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CONCEPTS AND QUESTIONS A role for assisted evolution in designing native plant materials for domesticated landscapes TA Jones* and TA Monaco Developers of native plant propagation materials for wildland restoration may emphasize naturally occurring genetic patterns or, in contrast, the material’s empirical performance in comparative field trials. We contend that both approaches have value and need not be mutually exclusive. Anthropogenic influences have pushed many ecosystems across ecological thresholds, to less desirable states, so that actively managing for “domesticated nature” – nature as modified, either intentionally or inadvertently, by humans – is more realistic and more likely to succeed than recreating the original ecosystem. Furthermore, when domesticated nature is the most reasonable objective, empirical performance, together with geographical origin, are plausible criteria for choosing restoration plant material. For altered ecosystems, we suggest that evolution should be assisted by the inclusion of plants that (1) reflect general historical evolutionary patterns, (2) are particularly suited to the modified environment, (3) are able to adapt to contemporary selection pressures, and (4) contribute to the restoration of ecosystem structure and function. Front Ecol Environ 2009; doi:10.1890/080028 A s a matter of policy, scientists and land managers sometimes emphasize protection and sustainability of lands that have been only minimally altered by anthropogenic influences. The goal, with these less altered ecosystems, is to maintain endemic genotypes. On the other hand, many ecosystems have been drastically altered (ie domesticated), leaving very few truly wild places on Earth (Kareiva et al. 2007). Previous generations of conservationists have written off lands impacted by humans as “lost causes”, yet the discipline of restoration ecology has emerged, with the goal of restoring ecosystem processes to such lands (MacMahon 1997). Many scientists believe that these domesticated systems must be managed at some point (Gallagher and Carpenter 1997). Hobbs et al. (2006) suggest managing highly modified systems for utilitarian purposes when a In a nutshell: • Indigenous genetic material may no longer be adapted in modified ecosystems that have crossed ecological thresholds • Genetically manipulated plant materials developed to overcome common biotic and abiotic stresses are useful for restoring ecosystem structure, function, and biodiversity in modified ecosystems • Considering both empirical performance and geographic origin is important when fashioning plant materials to assist evolution along a desirable trajectory USDA-Agricultural Research Service, Forage and Range Research Laboratory, Utah State University, Logan, UT *(thomas.jones@ ars.usda.gov) © The Ecological Society of America return to their previous state is not feasible. Kareiva et al. (2007) challenge scientists to manage these landscapes by balancing trade-offs between ecosystem services. Ultimately, to be successful, management objectives must be based on pragmatism (Hobbs et al. 2006) and sound scientific principles (Gallagher and Carpenter 1997). Here, we consider issues surrounding the development and choice of plant propagation materials for restoring domesticated ecosystems. First, as an example of a domesticated landscape, we describe the modification of western North America’s sagebrush steppe by an invasive plant–wildfire cycle. Next, we make the argument for plants that restore evolutionary and ecological processes under such dramatic circumstances. Finally, we contend that human-assisted evolution offers the best hope for repairing ecosystem structure and function when landscapes have become domesticated. We use the term “ecological restoration” in the broad sense – that is, “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed”, as defined by the Society for Ecological Restoration International Science and Policy Working Group (SER 2004). A controversial topic: options for restoration plant materials Two paradigms have long vied for the allegiance of scientists who develop plant materials for restoration. The “evolutionary” paradigm seeks to restore putative natural patterns of genetic variation, in order to generate an evolutionary trajectory as similar as possible to that which prevailed before the advent of any anthropogenic disturwww.frontiersinecology.or g Assisted evolution in domesticated landscapes bance, such as weed invasion or soil degradation. Great importance is therefore placed on genetic identity. In contrast, the “resource” paradigm depends on the delivery of economic products and ecosystem services, with minimal regard for species or genotype. These two models have arisen within scientific traditions with contrasting attitudes toward the conservation of biological resources and with distinct philosophical beliefs as to how the world should be. In the US, this distinction corresponds to the historic division between the biocentric philosophy of John Muir and the anthropocentric philosophy of Gifford Pinchot. These contrasting viewpoints influence the choice of seeds used in restoration projects; indeed, there has long been a healthy diversity of opinions regarding standards for the genetic identity of plant materials with the ecosystem restoration community. But what is the best approach in choosing plants for the restoration of modified environments where domesticated nature is the most viable remaining option? Vast