Genetic variation in wild plantsand animals in Sweden : A review of case studies from the perspective of conservation genetics

• Main Authors: Lundqvist, Anna-Carin, Andersson, Stefan, Lönn, Mikael
• Format: Report
• Language: English
• Published: Utförare miljöövervakning, Uppsala universitet 2008
• Subjects: Environmental Sciences; Miljövetenskap; Sickle; Gadus morhua; mountain hare
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:naturvardsverket:diva-9800

By ratifying the Convention of Biological Diversity, Sweden has agreed toconserve the biological diversity at the ecosystem, species and genetic levels.One common assumption is that the conservation of ecosystems and habitatsalso conserves species, and that the conservation of species also conservesgenetic diversity. However, there is a growing realization that the conservation of species does not necessarily conserve the genetic diversity within species. Until now, the conservation of genetic resources has attracted relativelylittle attention in practical nature conservation.In recent years several scientists have argued that the conservation of biological diversity should focus on preventing the disappearance of geneticallydistinct populations rather than to solely prevent the extinction of species. Toconserve genetically distinct populations could be a better way of conservingthe evolutionarily potential of species. This will also reduce the risk that species go extinct, even in a longer time perspective.In 2006, The Swedish Environmental Protection Agency received agovernment commission to develop a national action plan for the conservation of genetic diversity in wild plants and animals, in consultation with theSwedish Board of Agriculture, the Swedish Forest Agency, the Swedish Boardof Fisheries and the Swedish University of Agricultural Sciences. This reportconstitutes a part of this work and is the internationally adapted version ofthe original report in Swedish, published in 2007 (Andersson et al. 2007).The objective of the report is to characterize the genetic variation in Swedishpopulations of wild plant and animals from the perspective of several themes.These themes were chosen to illustrate general issues currently identified inthe international research field of conservation genetics, exemplified withrelevant genetic studies of Swedish organisms. The aim of this report is not to summarize all studies of genetic variationof wild animals and plants performed in Sweden. Studies performed before1997 have been reviewed in two earlier reports, one about genetic monitoring(Laikre & Ryman 1997) and the other about genetically distinct populationsin Sweden (Lönn et al. 1998). These reports are still of immediate importance. Because of the amount of genetic research performed in Sweden and othercountries, the present report is not an exhaustive review of all recent progressin the field. Instead it focuses on illustrative examples and on relevant processes that may change the genetic diversity of wild populations in Sweden.Benefits of genetic variation. The genetic differentiation between individuals is the basis of evolution and adaptation. If all individuals of a species wereidentical, the species could not adapt genetically to a changing environmente.g. climate change anticipated due to anthropogenic emissions of greenhousegases. To measure genetic variation and relate it to evolutionary change andecological function in wild populations is challenging. Recently, some studieshave revealed direct effects associated with the genetic variation of a population. A review summarising the results from several plant-studies shows a positive correlation between population size, genetic variation and differentmeasures of viability and fertility. Experiments with artificial plant communities having different levels of genetic diversity have also produced evidence.Eelgrass stands consisting of many genotypes were less sensitive to environmental change, grew denser, and had larger numbers of small animals associated with them than stands with low levels of genetic diversity.Human activities may have negative consequences for the genetic diversity, and consequently for the adaptive potential, of wild populations. Regardless of whether the genetic variation is useful or not in the present-day populations, we cannot anticipate which traits will be essential for survival in arapidly changing environment.Loss of genetic diversity in small populations. Decreasing population sizeis a problem for many animals and plants that inhabit areas affected byhuman activities. Small isolated populations are expected to lose geneticdiversity through local random processes (genetic drift). Loss of genetic diversity may ultimately have effects on the ability to cope with environmentalchanges (evolutionary potential). Studies show that many populations are sosmall and isolated that they will be affected by the loss of genetic diversity.Furthermore, populations with low genetic diversity may have reduced fertility and viability either as a result of inbreeding, or because valuable alleleshave been lost. Several studies suggest that a few immigrants are sufficient toeliminate or reduce the negative effects of inbreeding. However, so far it isdifficult to draw general conclusions about how the loss of genetic diversityaffects the long-term evolutionary potential of populations.Genetic diversity after gene flow and hybridisation. Although anthropogenic habitat fragmentation usually causes negative isolation effects, humanactivities may also result in increased gene flow between natural populationsof animals or plants. A too extensive or too distant gene flow can have negative effects on the recipient populations. Occasionally, human activities haveincreased the gene flow by creating zones or ”hybrid environments” wheregenetically dissimilar populations or closely related species can meet andexchange genes. In several species it is known that genes from introduced ordomesticated species have spread to Swedish populations, as in the cases ofthe mountain hare (genes from the brown hare) and two subspecies of theplant lucerne (genes from the cultivated Medicago sativa spp. sativa to thewild relative sickle medick M. sativa spp. falcata). There is only sporadicknowledge about gene flow that occurs when alien populations of trees, birdsand fish are released into the wild and come into contact with indigenouspopulations, or when foreign grass seeds are sown on road verges. Studies ofsalmon show that gene flow can be harmful by creating hybrids with lowlevels of viability and fertility (i.e. outbreeding depression). In other casesgene flow has been so intense that the genetic integrity of species is threatenedas in the case of the low-density populations of the plant Viola alba on theisland of Öland, a species that easily hybridises with other related species.Sometimes populations with the capability to invade natural ecosystems havebeen created due to human-mediated hybridisation between closely relatedspecies. Potentially negative effects of gene flow must also be taken into consideration in conservation biology, e.g. when populations are supplementedwith individuals raised in captivity or with individuals from distant populations.Genetic effects of harvesting. Many Swedish animal and plant species areexposed to regular harvesting, such as fishing, hunting and forestry. Harvesting is expected to increase the random loss of genetic variation by decreasingthe effective population size. In a study of cod (Gadus morhua), local harvesting