Cooperation, and the Stability of Plant-Rhizosphere Mutualisms
Vol. 39 (2008), pp. 215–236More LessThere are both costs and benefits for host plants that associate with microbes in the rhizosphere. Typically, an individual plant associates with multiple microbial genotypes varying in mutualistic benefit. This creates a potential tragedy of the commons where less-mutualistic strains potentially share in the collective benefits, while paying less of the costs. Therefore, maintaining cooperation over the course of evolution requires specific mechanisms that reduce the fitness benefits from “cheating.” Sanctions that discriminate among partners based on actual symbiotic performance are a key mechanism in rhizobia and may exist in many rhizosphere mutualisms, including rhizobia, mycorrhizal fungi, root endophytes, and perhaps free-living rhizosphere microbes. Where they exist, sanctions may take different forms depending on the system. Despite sanctions, less-effective symbionts still persist. We suggest this is because of mixed infection at spatial scales that limit the effects of sanctions, variation among plants in the strength of sanctions, and conflicting selection regimes.
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Shade Tolerance, a Key Plant Feature of Complex Nature and Consequences
Vol. 39 (2008), pp. 237–257More LessLight gradients are ubiquitous in nature, so all plants are exposed to some degree of shade during their lifetime. The minimum light required for survival, shade tolerance, is a crucial life-history trait that plays a major role in plant community dynamics. There is consensus on the suites of traits that influence shade tolerance, but debate over the relative importance of traits maximizing photosynthetic carbon gain in low light versus those minimizing losses. Shade tolerance is influenced by plant ontogeny and by numerous biotic and abiotic factors. Although phenotypic plasticity tends to be low in shade-tolerant species (e.g., scant elongation in low light), plasticity for certain traits, particularly for morphological features optimizing light capture, can be high. Understanding differential competitive potentials among co-occurring species mediated by shade tolerance is critical to predict ecosystem responses to global change drivers such as elevated CO2, climate change and the spread of invasive species.
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The Impacts of Fisheries on Marine Ecosystems and the Transition to Ecosystem-Based Management
Vol. 39 (2008), pp. 259–278More LessFishing remains one of the largest factors modifying marine ecosystems. Because fisheries constitute only one of many anthropogenic effects, management is shifting from single-species approaches toward ecosystem-based management. Interaction webs are a critical nexus to understand linkages, to model ecosystem change, and to apply management directives. Ecosystem-based management requires consideration of both direct and indirect effects of commercial fisheries. But it must also include impacts of bycatch, recreational fisheries, artisanal fisheries, and environmental change that can be large but unanticipated. Synergistic effects of fishing, environmental variation, and climate change increasingly threaten marine ecosystems and complicate management. Here we review the global effects of fisheries and propose an integrated framework for managing biophysical processes and human ecology. To incorporate the multitude of effects, this emerging approach focuses on the dynamics of interaction webs in a spatially explicit or place-based framework.
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The Performance of the Endangered Species Act
Vol. 39 (2008), pp. 279–299More LessArguably the most notable success of the Endangered Species Act (ESA) is that listed species improve in status through time. More species are downlisted than the converse; more species transition from stable to improving status than the converse. Although some listed species have gone extinct, this number is smaller than expected. Given modest recovery funding, the fraction of listed species responding positively is remarkable. Several factors have been linked to improving species status including recovery expenditures, critical habitat listing, and time spent under protection. The inability of government to fully empower the agencies to implement the law has been the most notable failure of the ESA. Listing of species has not matched need, recovery expenditures do not match need or agency-set priorities, and critical habitat determinations have lagged. Alternative protection strategies to listing may be having a positive effect, but are difficult to assess because of sparse data.
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Phylogenetic Approaches to the Study of Extinction
Vol. 39 (2008), pp. 301–319More LessSpecies extinction is both a key process throughout the history of life and a pressing concern in the conservation of present-day biodiversity. These two facets have largely been studied by separate communities using different approaches. This article illustrates with examples some of the ways that considering the evolutionary relationships among species—phylogenies—has helped the study of both past and present species extinction. The focus is on three topics: extinction rates and severities, phylogenetic nonrandomness of extinction, and the testing of hypotheses relating extinction-proneness to attributes of organisms or species. Phylogenetic and taxic approaches to extinction have not fully fused, largely because of the difficulties of relating discrete taxa to the underlying continuity of phylogeny. Phylogeny must be considered in comparative tests of hypotheses about extinction, but care must be taken to avoid overcorrecting for phylogenetic nonindependence among taxa.
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Adaptation to Marginal Habitats
Vol. 39 (2008), pp. 321–342More LessThe ability to adapt to marginal habitats, in which survival and reproduction are initially poor, plays a crucial role in the evolution of ecological niches and species ranges. Adaptation to marginal habitats may be limited by genetic, developmental, and functional constraints, but also by consequences of demographic characteristics of marginal populations. Marginal populations are often sparse, fragmented, prone to local extinctions, or are demographic sinks subject to high immigration from high-quality core habitats. This makes them demographically and genetically dependent on core habitats and prone to gene flow counteracting local selection. Theoretical and empirical research in the past decade has advanced our understanding of conditions that favor adaptation to marginal habitats despite those limitations. This review is an attempt at synthesis of those developments and of the emerging conceptual framework.