landscapes in the North American Intermountain West, for example, are increasingly being modified by plant invasions, degradation of soil and hydrological processes, and loss of biodiversity. Under such conditions, the evolutionary paradigm alone will not be able to overcome the problems encountered when modified conditions render local populations less adapted than before. Here, we suggest that both paradigms have an important role in the restoration of damaged wildlands. Wise land managers will try to combine principles from both traditions to effectively realize their restoration objectives. We developed the Restoration Gene Pool (RGP) concept (Jones 2003; Jones and Monaco 2007) to clarify plant options for restoration. The four RGPs – primary, secondary, tertiary, and quaternary – are placed in descending order of preference to the restoration practitioner. The primary RGP includes materials that are indigenous to the restoration site and to ecologically similar sites, as well as material from sites that are genetically connected to the site. Typically, seeds are either collected directly from the site or propagated under cultivation from wildland seed collections. The secondary RGP consists of target-species material that is disqualified from the primary RGP, based on the definition given above. Typically, secondary RGP materials are intended for broader geographical use than are primary RGP materials; they are often “releases”, either cultivars or pre-variety germplasms (propagating material that has not yet been released as a variety), meaning that they have undergone an administrative approval process based on evaluation of adaptive performance before distribution to seed growers for commercial production. The tertiary RGP consists of material that has been intentionally manipulated through hybridization across a natural genetic barrier, in order to introduce traits of adaptive or economic importance. The quaternary RGP is composed of functional surrogate species that are used when the target species is no longer adapted to the modified environment. www.fr ontiersinecology.or g TA Jones and TA Monaco When are genetically manipulated plants an appropriate choice? Anthropogenic disturbances may cause plant communities to cross one or more ecological thresholds, which will be difficult or impossible to reverse. We suggest that the decline of local genotypes under such conditions indicates that the primary RGP is no longer as well adapted to the local environment as it once was, and that this decline is a consequence of natural selection, a process that under “natural” circumstances is considered desirable (Meffe and Carroll 1997). The anthropogenic perturbation of the sagebrush–steppe ecosystem is unnatural, but the biological response to the perturbation is in accordance with the normal workings of nature (Botkin 1990). Although the “local is best” assumption (Johnson et al. 2004) that underpins the general preference for the primary RGP is arguably valid when thresholds have not yet been crossed, we contend that, for altered ecosystems, this debate may be moot. Instead, the appropriate question becomes, “is local still good enough to confer acceptable ecological function now that novel evolutionary forces are at work?”. We believe that local may no longer be good enough, based on simple observation; that is, if local was still good enough, we would not have an invasive plant problem. The essential question then becomes, “can we design better plant materials for the monumental challenge sitting on our doorstep?”. Costs and risks If we are to pursue a comprehensive plant materials approach, such as that described above, the associated costs and risks must be considered. The first cost results from the research infrastructure required to develop the plants. A multidisciplinary, problem-solving approach and a long-term commitment are essential. Specific goals for improving ecological function in western North America include controlling wildfire, inhibiting invasive plant populations, and restoring soil structure and nutrient dynamics. To be successful, plant ecologists, physiologists, soil scientists, rangeland scientists, seed scientists, geneticists, plant breeders, weed scientists, and engineers must all work together to confront the problem at various ecological scales. The research objective must be to develop seeding technologies, weed management protocols, and plant materials that may be prescribed for rehabilitating the land and restoring ecological structure, function, and diversity. Prerequisites for success include an understanding of how desirable species interact with invasive plants at all life stages, and how these interactions may favorably enhance the population demographics of desirable species. It is also important to understand how these communities assemble (Nuttle et al. 2004). In this regard, a good working knowledge of (1) functional groups and the concomitant redundancy among species and (2) the traits that confer such functional attributes is useful, especially © The Ecological Society of America TA Jones and TA Monaco Assisted evolution in domesticated landscapes Oregon Oregon Idaho Burns Idaho Burns Idaho Falls Idaho Falls Boise Boise Twin Falls Twin Falls Klamath Falls Klamath Falls Elko Elko Salt Lake City Nevada Reno Utah Ely Nevada Utah Ely or lif Ca a ni a ni or lif Ca Cedar City Cedar City Las Vegas Las Vegas (a) (b) Figure 1. Maps of the floristic Great Basin showing (a) BLM-administered lands (yellow) and (b) sagebrush–steppe rangelands (green). following disturbance. The ultimate objective is to restore native plant populations in order to maximize resource use over time and space in order to assemble weed-resistant plant communities (Sheley and Carpinelli 2005). To place plant material development efforts on an ecological footing, we integrate them with the successional management model developed by Pickett et al. (1987). This model identifies three general causes of succession: site availability, species availability, and species performance. Sheley et al. (2006) provided data to support the hypothesis that establishment and persistence of desirable native plants are enhanced when weed management protocols address factors that modify or repair processes that influence Pickett et al.’s (1987) three causes of succession. Some biologists believe that the release of novel genetic material is inherently risky. For example, outbreeding depression (ie offspring from crosses between individuals from different populations have lower fitness than offspring from crosses between individuals from the same population) may result when seeded material hybridizes with remnants of naturally occurring populations (Hufford and Mazer 2003). This potential risk is reduced by choosing plant materials that reflect the most current scientific understanding of infraspecific taxonomy and metapopulation structure. It is also important to remember that when outbreeding depression is expressed in initial hybridization products, it may still be overcome as a result of the processes of natural selection and/or introgression of small portions of one genotype into another (Carney et al. 2000), ultimately resulting in adapted plant material. © The Ecological Society of America A second concern is the maladaptation of novel plant material over the long term. In fact, restoration is necessitated because the indigenous plants have proven to be maladapted in the short term. Conversely, some researchers consider genetically altered materials to be too well adapted and therefore potentially invasive, but this argument does not hold in stressful or competitive environments, where unaltered genetic material of the same species has been unsuccessful in coping with abiotic stress or competition. Finally, because of serious concerns regarding the genetic integrity of threatened and endangered species, genetic manipulation of closely related plants must be avoided, to preclude undesirable hybridization. Ecosystem degradation in the sagebrush steppe The US Department of the Interior’s Bureau of Land Management (BLM) administers 73 million acres of rangeland in the 135 million-acre floristic Great Basin (Figure 1a). Fifty-seven million acres (54%) in this region are occupied by sagebrush–steppe ecosystems, with the iconic big sagebrush (Artemisia tridentata Nutt) as the primary sagebrush species (Figure 1b). Invasive annual grasses, particularly cheatgrass (Bromus tectorum), are an increasing threat to the integrity of these wildlands. Cheatgrass has come to dominate large areas of this region, and even more extensive areas are believed to be at risk of future invasion (Wisdom et al. 2005; Bradley and Mustard 2006; Figure 2). Cheatgrass can adapt to novel environments, possibly because its www.frontiersinecology.or g M Pellant/BLM Reno Salt Lake City M Pellant/BLM Assisted evolution in domesticated landscapes Cheatgrass risk Decile Low High TA Jones and TA Monaco N 0 200 400 Kilometers Figure 2. Map of risk of cheatgrass distribution, based on soil, elevation, and precipitation variables. substantial genetic diversity is highly correlated with its habitat and ecological traits (Ramakrishnan et al. 2006). Moreover, it has a high degree of phenotypic plasticity (Novak et al. 1991). Wildfire frequency has greatly increased in the floristic Great Basin, as a result of cheatgrass invasion (Figure 3). Cheatgrass generates horizontally continuous fuels with surface-to-volume and fuel-packing ratios that favor ignition and perpetuate a cheatgrass–wildfire cycle (Brooks et al. 2004). This cycle results in conversion of sagebrush–steppe ecosystems to cheatgrass-dominated annual grasslands that experience frequent wildfires, resulting in extirpation of local populations of fire-intolerant shrubs, such as big sagebrush (Whisenant 1990). From 1990 to 2007, over 16 million acres burned in the Great Basin region, and nearly 2 million of these involved repeated burns of the same acreage (M Pellant pers comm). The most extensive fire years in this period have been 1996 (2.2 million acres), 1999 (2.7 million), 2000 (1.9 million), 2001 (1.6 million), 2006 (1.8 million), and 2007 (2.7 million). On the other hand, in some years, fire in the region is relatively infrequent, eg 1993 (43 000 acres), 1997 www.fr ontiersinecology.or g (92 000), and 2004 (17 000), highlighting the vacillating nature of the problem. Whisenant (2002) provides a three-stage model that reflects this ecosystem degradation. Degradation increases, initially, as a biotic threshold is crossed (A¡B) and, subsequently, as an abiotic threshold is crossed (B¡C; Figure 4). At stage A, recovery of damaged ecosystem function is facilitated by removal of the causal agent (eg excessive grazing pressure). After crossing the biotic threshold from stage A to B, manipulation of biota – seeding, for instance – is required, but abiotic function remains mostly intact. Large expanses of the sagebrush–steppe ecosystems have crossed the biotic threshold (A¡B) as a result of cheatgrass invasion (Figure 5). After crossing the