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      Infectious disease, shifting climates, and opportunistic predators: cumulative factors potentially impacting wild salmon declines

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          Abstract

          Emerging diseases are impacting animals under high-density culture, yet few studies assess their importance to wild populations. Microparasites selected for enhanced virulence in culture settings should be less successful maintaining infectivity in wild populations, as once the host dies, there are limited opportunities to infect new individuals. Instead, moderately virulent microparasites persisting for long periods across multiple environments are of greatest concern. Evolved resistance to endemic microparasites may reduce susceptibilities, but as barriers to microparasite distributions are weakened, and environments become more stressful, unexposed populations may be impacted and pathogenicity enhanced. We provide an overview of the evolutionary and ecological impacts of infectious diseases in wild salmon and suggest ways in which modern technologies can elucidate the microparasites of greatest potential import. We present four case studies that resolve microparasite impacts on adult salmon migration success, impact of river warming on microparasite replication, and infection status on susceptibility to predation. Future health of wild salmon must be considered in a holistic context that includes the cumulative or synergistic impacts of multiple stressors. These approaches will identify populations at greatest risk, critically needed to manage and potentially ameliorate the shifts in current or future trajectories of wild populations.

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          Most cited references 352

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          Marine viruses--major players in the global ecosystem.

          Viruses are by far the most abundant 'lifeforms' in the oceans and are the reservoir of most of the genetic diversity in the sea. The estimated 10(30) viruses in the ocean, if stretched end to end, would span farther than the nearest 60 galaxies. Every second, approximately 10(23) viral infections occur in the ocean. These infections are a major source of mortality, and cause disease in a range of organisms, from shrimp to whales. As a result, viruses influence the composition of marine communities and are a major force behind biogeochemical cycles. Each infection has the potential to introduce new genetic information into an organism or progeny virus, thereby driving the evolution of both host and viral assemblages. Probing this vast reservoir of genetic and biological diversity continues to yield exciting discoveries.
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            Climate warming and disease risks for terrestrial and marine biota.

            Infectious diseases can cause rapid population declines or species extinctions. Many pathogens of terrestrial and marine taxa are sensitive to temperature, rainfall, and humidity, creating synergisms that could affect biodiversity. Climate warming can increase pathogen development and survival rates, disease transmission, and host susceptibility. Although most host-parasite systems are predicted to experience more frequent or severe disease impacts with warming, a subset of pathogens might decline with warming, releasing hosts from disease. Recently, changes in El Niño-Southern Oscillation events have had a detectable influence on marine and terrestrial pathogens, including coral diseases, oyster pathogens, crop pathogens, Rift Valley fever, and human cholera. To improve our ability to predict epidemics in wild populations, it will be necessary to separate the independent and interactive effects of multiple climate drivers on disease impact.
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              Adaptation from standing genetic variation.

              Populations adapt to novel environments in two distinct ways: selection on pre-existing genetic variation and selection on new mutations. These alternative sources of beneficial alleles can result in different evolutionary dynamics and distinct genetic outcomes. Compared with new mutations, adaptation from standing genetic variation is likely to lead to faster evolution, the fixation of more alleles of small effect and the spread of more recessive alleles. There is potential to distinguish between adaptation from standing variation and that from new mutations by differences in the genomic signature of selection. Here we review these approaches and possible examples of adaptation from standing variation in natural populations. Understanding how the source of genetic variation affects adaptation will be integral for predicting how populations will respond to changing environments.
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                Author and article information

                Journal
                Evol Appl
                Evol Appl
                eva
                Evolutionary Applications
                BlackWell Publishing Ltd (Oxford, UK )
                1752-4571
                1752-4571
                August 2014
                27 May 2014
                : 7
                : 7
                : 812-855
                Affiliations
                [1 ]Pacific Biological Station, Fisheries and Oceans Canada Nanaimo, BC, Canada
                [2 ]Forest and Conservation Sciences, University of British Columbia Vancouver, BC, Canada
                [3 ]Biology Department, University of Victoria Victoria, BC, Canada
                [4 ]Fisheries and Oceans Canada, School of Resource and Environmental Management, Simon Fraser University, Science Branch Burnaby, BC, Canada
                [5 ]Environment Canada, Wildlife Research Division Delta, BC, Canada
                [6 ]Fish Ecology and Conservation Physiology Laboratory, Department of Biology, Carleton Univerisy Ottawa, ON, Canada
                Author notes
                Correspondence: Kristina M. Miller, Pacific Biological Station, Fisheries and Oceans Canada, Nanaimo, BC V9T 6N7, Canada, V9T 6N7., Tel: (250) 756-7155;, fax: (250) 756-7053; e-mail: Kristi.Miller@ 123456dfo-mpo.gc.ca
                Article
                10.1111/eva.12164
                4227861
                © 2014 John Wiley & Sons Ltd

                This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                Categories
                Reviews and Synthesis

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