Information and Advisory Note Number 15 Back to menu
Direct management of populations of rare plants is becoming increasingly
important as a means to maintain or restore species in the face of modem land
use. The results of genetic research have rarely been used to inform and guide
the development of translocation guidelines or recovery attempts. Yet the
diversity and movement of genes within created or existing populations may have
profound implications for their long term persistence. Equally, genetic
diversity with its patterns of local adaptation, and evolutionary lineage and
potential, is a significant element of biodiversity and worthy of conservation
in its own right.
In this note, we present a set of guidelines that inform conservation managers how genetic principles can be applied at different stages in the recovery of wild plant populations. They are meant to complement, not supersede, guidelines on translocations already in existence and management principles based on demography. They will be applied to plant recovery work being done as part of the SNH Species Action Programme
Most species which are candidates for recovery are likely to consist of small, isolated populations (75% of a sample of 351 populations of rare Scottish plants had less than 100 individuals). These are known to be at greatest risk from demographic and environmental stochasticity. Such populations are also at great risk from processes, such as drift, founder effects or inbreeding depression, which lead to rapid loss of genetic variation. Detailed guidance on populations most likely to be a priority for screening to detect these effects, can be obtained from RASD.
Methods of genetic screening include (a) assessing markers in the genetic material of the plants by isoenzyme electrophoresis or by analysis of DNA polymorphism; and (b) assessing quantitative characters expressed in the phenotype of individuals in population/ family trials. The first are relatively expensive in equipment and materials but are now widely used by geneticists. The second is time consuming but less technically demanding and may tell us about the variation in characters that could be of direct ecological importance. The important question of whether inbreeding or outbreeding depression occurs in target populations can be assessed by carrying out crosses between populations and measuring the performance of the progeny preferably when planted out in the field.
Ex situ conservation measures have an important role in the conservation of rare
plants. First, they are an important resource for fundamental research,
education and publicity. Second, they act as a safeguard against the extirpation
of species or populations in the wild. Third, they may be an
important source of plant propagules to return to the wild as part of any future
co-ordinated recovery attempts. Ex situ conservation, however, poses some risk
to the capture or maintenance of genetic variation without loss or change.
The three main methods in ex situ conservation are maintaining living plants in cultivation, maintaining them in vitro, and storing frozen seeds or spores. The success of any ex situ method in conserving the genetic variation of a species or population, critically depends on the collection of an adequate sample of the in situ material. The smaller the original population, the greater the proportion of the individuals that should be sampled to capture the available variation. Guidelines are presented in Falk & Holsinger (1991) which should be followed to prevent sampling error or bias towards, for example, the most vigorous plants or those with most prolific seed production. Token, ad hoc, collections are likely to be of little value in recovery programmes compared to those derived from a planned sampling programme. It is clearly essential that the survival of a source population should be paramount in the design of any sampling scheme.
There are also risks during ex situ maintenance. If sexual reproduction occurs in an outbreeding species, then chance loss of traits may occur in the offspring due to genetic drift and adaptive selection to the artificial conditions of the ex situ environment. Unless rigorous precautions are taken, living collections may hybridise with other material of different provenance and garden strains or hybrids of the same species. Traits may also be favoured by anthropogenic selection in the ex situ environment, including precocious growth and flowering, reduced seed dormancy and abundant seed production. Rigorously preventing sexual reproduction during ex situ conservation could largely prevent undesirable genetic change but this has rarely been applied.
Unlike most animals, plants are capable of vegetative regeneration and many species can be maintained this way indefinitely in cultivation without suffering any major genetic change. However a strict management programme is still required. Clones less suited to the garden environment may be gradually eliminated or swamped by more successful clones if not spatially separated. Sampling and maintaining the full range of genets available in surviving populations is therefore likely to be expensive.
If species or populations chosen for recovery have reached a very low ebb, material for restoration is likely to be limited. Bulking up is a normal horticultural technique and could be necessary. However, here the possibility of rapid genetic selection is particularly acute. Therefore to maintain the maximum range of the genetic variation found in a wild population, plant material being produced for return to the wild should be grown from propagules derived directly from wild populations.
Plants resulting from any ex situ programme which are intended for use in translocations must be randomly selected and not biased, as is normal in horticulture, towards the largest and most vigorous. These may not necessarily be best suited to the wild and their use may mean the loss of part of the full range of genotypes present.
The ultimate aim of most recovery attempts is to restore long-term viability to
populations in their native localities. Methods to achieve this range from
habitat management to direct manipulation of populations.
4.1 Habitat management
This technique involves least intervention in normal population processes and should, therefore, be considered as a first option in any recovery attempt. It is least likely to modify the genotype of the target species yet genetic effects may still be encountered and should be considered in planning. For example, there are likely to be founder effects if the original population has been reduced to a few, perhaps genetically attenuated, individuals to act as the source of new recruits.