abiotic threshold from stage B to C, biotic processes are severely impaired, and abiotic processes (eg soil stability, hydrology, and nutrient dynamics; King and Hobbs 2006) recover only with intensive human intervention. Emerging evidence suggests that cheatgrass dominance is moving sagebrush–steppe ecosystems across an abiotic threshold (B¡C) as soil structure (Norton et al. 2007) and nutrient dynamics (Saetre and Stark 2005) are altered. Resource managers recognize that distinct seral stages are a consequence of the current disturbance regime (Allen-Diaz and Bartolome 1998). Restoration plans in relation to evolutionary and ecological processes Many have lamented that restoration plans commonly fail to consider evolutionary processes, particularly for drastically altered, domesticated ecosystems (Smith and Bernatchez 2008). Ashley et al. (2003) discuss the advisability of permitting evolution to confer adaptation to disturbed habitats, a relatively rapid process referred to by Hendry and Kinnison (1999) as “contemporary evolution”. A functional, altered state may be preferred over the predisturbance state, when the latter is problematic because of an ongoing need for human intervention (Ashley et al. 2003), financial limitations, or shifting priorities (Schlaepfer et al. 2005). Ashley et al. (2003) attribute preference for the predisturbance state to typological thinking – that is, the idea that species are fixed entities. This viewpoint seems to be motivated by the equilibrium (“balance of nature”) paradigm, a view of nature that has lost favor in recent years (Botkin 1990; Pickett et al. 1992). We propose a comprehensive view for plant materials development for the greatest restoration challenges – those domesticated landscapes that have moved to a per© The Ecological Society of America Assisted evolution in domesticated landscapes Ecosystem function manently altered state, as has occurred in the Montana floristic Great Basin. Our comments are based on this region as a case study and must not be Oregon construed as generally applicable to more malBoise leable environments that are relatively responsive to restoration treatments; nevertheless, our remarks may apply to other, highly modiIdaho fied systems. In order to determine whether they apply to some other recalcitrant systems, we suggest posing the following questions: Are undesirable modifications reversible? Is it possible to restore species composition, ecological Salt Lake function, and successional and evolutionary City processes that prevailed prior to disturbance? If Carson City so, B¡A conversion may be achieved by reintroduction of the primary RGP. Under such Nevada circumstances, we believe that it is appropriate to focus on genetic identity when choosing Utah plant materials. In contrast, when ecosystem functions and California Fresno services targeted for restoration have been severely impaired, the primary RGP may be inadequate to effect a successful restoration Las (Jones 2003). The primary aim, then, is to Vegas trigger natural processes that re-establish ecosystem form and function, such as in Figure 3. Burn areas of the floristic Great Basin from 1990 to 2007, with Whisenant’s (2002) stage B and especially 1990 to 2006 in orange and 2007 in red. stage C, commonly referred to as “rehabilitation”. Because a C¡B conversion is more difficult than a longer well adapted. Rice and Emery (2003) concurred, B¡A conversion, the former may require use of the ter- recommending the use of a mixture of genotypes from tiary or quaternary RGPs, whereas the primary or sec- multiple indigenous populations from a region with a similar climate (primary RGP), with genotypes from the ondary RGPs may suffice for the latter. When the intent is to mirror historical genetic patterns, fringe of the species’ distribution (secondary RGP). the primary RGP is naturally preferred, because of its asso- Such a strategy balances the needs for current and future ciation with historical (long-term) evolutionary processes adaptation. (Meffe and Carroll 1997). Nevertheless, as demonstrated experimentally by Kinnison et al. Increasing degradation (2008) with respect to fitness-related traits in Chinook salmon (Oncorhynchus tshawytscha), contemporary evolution acting on non-local Abiotic Biotic genetic material may rescue threatened poputhreshold threshold Primary processes lations (Stockwell et al. 2003). Projects that fully functional place a priority on increasing effective populaPrimary processes partially functional tion size and precluding selection often fail to Primary processes consider that adaptation entails selective loss nonfunctional of genetic variation, increased inbreeding among individuals with higher fitness, and ini(A) (C) (B) tial reductions in effective population size Improved Physical (Kinnison et al. 2007). management modification Manipulation of Microevolutionary processes may generate required required biota required site-adapted genotypes from generally adapted ones (Moritz 2002). Priming these Time or disturbance intensity processes with sizable quantities of adaptive genetic variation may be more beneficial than insisting on conformity to naturally Figure 4. Three stages of degradation of ecosystem function (A, B, and C) occurring genetic arrays (ie the primary separated by biotic (A¡B) and abiotic (B¡C) thresholds. Modified from RGP), particularly if such material is no Whisenant (2002). © The Ecological Society of America www.frontiersinecology.or g M Pellant/BLM TA Jones and TA Monaco Assisted evolution in