J. Range Manage. 55: 571-575 November 2002 Viewpoint: The ecological value of shrub Islands on disturbed sagebrush rangelands

WILLIAM S. LONGLAND AND SHERYL L. BATEMAN

Authors are Ecologist, USDA, Agricultural Research Service, 920 Valley Road, University of Nevada, Reno, Nev. 89512; and Graduate Research Assistant, Department of Biology, University of Nevada, Reno, Nev. 89557.

Abstract Resumen

Undisturbed plant communities dominated by shrubs or trees Comunidades vegetales sin disturbio dominadas por arbustos are often left isolated within landscapes otherwise devoid of y arboles a menudo se quedan aisladas dentro de los paisajes woody vegetation following large-scale disturbances such as wild- desprovistos de vegetacion lenosa despues de ser sujetos a distur- fires. We discuss potential ecological benefits associated with bios de gran escala como fuegos naturales. Discutimos los benefi- these terrestrial vegetation "islands", giving special attention to cios ecologicos potenciales asociados con esta "islas" terrestres de islands in disturbed shrub systems dominated by big sagebrush vegetacion, poniendo especial atencion a las islas en los sistemas ( Nutt.). Shrub habitat islands provide impor- arbustivos disturbados dominados por "Big sagebrush" tant refugia for plant and species that are associates of (Artemisia tridentata Nutt.). Las islas de arbustos se proveen un shrubs-from those that generally require shrub cover to those refugio importante para especies de plantas y animales que estan that have evolved obligate symbioses with a particular shrub asociadas a los arbustos, desde aquellas que generalmente species. Even if islands are not able to support breeding popula- requieren de una cobertura de arbustos hasta aquellas que han tions, they may provide essential temporary habitat for main- evolucionado simbiosis obligadas con especies arbustivas partic- taining a plant or animal metapopulation or for dispersing ani- ulares. Aun si las islas no son capaces de soportar poblaciones en mals. Habitat islands are likely to enhance local biological diver- reproduction ellas pueden proveer un habitat temporal esencial sity of plants and , because they harbor species that are para mantener una poblacion de plantas o animales o para ani- lacking in disturbed areas, and because abrupt structural males dispersantes. Los habitats de las islas probablemente changes from disturbed to undisturbed vegetation provide a aumentaran la diversidad biologica de plantas y animales porque habitat mosaic that facilitates high levels of species turnover. A ellas refugian especies de las que carecen las areas distrubadas y previous study confirmed that small mammal species richness in porque los cambios estructurales abruptos de una vegetacion dis- sagebrush islands is intermediate to the high species richness in turbada a una sin disturbio proveen un mosaico de habitats que undisturbed sagebrush "mainlands" and the low richness associ- facilitan altos niveles de movimiento de especies. Un estudio pre- ated with burned sagebrush habitats. In re-analyzing some of the vio confirmo que la riqueza de especies de pequenos mamiferos data from the latter study, we found that small mammal richness en las islas de "Sagebrush" es intermedia entre la alta riqueza de in sagebrush islands increases with time since the surrounding especies de las areas sin disturbio y la baja riqueza asociada con habitat burned. Finally, habitat islands provide more evenly dis- los habitats de "Sagebrush" quemados. Al reanalizar los datos persed seed sources for re-establishment of decimated vegetation del ultimo estudio encontramos que la riqueza de pequenos within disturbed areas, and they may harbor animal species that mamiferos en las islas de "Sagebrush" aumenta con el tiempo a provide seed dispersal services. Thus, they should accelerate veg- partir de los habitats quemados que las rodean. Finalmente, los etation recovery after disturbance. Managers, fire crews, and habitats de las islas proveen fuentes de semilla con una disper- others who may influence how disturbance patterns affect habi- sion mas uniforme para el reestablecimiento de la vegetacion tat heterogeneity should be aware of these ecological benefits of diezmada dentro de las areas disturbadas y eilas tambien pueden habitat islands. refugiar especies animales que pueden dispersar la semilla. Asi, ellos deben acelerar la recuperation de la vegetacion despues de un disturbio. Manejadores, cuadrillas de bomberos y otros Key Words: Artemisia tridentata Nutt., biological diversity, dis- quienes pueden influir en como los patrones de disturbio afectan turbance, Island Biogeography Theory, succession, terrestrial la heterogeneidad de habitats debe estar alerta de estos benefi- habitat islands cios ecologicos de las islas de habitat.