Habitat manipulation may be used to exploit the recovery of individuals from in situ seed or spore banks. Such recruitment is likely to be near natural and may even enable the restoration of genotypes, and thus heterozygosity, lost from the contemporary population. Spore or seed banks have been detected for a wide variety of ferns and flowering plants.
Genetic impacts from conservation management are probably far more likely in the restoration or creation of populations than the conservation of existing ones. For example, re-stocking could destroy local adaptation and co-adapted gene complexes so causing outbreeding depression, but could also be used to counter inbreeding depression
founder effects. It might damage that part of the scientific value of populations related to their genetic lineage and history. It should, therefore, be approached with a considerable
degree of caution and as full an understanding as possible of the demography, genetics and breeding system of the target plant.
Three methods are potentially available to establish plants in the wild, namely, (i) direct planting of cultivated material; (ii) direct sowing of seed or other propagules; (Hi) direct transfer of material from wild populations. Each of these are discussed in turn.
(i) Direct planting. This is the approach most commonly used in recovery attempts. Typically, cultivated material is planted out at field locations after suitable hardening off. Such plantings are likely to be given considerable aftercare by watering, weeding or protection from grazing until they are fully established. Yet this may impose considerable artificial selection on transplants. For some species, this may be the only option available to conservation managers but the extent to which natural selection pressures are overridden should be kept to a minimum (even though this may result in greater losses). In some cases these losses may be minimised by careful planning; for example, a perceived need for watering might be avoided by shifting from spring to autumn outplanting.
(ii) Direct seeding of field sites. This has significant advantages because anthropogenic selection pressures will be reduced if plants are established in the field from wild-collected seed. If seed has to be derived from garden stock because of extreme scarcity in the wild (see earlier guideline on the importance of minimising the number of sexual generations in cultivation), then at least the selection that takes place during its establishment will be near natural. Costs are likely to be low because seed is relatively easy to handle and aftercare can be minimal. Plants established from seed in the wild are likely to be able to exploit crevices and other niches in a way that plants grown in containers may not do so readily. Therefore it may be a useful method for establishing plants in sites with shallow soils - a relatively common niche for threatened plants. The risk of importing novel pests and diseases from cultivation will also be reduced.
Drawbacks to this method include the requirement for a large supply of seed which may not be available from the wild population.
We may need to carefully select or create the areas used for sowing to make best use of scarce seed resources. The balance of genotypes needs to be equally represented in any sowings and should not be biased by containing larger amounts of seed derived from more prolific individual seed producers. The approach may be considered to be inefficient if the resource is very scarce and if significant losses are likely. It is also likely to be slow to produce demonstrable results, a problem where resources are time constrained or where there is pressure for early achievements. However, this slow response may itself allow the expression of individuals that are slow to germinate and which would be discarded in garden cultivation.
(iii) Direct transfer of wild material. This third option is direct transfer of material (other than seed) from wild populations. This has potentially a direct and damaging impact on donor populations if individuals are to be uprooted, but may be less so if cuttings or other propagules, such as bulbils or corms, are used. The material can be screened beforehand so that its genetic constitution is known. This method is likely to be particularly applicable where transfer of propagules or clones between populations is required to enable outcrossing between genetically different individuals.
4.3 Provenance of source material
The most important consideration in the genetic management of populations is the source and genetic composition of translocated material. Traditionally, the conservation agencies have adopted the precautionary principle of advocating the use of local genotypes in all plant translocations. This will tend to maintain any local adaptation and co-adapted gene complexes, and preserve the genetic history and integrity of populations. It assumes that the greatest threat to the survival of small isolated populations subject to conservation management is from out-breeding depression with less-fit progeny resulting from the introduction of any new genetic variation. However, if a population is in decline because it is subject to inbreeding depression, or if variation is reduced such that only sexually incompatible individuals remain, adopting this precautionary approach will only compound these problems.
On balance, the precautionary approach remains that direct re-stocking of an existing population should only use genotypes selected from that population unless there is evidence to suggest the contrary. If there is a subsequent continuing decline, the site manager must consider examining the possibility of these other genetic impacts.
If a small population is surviving because of successful adaptation to its local environment it might also be at considerable risk if a new and more variable population is established within easy pollen (or conceivably seed) transfer distance. Problems may be especially acute if the new population is larger than the native one with the risk that foreign pollen will lead to less well adapted offspring in the original population.
There are, however, an increasing number of field and glasshouse studies demonstrating the deleterious effects of inbreeding, or breeding incompatibility in outbreeding and even highly selfing populations, if we have reason to believe that the detrimental effects of inbreeding depression or similar effects override the advantages of local adaptation or co-adapted gene complexes, then the restoration of heterozygosity to an existing population is a priority even if this damages local adaptation.