domesticated landscapes TA Jones and TA Monaco (d) (a) M Pellant/BLM M Pellant/BLM USDA-NRCS ural selection operates; (2) subjecting populations that reflect primary RGP boundaries, as established by phylogeographic, ecoregion, or seed-transfer zone research, to artificial selection to confer enhanced fitness; or (3) hybridizing indigenous material with non-indigenous material tolerant to biotic and/or abiotic stress. We are intrigued that such approaches have (b) (e) also been proposed for animal populations (Schlaepfer et al. 2005). In this vein, Ferrière et al. (2004) recommended harnessing the forces of evolution to restore domesticated ecosystems. Stockwell et al. (2006) have sought to link historically established genetic diversity (the past) with contemporary evolution (the present) and long-term evolutionary potential (f) (c) (the future). They argue that gene flow influences evolutionary change and enhances long-term persistence and that the positive effects of reduced inbreeding depression, increased genetic variation, and greater potential for future adaptation balance the negative impacts of gene flow. Emphasizing empirical performance Figure 5. Four stages of degradation of a sagebrush–steppe ecosystem: (a) fully in addition to geographic origin is a functional, with big sagebrush and perennial bunchgrass components; (b) crossing the pragmatic approach that focuses on biotic threshold (stages A¡B), as triggered by overgrazing and resultant loss of the what is technically feasible, yet respects perennial bunchgrass component; (c) stage B – invasion by cheatgrass; (d) high wildfire naturally occurring genetic arrays. The frequency, the trigger of the abiotic threshold (stages B¡C); (e) resultant loss of the big result may be plant materials that are sagebrush component; and (f) stage C – a cheatgrass-dominated annual community. better suited to domesticated landscapes under current and future conditions. Genetically manipulated material, based on local geno Conclusions types, may be incorporated into the Jones and Monaco Geographic origin is important because it confers adaptive (2007) flowchart as the primary-C RGP. The flowchart traits that are geographically relevant. Nevertheless, we already supports an analogous, genetically manipulated catbelieve that the tacit assumption that local material will egory for the secondary RGP, termed secondary-C. demonstrate optimal performance, adaptation, and fitness, Although this approach will not appeal to everyone, it is despite severe disturbance, is unwarranted. The critical less manipulative than other options mentioned by Brooks issue for restoration plant material must be its adaptation et al. (2004). These include creating novel ecosystems from to the modified environment, which encompasses both native species, corresponding to the quaternary RGP (Jones genetic identity and empirical performance. The questions 2003), or using exotics to suppress invasive plants (such as must be: Is the material physiologically able to tolerate the cheatgrass), corresponding to the category of reclamationenvironmental conditions and disturbance regime of the grade material (Jones and Monaco 2007). Finally, we believe modified site? Is it able to establish and persist, despite the that assisted evolution for domesticated ecosystems is compresence of other organisms, including invasive plants? patible with Moritz’s (2002) strategies that seek to maximize Does it have the adaptive genetic variation necessary to both representation and persistence of genetic diversity. allow natural selection to assemble site-adapted genotypes Moritz advocates both historical genetic variation for which as needed? In essence, the answers to these questions will adaptive importance is not immediately apparent (represenpredict the plant material’s prospects for success. tation) and genetic variation for adaptive traits (persisFor domesticated nature, assisted evolution is an attrac- tence). We advocate a mixture of caution and pragmatism, tive option. It is implemented by (1) augmenting genetic meaning the least obtrusive approach resulting in improved variation in locally adapted populations upon which nat- ecosystem function. Assisted evolution offers these qualities www.fr ontiersinecology.or g © The Ecological Society of America TA Jones and TA Monaco under particular circumstances and deserves serious consideration as an option for restoration. Acknowledgments The contributions of S Knick (USGS) and M Pellant (BLM) and the helpful critiques of M Kinnison, P Adler, E Leger, B Roundy, and A Svejcar are gratefully acknowledged. References Allen-Diaz B and Bartolome JW. 1998. Sagebrush-grass vegetation dynamics: comparing classical and state-transition models. Ecol App 8: 795–804. Ashley MV, Willson MF, Pergams ORW, et al. 2003. Evolutionarily enlightened management. Biol Conserv 111: 115–23. Botkin DB. 1990. Discordant harmonies: a new ecology for the twenty-first century. New York, NY: Oxford University Press. Bradley BA and Mustard JF. 2006. Characterizing the landscape dynamics of an invasive plant and risk of invasion using remote sensing. Ecol Appl 16: 1132–47. 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Handbook of ecological restoration. Vol 1. Principles of restoration. Cambridge, UK: Cambridge University Press. Wisdom MJ, Rowland MM, and Suring LH. 2005. Habitat threats in the sagebrush ecosystem: methods of regional assessment and applications in the Great Basin. Lawrence, KS: Alliance Communications Group. www.frontiersinecology.or g