An alarmingly high proportion of arid western rangelands has been and continues to be converted to weedy monocultures as a on the most extensive spatial scale is undoubtedly wildfire. Dense result of disturbances that devastate native plant communities. understories of non-indigenous, herbaceous weeds among native The specific disturbance that has caused this floristic conversion and shrub communities have permitted fire to become common- place in contemporary desert or semi-desert environments, where Authors wish to thank Sanjay Pyare, James Young, and three anonymous it was historically infrequent or even virtually absent (Billings reviewers for commenting on the manuscript. This study is a contribution of the 1990, Longland and Young 1995). Although the scale of cata- USDA, ARS, Exotic and Invasive Weed Research Unit, Reno, Nev. strophic fires can denude Manuscript accepted 4 Mar. 2002. the vast majority of an entire landscape

JOURNAL OF RANGE MANAGEMENT 55(6) November 571 of native vegetation, patches or "islands" of vegetation are often left isolated within the burned matrix due to natural or human- made firebreaks, changes in wind direction while a fire burns, or other fortuitous caus- es (Fig. 1). We believe that such islands of unburned native vegetation embedded within an extensive disturbance have poten- tially great ecological value, and we discuss those values here. Although we focus on fire as a disturbance agent and on shrub habitat islands dominated by big sagebrush (Artemisia tridentata Nutt.), the benefits of such islands are applicable to other agents of disturbance and perhaps to islands of native herbaceous vegetation, as well.