led to increased migration of individuals (and genes) from nearby populations, which also resulted in a change of the large-scale pattern of geneticvariation. There are many examples of harvested animal populations thathave undergone directional evolution as a result of selective harvesting. Inseveral cases this change has decreased the ability for the population to recover after a period of intense harvesting.Genetic diversity and change of habitat and climate. Local adaptationoccurs when populations become genetically adapted to different environments. Generally, the ability to adapt is larger the more genetic variation ispresent in a population. There are many examples of species in Sweden withlocally adapted populations in certain environments, e.g. the rough periwinkle (Littorina saxatilis), the common mussel (Mytilus edulis), herring (Clupeaharengus), three-spined stickle-back (Gasterosteus aculeatus), Scots pine(Pinus sylvestris) and white clover (Trifolium repens). Local adaptation implies that individuals from different populations are not interchangeable –locally adapted populations have a conservation value of their own. For thisreason, local adaptation is an important issue in e.g. reintroduction and supplementation strategies. For each type of adaptation, specific genetic variation is needed. How this variation is distributed and exchanged betweenpopulations through gene flow is largely unknown. Genetically distinct populations in Sweden. There are few endemic taxa atthe species level in Sweden, and those that exist have arisen relatively recentlythrough local processes such as hybridisation and polyploidisation. At thesame time there are many genetically distinct populations in Sweden. Populations are different due to different origins and colonisation routes or becausethey are adapted to their local environments. Taxonomic units as species,varieties and forms, together with informal genetic entities such as evolutionary significant units and management units reflect genetic differentiationthat has arisen within or outside the borders of Sweden. Genetically differentiated groups can be difficult to distinguish morphologically (they are cryptic), but molecular genetic studies have provided strong evidence for ”hidden” genetic structure in Swedish taxa. Lönn et al. (1998) called attention tothe fact that populations with their main distribution in Sweden are not marginal populations from a genetic perspective. In contrast, species that aremainly distributed in southern areas and are represented by marginal populations in Sweden are less genetically variable in this region. In many cases,Swedish populations are as genetically diverse as the populations in areas thatwere not covered with ice during the last glaciation. This pattern is also confirmed by more recent studies. Furthermore, recent investigations also verifythat populations from the islands Öland and Gotland, the Baltic Sea with the surrounding coastal areas, the mountain areas and some traditionally managed landscapes, are genetically distinct. Each population that is lost meansloss of genetic variation and consequently loss of adaptive potential. For thepurpose of conserving genetic resources, populations or groups of populations are the natural conservation units, because genetic variation occurs bothwithin and between populations.Genetic monitoring. The need for a genetic monitoring programme inSweden was the main conclusion by Laikre & Ryman (1997). We concur thata centrally organised genetic monitoring programme is still needed. Our suggestion is largely based on the proposal made in 1997 and is meant as anupdated starting point for more detailed discussions of programme design.We propose that the monitoring programme should focus on six differenttypes of taxa (species or groups of populations within species), for exampletaxa with negative population trends and taxa that are harvested by humans.One of the most important issues for the monitoring programme is toestablish procedures for collecting and storing different types of biologicalmaterial, e.g. tissue samples, which can be used in genetic investigations. It isvery important that the storing of the biological material does not in any waylimit which methods that can be used in future investigations. We suggest theestablishment of a common database for biological material stored in different museums, and we also suggest a formalised system where researchers canreport when collected biological material no longer can be stored locally andtherefore could be offered to the museums.Lack of knowledge and suggestion for research areas. In spite of the factthat many genetic studies have been performed within the focal research fieldof this report, we have identified several issues with significant knowledgegaps. We would like to see more genetic research regarding the followinggeneral questions:(i) Global warming is likely to result in large changes for Swedish populations and ecosystems. Processes like gene flow and local adaptation will beimportant for Swedish populations in order to meet these changes. We needmore knowledge to address questions such as: What role may geneticallydistinct populations in Sweden play to enable species to meet large-scale climatic and environmental changes? Is there sufficient genetic variation in relevant ecological traits to enable species to adapt to rapidly changing environments? Which populations are most valuable in this respect – central populations or those at the periphery of the distribution? Is there a risk that genetically distinct populations will disappear in those habitats that supposedly willbe most affected by global warming? Which genetic methods are most relevant for assessing evolutionary potential? How will genetic variation of keyspecies in important Swedish ecosystems affect the function, species composition and stability of these ecosystems?(ii) It is important to understand how the human-mediated gene flowaffects the gene pool of the genetically distinct populations present in Sweden.What genetic effect does the release of alien populations that takes place ine.g. the forestry and fisheries have? How will grass-species of natural grasslands be affected by the gene flow from foreign provenances sown on road verges? When is it appropriate to supplement small or inbred populations, orpopulations that are poorly adapted to a changing environment? When willsuch measures be harmful? From which populations should the individualsused in the supplementation be taken?(iii) There are several scientific questions that are in need of answers whendeveloping a genetic monitoring programme. One of the most central questions is: How can we differentiate natural changes in genetic diversity fromthe un-natural, which might constitute a threat to the genetic diversity?What should the national action plan include? The national action planfor conserving genetic diversity in wild plants and animals in Sweden shouldfirst and foremost include a genetic monitoring programme and also prioritise the suggested research areas. Furthermore we suggest that the action planproposes guidelines for how to deal with genetic problems in monitoredpopulations and in the supplementation of wild populations, an action already suggested in several of the recovery and action plans for red-listed speciesin Sweden. The national action plan should also aim to develop guidelinesand effective systems for the registration of the release of alien populationsthat take place e.g. in forestry, fish management and in road management.