The Restoration Gene Pool Concept: Beyond the Native Versus Non-Native Debate T
Total Page：16
File Type：pdf, Size：1020Kb
The Restoration Gene Pool Concept: Beyond the Native Versus Non-Native Debate T. A. Jones Abstract cision can be made individually for each plant species in Restoration practitioners have long been faced with a di- the restored plant community in the scientific context that chotomous choice of native versus introduced plant mate- ecosystem management demands. The restoration gene rial confounded by a general lack of consensus concerning pool concept provides a place for cultivars of native spe- what constitutes being native. The “restoration gene pool” cies and noninvasive introduced plant material when use concept assigns plant materials to one of four restoration of native-site material is not feasible. The use of metapop- gene pools (primary to quaternary) in order of declining ulation polycrosses or composites and multiple-origin genetic correspondence to the target population. Adapta- polycrosses or composites is encouraged as appropriate. tion is decoupled from genetic identity because they often The restoration gene pool concept can be implemented as do not correspond, particularly if ecosystem function of a hierarchical decision-support tool within the larger con- the disturbed site has been altered. Because use of plant text of planning seedings. material with highest genetic identity, that is, the primary restoration gene pool, may not be ultimately successful, Key words: genetic adaptation, genetic identity, metapop- material of higher order pools may be substituted. This de- ulation, multiple-origin polycross. Introduction Genetic Identity Versus Adaptation Uncertainty is often characteristic of the decision-making To begin this discussion I define genetic identity and ge- process when plant materials must be chosen for restora- netic adaptation. Nei (1972) measures the genetic identity tion. When one or more elements of the disturbed ecosys- between two populations on a scale from 0 to 1. If the ge- tem are dysfunctional or missing altogether, description of netic identity of populations X and Y is 1, then they have pristine form and function is usually speculative. The pri- identical alleles with identical frequencies, that is, they are mary objective of ecological restoration is “the reinitiation genetically synonymous. If their genetic identity is 0, they of natural succession that will lead to the reestablishment have no alleles in common at any locus, that is, X has one of ecosystem form and function” (Brown & Amacher or more alleles at each locus, all of which are different 1999). Following this logic a “native” is fundamentally from Y’s alleles at each locus. whatever contributes to this end.1 Although native mate- For a plant population to be adapted to a site, it must be rial may be preferred by the restorationist (Lesica & Al- able to persist and reproduce on the site and its progeny lendorf 1999), whether material is native or not is a matter likewise. Brown and Amacher (1999) asserted that adapt- of scale (is it native at the species, race, or population ability is defined by a physiological range of stress toler- level?) as well as a matter of adaptation, that is, ecological ance defined by genotype. It reflects the ability of the plant (does it interact with the biotic and abiotic elements of the to adjust to a fluctuating environment by structural modifi- ecosystem as it did before disturbance?) and physiological cation and physiological adjustment (Conrad 1983). There- (what are its tolerance limits?) (Brown & Amacher 1999). fore, adaptability is not a unit of measure per se. Because no unambiguous answer can be offered to the To understand the RGP concept one must first under- question of what is native, these authors even suggested stand that maximizing genetic identity between the target abandoning the use of the terms “native” and “intro- plant population and the restoration plant material does duced,” but they doubted that even that radical step would not necessarily maximize genetic adaptation. Considering lead to a conceptual improvement. Here I attempt to a target population from a particular site, a broad sample make that conceptual improvement through delineation of of seed taken from that site can be considered essentially what I term “restoration gene pools” (RGP). genetically representative of the target, that is, their ge- netic identity approaches 1 except for sampling error. A sample of seed of the same species from a site genetically USDA-ARS Forage and Range Research Laboratory and Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, U.S.A., connected to the target site via pollen transfer or seed dis- email [email protected] persal also has a relatively high genetic identity with the 1Professionals in plant-related field tend to define “native” less strictly than the target site. Genetic identities of local populations discon- lay public (Smith & Winslow 2001). nected from the target site may be lower but likely not as © 2003 Society for Ecological Restoration International low