Habitat Islands as Refuges

There are several species that are obligate associates of the shrub species that domi- Fig. 1. Large > 37 ha) sagebrush island isolated within a burned landscape near Pyramid nate undisturbed sagebrush rangelands. Lake, Nev. These include understory plant species that utilize shrubs as nurse plants, species that degree of habitat complexity, such as that islands in high densities following fires, are parasitic on shrubs, herbivores on provided by shrub cover. One such species, and the islands may therefore serve an shrubs, and animal species that require the the pygmy rabbit (Sylvilagus idahoensis), important function in the persistence of a habitat structure provided by shrubs. is an obligate associate of sagebrush habi- metapopulation of these species (Bateman herbivores are often highly spe- tats (Weiss and Verts 1984), and, like the 1999). For such species, sagebrush islands cialized on particular plant species, includ- sage grouse; may be given threatened sta- can not only maintain metapopulation ing sagebrush (Christiansen et al. 1989, tus soon. There are many other examples structure by facilitating recolonization of Wiens et al. 1991). Various species of of small vertebrates that routinely avoid extinct subpopulations through dispersal; (Hsiao 1986, Strenge and Zack disturbed shrub communities, not because these habitat islands may also serve to 2001) and beetles (Pringle 1960), for of specialization on sagebrush or some reduce local extinction probability, which example, are known to impact sagebrush, other plant species that is lacking after the appears to be relatively more important to sometimes causing significant mortality. disturbance, but because they would be at metapopulation persistence than an Any such phytophagous species that feeds greater risk of predation due to increased increase in the probability of recoloniza- specifically on sagebrush will obviously conspicuousness where shrub cover is tion (Etienne and Heesterbeek 2001). be doomed locally wherever sagebrush has absent (Longland and Price 1991). These been removed by disturbance. Such spe- examples illustrate that, in addition to the cialization is less common and usually less shrub species that are directly impacted by Biological Diversity among terrestrial vertebrates than extreme disturbance, many closely associated in , but it does still occur. Sage species may also persist only within shrub grouse (Centrocercus urophasianus) and Island Biogeography Theory (IBT) is a mainland and island habitats following a sagebrush voles (Lagurus curtatus), for well developed set of ideas that generate large-scale disturbance. example, require sagebrush in their diets, predictions concerning how physical Shrub habitat islands provide benefits and would quickly perish where sagebrush attributes of islands should affect the to dispersing animals as well as residents has been removed by fire (cf., Nelle et al. diversity of species occurring on those within the islands. For any shrub-associat- 2000). Refugia provided by sagebrush islands. The theory reasons that the num- ed species with limited dispersal ability, islands could be very important to such ber of species that are able to successfully shrub islands provide "stepping stones" by species. This has particular practical sig- colonize an island from a mainland source acting as temporary refuges to facilitate nificance in the case of sage grouse, a is positively related to the size of the dispersal. Shrub islands may be useful in species that may be granted threatened sta- island, because more species can potential- this regard even when they are too small tus in the near future. Some herbivores ly coexist in a larger area, and is negatively to retain a breeding population. Similarly, that are more generalized, such as mule related to the distance of the island from habitat islands can provide essential tem- deer (Odocoileus hemionus), frequently the mainland, because successful dispersal porary habitat for transient animals or for include certain subspecies or varieties of is less likely as distance increases. Island the maintenance of a metapopulation, a sagebrush in their diets (Wambolt 2001, Biogeography Theory, as conceived by group of spatially disjunct subpopulations Welch and McArthur 1986), and thus MacArthur and Wilson (1967), was intend- that are interlinked and maintained by would also be expected to be negatively ed for application to true oceanic islands, occasional dispersal among the subpopula- impacted by sagebrush removal. but its concepts were soon applied to the tions. For example, certain small mammal species with insularization of terrestrial environments, There are also many animal species concentrate within sagebrush niche requirements that include some such as forested mountains surrounded by