In this case how should new genotypes be selected to mix in with native genotypes? The more distant a population the more likely it is to differ genetically and the greater the chance of different ecotypes being involved. Therefore alternative sources of genetic variation should be derived from relatively close populations. This will not always be feasible as some of our rarest plants are now many tens of kilometres from their nearest neighbour.
A different approach may be appropriate when creating new populations or restoring lost, now isolated, colonies. Crosses from individuals of widely separated populations may provide a significant advantage for the exploitation of such novel environments. There is also some evidence that crosses between distant populations can increase fitness in populations whether they are inbred or not. Foreign genes which are not suited to their new locality will be rapidly selected out and the new population become suited to the site. When restoring metapopulations in particular (see below) this approach can be tested experimentally using populations of pure local or distant origin and mixtures of the two.
Although we usually assume that establishing a new population from a local source must bring benefits in local adaptation, the existing experimental evidence indicates that local adaptation and co-adapted gene complexes break down only short distances away On some cases <100m) from the native locality. Plants may also have optimal outcrossing distances. Although the scale of these effects will vary between species, depending upon their breeding system and mechanisms of pollen dispersal, this research suggests that co-adapted gene complexes do not confer an advantage in the establishment of plants beyond a very local area.
Fragmentation of landscapes and the loss of plant populations means that the
survivors are often now increasingly isolated. Where the
species formerly existed as part of a metapopulation, then surviving populations are at increased risk of extinction, with loss of genetic variability possibly contributing to this.
A significant number of recovery programmes aim to restore extinct populations. In doing so, it may be that conservation managers may be creating further isolated populations with a low probability of persistence. When undertaking (re) introductions, managers should aim to create clusters of populations within dispersal range of one another, thus increasing the effective population (Ne) size overall. Occasional gene flow between the elements of a cluster is known to be sufficient to counter the effects of drift within small, isolated populations. Populations should be created or restored to an optimum density, to enable gene flow, matching that observed within viable native populations. Planting schemes may need to ensure that compatible mating types are grouped together or genotypes spatially arranged to minimise inbreeding.
Newly created or augmented populations should be made as large as possible to counter extinction risk. Guidelines to minimum effective population size may be useful in the absence of specific studies. Given that gene flow by pollen is mainly from large to small populations, and that the latter receive external pollen at higher rates than larger populations, it may be possible to influence pollen transfer by manipulating the respective size of elements of a cluster.
Recovery work has been a minor part of plant conservation in the past Recovery attempts offer an ideal opportunity to gather empirical data that can be applied more widely to the conservation of other threatened plants. Many attempts at re-establishment have been poorly documented. All recovery attempts should be regarded as experiments and documented fully. This need is particularly relevant to the genetics of recovery because it has been applied so rarely in practice.
Birkinshaw, CR. 1991. Guidance notes for translocating plants as part of recovery plans. CSD Report No. 1225. Nature Conservancy Council, Peterborough.
Botanic Gardens Conservation International. 1992. A study on the guidelines that should be followed in the design of plant conservation or recovery plans. Paper to the Bern Convention. Council of Europe, Strasbourg.
Falk, DA & Holsinger, K.E. (Eds.) 1991. Genetics and conservation of rare plants. Oxford University Press, New York.
Fleming, LV. & Sydes, C. On press). Genetics and rare plants: guidelines for recovery programmes. JNCC/BES
IUCN. 1995. Guidelines for reintroductions.
IUCN/SSC reintroduction specialist group.
Co-adapted gene complexes: a group of genes whose combined effects confer an
advantage in fitting different conditions.
Effective population size (Ne): the average number of individuals in a population which contribute genes equally to succeeding generations (note that the effective size is usually much lower than the total population size).
Founder effect: the result of only a fraction of the genetic variation of a parent population being present in the small number of founder members of a new colony or population. Genetic drift random changes in gene frequency due to factors other than natural selection.
Heterozygosity: proportion of heterozygotes for a given locus in a population, i.e. an expression of genetic variation.
Inbreeding depression: loss of vigour following breeding between related individuals due to expression of deleterious genes in the homozygous state.
Mutation: a change in the amount, or chemical structure, of DNA resulting in a change in a characteristic of the organism.
Outbreeding depression: loss of vigour following breeding between unrelated individuals due to loss of local adaptation or break up of co-adapted gene complexes.
Stochastisity: a range of outcomes that arise due to chance alone.
Dr Vin Fleming/Dr Chris Sydes
International and Biodiversity Branch
Scottish Natural Heritage
2 Anderson Place
Edinburgh EH6 5NP
Tel: 0131-447 4784
Back to menu