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Conspecific Brood Parasitism in Birds: A Life-History Perspective
Vol. 39 (2008), pp. 343–363More LessConspecific brood parasitism (CBP), whereby females lay eggs in the nests of other conspecifics, occurs in over 200 species of birds. As an alternative tactic to typical nesting, CBP expands and enriches the classic avian clutch size problem. It is an integral component of a flexible life-history strategy and, consequently, many intriguing aspects of this behavior—adaptive benefits to parasites, host-parasite interactions, population and evolutionary dynamics—can be understood best from a life-history perspective. Because parasite fitness depends on hosts, yet parasitism potentially reduces host fitness, CBP offers a novel opportunity to explore conflicts of interest within species. The intersection of life-history evolution, conflicts of interest, and frequency-dependent fitness provides much scope for theoretical exploration, and recent models indicate a complex range of evolutionary dynamics is possible, including consequences of CBP for population dynamics and conservation. CBP may also be a macroevolutionary stepping stone to diverse breeding systems.
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Stratocladistics: Integrating Temporal Data and Character Data in Phylogenetic Inference
Vol. 39 (2008), pp. 365–385More LessDebate has long simmered over whether data on the order of appearance of taxa in the stratigraphic record should play any role in analyses of phylogenetic relationships among those taxa. Critics argue that temporal data are in principle inapplicable to questions of cladistic relationship, but specific versions of this claim all seem flawed. Stratocladistics offers a methodological context (patterned after that of cladistics itself) within which temporal data participate along with conventional character data in selecting most-parsimonious hypotheses. Stratocladistics outperforms cladistics in tests based on simulated histories, and additional testing will be facilitated by new software automating stratocladistic searches. As with any body of data, we may decide to include or exclude temporal data for specific reasons, but the explanatory power of hypotheses that use both temporal and conventional character data exceeds that of hypotheses based on character data alone.
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The Evolution of Animal Weapons
Vol. 39 (2008), pp. 387–413More LessMales in many species invest substantially in structures that are used in combat with rivals over access to females. These weapons can attain extreme proportions and have diversified in form repeatedly. I review empirical literature on the function and evolution of sexually selected weapons to clarify important unanswered questions for future research. Despite their many shapes and sizes, and the multitude of habitats within which they function, animal weapons share many properties: They evolve when males are able to defend spatially restricted critical resources, they are typically the most variable morphological structures of these species, and this variation honestly reflects among-individual differences in body size or quality. What is not clear is how, or why, these weapons diverge in form. The potential for male competition to drive rapid divergence in weapon morphology remains one of the most exciting and understudied topics in sexual selection research today.
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Unpacking β: Within-Host Dynamics and the Evolutionary Ecology of Pathogen Transmission
Vol. 39 (2008), pp. 415–437More LessRather than being fixed, pathogen transmission varies and is thus an object of natural selection. I examine how opportunities for selection on pathogen transmission depend on (a) pathogen fitness, (b) genetic variability, and (c) forces acting at within- and between-host levels. The transmission rate, β, influences processes such as epidemic spread, postepidemic fade-outs, and low-level persistence. Complexity of infection processes within hosts leads to different transmission rates among hosts and between types of pathogens (viruses, bacteria, eukaryotic Protozoa). Generality emerges, however, by “unpacking” β into within- and between-host opportunities for selection. This is illustrated by evolutionary biology of the bacterium Yersinia pestis, which causes plague in mammals, remains highly virulent and is transmitted by multiple routes, including fleas and direct contacts with infected hosts. The strength of within-host selection is manifested through infectivity, replication, pathogenicity, and dissemination from hosts. At the between-host level, responses to selection are less predictable because of environmental variation, whereas vector-borne transmission (usually by arthropods) provides additional opportunities for selection and trade-offs between vectors and hosts. In subdivided host populations, selection favors transmission before local pathogen extinction occurs, but key components (e.g. infectious periods of hosts) are determined by within-host dynamics. Pathogen transmission is often viewed in the context of transmission-virulence trade-offs, but within-host dynamics may cause host damage unrelated to transmission, and thus transmission-virulence trade-offs are not universal.
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Previous Volumes
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Volume 54 (2023)
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Volume 53 (2022)
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Volume 52 (2021)
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Volume 51 (2020)
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Volume 50 (2019)
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Volume 49 (2018)
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Volume 48 (2017)
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Volume 47 (2016)
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Volume 46 (2015)
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Volume 45 (2014)
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Volume 44 (2013)
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Volume 43 (2012)
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Volume 42 (2011)
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Volume 41 (2010)
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Volume 40 (2009)
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Volume 39 (2008)
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Volume 38 (2007)
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Volume 37 (2006)
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Volume 36 (2005)
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Volume 35 (2004)
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Volume 34 (2003)
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Volume 33 (2002)
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Volume 32 (2001)
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Volume 31 (2000)
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Volume 30 (1999)
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Volume 29 (1998)
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Volume 28 (1997)
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Volume 27 (1996)
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Volume 26 (1995)
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Volume 25 (1994)
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Volume 24 (1993)
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Volume 23 (1992)
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Volume 22 (1991)
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Volume 21 (1990)
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Volume 20 (1989)
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Volume 19 (1988)
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Volume 18 (1987)
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Volume 17 (1986)
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Volume 16 (1985)
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Volume 15 (1984)
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Volume 14 (1983)
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Volume 13 (1982)
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Volume 12 (1981)
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Volume 11 (1980)
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Volume 10 (1979)
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Volume 9 (1978)
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Volume 8 (1977)
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Volume 7 (1976)
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Volume 6 (1975)
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Volume 5 (1974)
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Volume 4 (1973)
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Volume 3 (1972)
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Volume 2 (1971)
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Volume 1 (1970)
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Volume 0 (1932)