572 JOURNAL OF RANGE MANAGEMENT 55(6) November 2002 desert valleys (Brown 1971). We might sagebrush habitats, species richness and on near than far islands. For terrestrial expect these areas to represent "biological evenness tended to be greater in the islands, however, isolation from their islands" to the species that are unique to unburned, intact shrub communities mainland species sources is not as difficult such environments. Island Biogeography (Longland 1995, Halford 1981). Second, to overcome as the isolation of true ocean- Theory may thus provide a useful frame- in an extensive study comparing small ic islands. Many animal species, especially work for considering effects of shrub insu- mammal communities in burned areas, those with generalized habitat affinities, larization on biological diversity. sagebrush islands within the burns, and in may be uninhibited from crossing large One assumption of IBT almost certain to adjacent, continuous sagebrush communi- disturbances to reach habitat islands. be true of shrub islands isolated by distur- ties, small mammal species richness typi- Furthermore, the degree of isolation bance is that the species occurring in the cally was greatest in continuous sage- decreases over time for a terrestrial island islands are a subset of those that occur in brush, lowest in the burned areas, and as the disturbed area that isolates it under- undisturbed mainland plant communities. intermediate in the islands (Bateman goes succession and becomes more similar Habitat islands may contain the majority 1999). Furthermore, such beneficial to the island and mainland habitats. Thus, of the mainland species, but except for effects of habitat islands may extend to the effect of isolation distance in habitat very large islands, they are likely to at higher trophic levels in a "bottom-up" islands may be much greater on a short- least lack uncommon mainland plant fashion; by providing refugia for prey lived species, especially one with poor dis- species. And, while they may lack certain species, islands could favor enhanced persal ability, such as some flightless species that could potentially exist there, local diversity of predator species as well insects, small mammals, or annual plants, these unburned shrub islands certainly har- (Hixon and Beets 1993). than on longer-lived species, such as bor species of woody shrubs that are lack- Another assumption of IBT that is likely woody shrubs. For long-lived taxa, the ing in surrounding burned areas. Like to hold true in the context of terrestrial disturbed habitat surrounding an insular- unburned mainland habitats, unburned islands isolated by large-scale distur- ized population may undergo succession islands also generally include (albeit in bances, is that island size should be posi- rapidly enough that the population is no lower densities) the same native herba- tively related to the number of species longer effectively isolated by the time ceous plants and the same introduced inhabiting islands. First, chance inclusions mortality within the island proceeds far weeds that quickly establish in and domi- of rare plant or animal species are simply enough to cause a risk of local extinction. nate the burned areas. By contrast to more likely to occur within large islands If one considers isolation of habitat unburned vegetation associations, early than smaller islands. Also, larger islands islands in terms of effects of time and suc- successional vegetation within burned offer more opportunities for heterogeneity cession, IBT may offer relatively more areas is usually composed of just a small in soil types, exposure, topography, and insight into the value of these islands than subset of the local plant species pool and other landscape features that are likely to it does through the usual considerations of often approaches a monoculture. promote plant species diversity. The isolation by distance. As an illustration, It is clear, then, that for some time dur- greater diversity of plants and physical we reanalyzed Bateman's (1999) data on ing early post-fire succession unburned features in larger islands should also trans- small mammal species richness in 23 shrub islands harbor both more plant late into greater animal species diversity. sagebrush islands created by fire versus species and consequently greater floristic Moreover, larger islands are more likely to paired sagebrush "mainlands". Small genetic diversity than does the surround- include larger animal species, because mammal species richness was reduced in ing disturbed area. Such measures of their home range requirements may 19 of the islands compared with local diversity within a relatively small patch exceed the amount of habitat offered by species pools; only 4 islands contained all are often referred to as a-diversity, where- many small islands. possible species. The mean age (i.e., time as 3- and y -diversity describe, respective- The utility of another assumption of since burning) of the latter 4 islands was ly, the degree of species turnover among IBT-that the distance an island is isolat- approximately double that of islands with different patches in an area and the dis- ed from mainland should be inversely reduced species numbers (16.0 vs. 8.1 tinctness of patches across a landscape related to species diversity harbored by the years, respectively), a significant differ- (Longland and Young 1995). Because island-is less clear for terrestrial habitat ence based on the 1-tailed expectation of entire plant taxa represented in unburned islands than the assumption concerning lower species richness resulting from areas are absent from surrounding burns, island size. When a disturbance, such as greater isolation (less time for succession, species turnover between burned and fire, leaves behind an isolated island of and therefore less similar vegetation unburned patches is dramatic, and the dis- habitat, there is no reason why the diversi- between burned and unburned areas; Fig. tinctness of burned versus unburned patch- ty of species contained within the island 2a; 1-tailed t = 2.01; df. = 21, P = 0.029). es across a landscape is apparent. Thus, should be influenced by the distance that Relaxing the requirement of all species unburned vegetation islands enhance the island is left isolated from undisturbed being sampled within the islands, we con- floristic 13- and y-diversity at these larger (i.e., mainland) habitat. A distantly isolat- sidered only those 10 sites in Bateman's spatial scales, as well as a-diversity within ed island is likely to contain as many data with < 5 small mammal species. At 4 the islands themselves. species after a fire as an island that is very of these sites where > 80% of the local In theory, animal biodiversity should close to the edge of the burned area. In small mammal species occurred in sage- also be greater in unburned plant commu- classical IBT, it is the long-term processes brush islands, mean time since the burn nities than in the more uniform habitats of colonization and extinction of species which isolated the islands was, again, sig- provided by early successional, post-fire on islands that leads to the expectation that nificantly greater (16.8 years) than at the 6 vegetation, and there is some empirical greater isolation yields lower species sites where < 60% of species were evidence that this is so. First, in compara- diversity. The extinction rate of species is retained in the island habitats (9.2 years; tive studies of small mammal species expected to be independent of isolation Fig. 2b; 1-tailed t = 2.78, d.f. = 8, P < diversity in burned and adjacent unburned distance, while colonization is more likely 0.012).