as populations subjected to altogether different selec- SEPTEMBER 2003 Restoration Ecology Vol. 11 No. 3, pp. 281–290 281 Restoration Gene Pool Concept tion pressures. And such populations of the given taxon The RGPs are ordered from primary to tertiary in de- have higher genetic identity to the target site than separate scending level of genetic identity to the target population. yet closely related taxa, which in turn have higher genetic The primary RGP consists of the target population itself identity to the target than disparate taxa. or material genetically connected to it via pollen flow or This is not to say that the genetically connected popula- seed dispersal, whereas the quaternary RGP consists of tion is better genetically adapted than the disparate taxon. material of a different taxon from the target population al- Indeed, it is commonly experienced that unrelated taxa together. Primary RGP material is preferred when it is from a different hemisphere may have superior genes for available and when the ecological function of the target adaptation to an altered site (and sites targeted for restora- site has not been fundamentally altered in a manner that tion are always altered). The primary objection to such a makes such material no longer adapted. But when either taxon is that it is not native; hence the long-standing empha- of these two conditions do not hold, materials from higher sis on the native/non-native dichotomy. However, this is an order RGPs may be substituted. In practice, secondary issue of genetic identity rather than genetic adaptation. RGP material will be most commonly substituted when My point is that although we may prefer high genetic primary RGP material is simply unavailable. Tertiary or identity for restoration in theory, use of this material may quaternary RGP materials will be substituted under the be problematic in practice. This may be because the mate- more challenging circumstances of major disruption of ec- rial of choice is unavailable, is difficult or expensive to prop- osystem function. agate, or is no longer adapted to the altered environment. When any of the above hold, and we should be alert for such circumstances, we should act on alternate possibilities without apology because inaction may be even less desirable An Explanation of the Concept Using Bluebunch than implementing an action other than the preferred alter- Wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve) native (Jones 1997). Lesica and Allendorf (1999) pointed out that choice of the correct strategy depends on under- Primary RGP standing trade-offs. The restoration practitioner must at- Genetic identity of the primary RGP with respect to the tempt to understand the degree and pattern of genetic vari- target population is high. The genetic structure of the pri- ation for the target species, which can be thought of as its mary RGP retains the original levels of heterozygosity “genetic personality.” The correct approach, that is, the (genetic variation within an individual that may be docu- choice of which RGP (as described below) to use, depends mented by phenotypic variation among progeny) and het- on the target species itself and on the target environment. erogeneity (genetic variation between individuals as re- flected by noncorrespondence of the progeny of one individual with those of another). The primary RGP also An Introduction to the Concept retains any biotypes (genetically controlled subpopula- Here I adapt a concept developed for the discipline of tions with different form or function), that may have been breeding of cultivated plants for use by the discipline of present in the original population. The primary RGP in- restoration ecology. Harlan and DeWet (1971) defined the cludes only material from the target site plus adjacent primary gene pool as the biological species, that is, all ma- areas that are genetically connected to the site via gene terials that easily cross, generating offspring with approxi- flow, that is, the metapopulation (Antonovics et al. 1994). mately normal fertility and segregation in succeeding Metapopulations can be thought of as subunits of a spe- generations. Harlan and DeWet’s secondary gene pool cies between which there is limited genetic exchange. A included all other biological species that have significant metapopulation may encompass sites that are ecologically genetic incompatibility barriers to crossing but may cross different but lack genetic isolation from the target popula- with the primary gene pool under natural, albeit excep- tion. Selection pressure exerted by a heterogeneous envi- tional, circumstances. This “greater species” is termed a ronment must overcome gene flow from adjacent popula- coenospecies (Clausen et al. 1939). Harlan and DeWet’s tions for natural selection to operate, which generally tertiary gene pool includes taxa that may be crossed with requires a landscape that permits isolation of local popula- the species of interest but only through extreme measures tions (Jones 1997). Moritz (1999) emphasized the impor- that would probably occur at most rarely in nature.