JOURNAL OF RANGE MANAGEMENT 55(6) November 573 models. For example, traditional IBT 30 -1 A assumes that islands are inhabited by colo- nization (MacArthur and Wilson 1967), in 25-1 whereas Bateman's (1999) sagebrush islands inhabitation can occur through sur- vival of residual populations within the 20 unburned islands. This is obviously the case for the plant populations occurring in 4 these islands immediately following isola- 15 tion by fire, but may also explain the pres- ence of certain animal species. Moreover,

10 most previous applications of IBT to ter- 19 restrial habitat islands (e.g., Brown 1971) involve habitats that have been insularized for sufficiently long time periods to permit repeated episodes of extinction and colo- nization to occur. By contrast, in the sage- < 100% 100% brush system, plant succession may restore shrubs to the burned areas rapidly % Local Species Pool in Islands enough to make effects of extinction and colonization within isolated habitat islands negligible even for those species that can-

20 -I not readily disperse across burns. B Regardless of these considerations, though, Bateman's (1999) data suggest that at least some predictions of IBT are 15 applicable to sagebrush habitat islands. Furthermore, terrestrial islands of any size or degree of isolation can provide essential habitat for certain species across a frag- mented landscape, and they can have important effects on succession within a surrounding disturbed area.

Successional Processes

Although we have already discussed potential effects of post-fire succession on < 60% > 80% habitat islands, we have not touched on % Local Species Pool in Islands with > 5 Species the how the presence of islands may, in turn, affect successional processes. Most Fig. 2. Effect of time (years) since sagebrush habitat insularization by fire on percentages of woody shrub species that occur in sage- local small mammal species pools occurring in habitat islands. A) All habitat islands sam- brush environments must reestablish in pled included-contrasts mean age of islands (± 1 SD) where 100% of small mammal species in local species pool occur in sagebrush islands versus islands with < 100% of disturbed areas from seeds; vegetative species. B) Only includes habitat islands with > 5 small mammal species- contrasts mean sprouting from roots following fire is rela- age of islands (± 1 SD) with > 80% of species pool versus those with < 60% of species. tively uncommon (Billings 1990). Thus, Numbers above bars are sample sizes of islands included in analyses. Data are from for successional recovery of a disturbance Bateman (1999). to proceed, a supply of viable seeds is nec- essary, and these seeds must find their It could be argued that habitat islands of the diversity of natural resources con- way to appropriate sites for germination lose ecological value with time, because as tained within or on conservation value, it and seedling establishment. A large-scale succession proceeds, burned areas become is relatively safe to say that "bigger is bet- disturbance that has islands of native veg- more similar to the unburned shrub island ter". It is not necessarily the case, howev- etation embedded in it provides a more and mainland areas they separate, effec- er, that less isolated islands are more use- evenly distributed seed source for succes- tively reducing isolation. While there is ful than more remotely isolated ones, and, sional recovery across the disturbed land- certainly some validity to this argument, in fact, the exact opposite may sometimes scape than simply relying on dispersal of Bateman (1999) still found more species be true. It is certainly possible that failure seeds from edges of the disturbance (i.e., in both islands and mainlands than in of some predictions of IBT to account for from the shrub mainland). This, alone, is burns, and this pattern held up in old as patterns in isolated sagebrush islands likely to speed the recovery process for well as recently burned sites. stems from fundamental differences seeds dispersed by either biotic or abiotic From the above reasoning, if one judges between terrestrial habitat islands and the agents. It is, perhaps, even more likely to the value of terrestrial islands on the basis oceanic islands that motivated early IBT enhance recovery for plant species whose

574 JOURNAL OF RANGE MANAGEMENT 55(6) November 2002 seeds are dispersed by small mammals, tion effort. Even in cases where large fires Brown, J.H. 1971. Mammals on mountaintops: which is probably a relatively common cleanly denude a landscape of shrubs, nonequilibrium insular biogeography. Amer. phenomenon in arid western shrub envi- restoration of native shrub islands within Natur. 105:467-478. Christiansen, T.A., J.A. Lockwood, and J. ronments (Vander Wall 1990). The shrub the burn may prove to be an effective Powell. 1989. community dynamics islands provide not only a ready source of means of accelerating natural successional in undisturbed and intensively managed moun- seeds for these animals to harvest, con- recovery. It is certainly less costly than tain brush habitats. Great Basin Natur. sume, and disperse, but also a refuge for active revegetation of an entire burned 49:570-586. various species that can act as effective environment, and the result of succession- Etienne, R.S. and J.A.P. Heesterbeek. Rules of thumb for conservation of metapopulations mice al expansion vegetation islands is likely seed dispersers. Deer (Peromyscus of based on a stochastic winking-patch model. maniculatus), and various chipmunks to yield a closer match to the preburn plant Amer. Natur.158:389-407. (Tamias spp.), ground squirrels (Spermo- community. The latter approach is being Halford, D.K. 1981. Repopulation and food philus spp. and Ammospermophilus spp.), attempted in the Buttermilk Winter Range habits of Peromyscus maniculatus on a burned and pocket mice (Perognathus spp. and Restoration Project at the eastern base of sagebrush desert in southeastern Idaho. NW. Chaetodipus for example, all tend to the Sierra Nevada Mountains in Round Sci. 55:44-49. spp.), Hixon, M.A. and J.P. Beets. 1993. Predation, have strong preferences for shrub-covered Valley, Calif. In 1995, 2,000 ha of critical prey refuges, and the structure of coral reef fish over disturbed areas with reduced shrub mule deer winter range was cleanly con- assemblages. Ecol. Monogr. 63:77-101. densities (such as burns), and all are sumed by fire in this area, but insufficient Hsiao, T.H. 1986. Biology and demography of the potentially important seed dispersers funds and concern about the introduction sagebrush defoliator and its impacts on big (Bateman 1999, Halford 1981, Longland of non-native plant germplasm prohibited sagebrush, p. 191-198. In: E.D. McArthur and B.L. Welch (comps.). Proc. Symp. on the 1995). By contrast, kangaroo rats attempts to actively revegetate the entire Biology of Artemisia and Chrysothamnus. (Dipodomys spp.) also disperse the seeds area. Instead, a large number of 10- x 10- USDA Forest Serv., Intermountain Res. Sta., of various desert plants, but they generally m plots have been planted with antelope Gen. Tech. Rep. INT-GTR-200, Ogden, Ut. increase in abundance in disturbed areas, bitterbrush (Purshia tridentata [Pursh] Longland, W.S. 1995. Desert rodents in disturbed preferring them to heavy shrub cover. DC) seedlings grown from local seed shrub communities and their effects on plant Even in this case, though, shrub islands stocks, and the plots have been made recruitment. p. 209-215. In: B.A. Roundy and E.D. McArthur (eds.). Proc. Symp. on Wildland can provide seed sources that allow these selectively accessible to mule deer and to Shrub and Arid Land Restoration. USDA Forest animals to disperse seeds of native plant seed-eating rodents. This project will thus Serv., Intermountain Res. Sta., Gen. Tech. Rep. species into the disturbed habitat matrix. simultaneously evaluate the utility of veg- INT-GTR-315, Ogden, Ut. etation islands for restoration and the Longland, W.S. and M.V. Price. 1991. Direct effects of 2 potential keystone animal taxa observations of owls and heteromyid rodents: can predation risk explain microhabitat use? on plant succession. Management Implications Ecol. 72:2261-2273. A more general message extending Longland, W.S. and J.A. Young. 1995. the issue of habitat islands is that What practical applications can be beyond Landscape diversity in the western Great Basin. theoretical constructs in ecology, conser- p. 880-891. In: N. West (ed.). Biodiversity on gleaned from the above concepts? First vation biology, or management are more Rangelands. Natural Resources and is our central message that and foremost - than complications or annoyances that Environmental Issues Series, Utah State Univ, native vegetation islands embedded within Logan, Ut. must be overcome during our years as stu- otherwise disturbed environments have MacArthur, R.H. and E.O. Wilson. 1967. The dents. They are heuristic tools that can high potential ecological and conservation theory of island biogeography. Princeton often instruct real life problems. University Press, Princeton, N.J. should be pre- value. Thus, these islands Predictions of Island Biogeography Nelle, P.J., K.P. Reese, and J.W. Connelly. served whenever possible following dis- Theory have been tested for a variety of 2000. Long-term effects of fire on sage grouse turbance. In fact, it may sometimes be habitat. J. Range Manage. 53:586-591. terrestrial habitat island situations, and possible or even advisable to manage for Pringle, W.L. 1960. The effect of a leaf feeding applied to the optimal design or location the maintenance or establishment of shrub beetle on big sagebrush in British Columbia. J. of wildlife reserves. Most such examples islands during the planning of prescribed Range Manage. 13:139-142. involve forested habitats isolated by sur- Strenge, D.L. and R.S. Zack. 2001. Observations fire or even during the effort to control a rounding non-forested lands, but there is on the life history of the sagebrush sheep , wildfire. For example, if an advancing line no reason why the same principles should hera hera (: of fire is too large and/or moving too ). NW Sci. 75:118-121. not be applicable to arid rangelands. There rapidly to allow establishment of a fire Vander Wall, S.B. 1990. Food hoarding in ani- can be useful, even valuable, empirical break along the entire advancing front, it mals. University of Chicago Press. Chicago, Ill. lessons hidden in sometimes seemingly Wambolt, C.L. 2001. Mule deer preference may still be possible to use smaller fire esoteric theories. among five sagebrush (Artemisia L.) taxa. breaks to facilitate the creation of Western North Amer. Natur. 61:490-494. unburned shrub islands within the larger Welch, B.L. and E.D. McArthur. 1986. burned matrix. We suggest that agency Wintering mule deer preference for 21 acces- handbooks should address these issues Literature Cited sions of big sagebrush. Great Basin Natur. explicitly in cases where policies regard- 46:281-286. Bateman, S.L. 1999. Effects of habitat fragmenta- Weiss, N.T. and B.J. Verts. 1984. Habitat and ing habitat islands are currently either distribution of pygmy rabbits (Sylvilagus ida- lacking or ambiguous. tion in Great Basin sagebrush-bunchgrass com- munities for small mammals and . M.S. hoensis) in Oregon. Great Basin Natur. If the agent of vegetation disturbance is 44:563-571. Thesis, Univ. of Nevada, Reno, Nev.. 72 pp. under more direct human control than dis- Billings, W.D. 1990. Bromus tectorum, a biotic Wiens, J.A., R.G. Cates, J.T. Rotenberry, N. turbance by fire, such as mechanical shrub cause of ecosystem impoverishment in the Cobb, B. Van Home, and R.A. Redak.1991. Arthropod dynamics on sagebrush (Artemisia removal, it may be possible to intentional- Great Basin. p. 210-222. In: G.M. Woodwell (ed.). The Earth in Transition: Patterns and tridentata): effects of plant chemistry and avian ly leave established shrub islands or to predation. Ecol. Monogr. 61:299-321. restore them later as part of the rehabilita- Processes of Biotic Impoverishment. Cambridge University Press, New York, NY.

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