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Prairie dogs and wildfires shape vegetation structure in asagebrush grassland more than does rest from ungulate grazing
L. C. CONNELL,1 J. D. SCASTA,1 AND L. M. PORENSKY 2,
1Department of Ecosystem Science and Management, University of Wyoming, Ag C 2004, Dept. 3354, 1000 E. University Ave, Laramie,Wyoming 82071 USA
2USDA-ARS Rangeland Resources Research Unit, 1701 Centre Avenue, Fort Collins, Colorado 80526 USA
Citation: Connell, L. C., J. D. Scasta, and L. M. Porensky. 2018. Prairie dogs and wildfires shape vegetation structure in asagebrush grassland more than does rest from ungulate grazing. Ecosphere 9(8):e02390. 10.1002/ecs2.2390
Abstract. Understanding drivers of vegetation structure has direct implications for wildlife conservationand livestock management, but the relative importance of multiple disturbances interacting within thesame system to shape vegetation structure remains unclear. We investigated the separate and interactiveeffects of multiple disturbance drivers on vegetation structure through a three-tiered, large-scale manipula-tive experiment in northeast Wyoming, USA. We used nested grazing exclosures to isolate the effects ofherbivory from livestock, wild ungulates, or small mammals within areas affected by either historical wild-fire, black-tailed prairie dog (Cynomys ludovicianus) colonies, or neither disturbance. We analyzed the inter-active effects of disturbance history and contemporary herbivory by either small mammals alone, smallmammals + native ungulates, or small mammals + native ungulates + livestock on vegetation structureby quantifying vegetation height, visual obstruction, shrub density, shrub canopy, and shrub leadergrowth. The exclusion of wild ungulates and lightly to moderately stocked livestock for two years did notsignificantly affect herbaceous vegetation structure, shrub density, or shrub canopy cover. Maximum vege-tation height, visual obstruction, and heights of grasses and forbs were ~50% lower on black-tailed prairiedog colonies than in undisturbed areas. Prairie dog colonies contained 71% lower shrub densities thanundisturbed sites. Sites with wildfire or black-tailed prairie dogs had 89% lower canopy cover of shrubsand Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), when compared to undisturbed sites.Shrub leaders experienced over 4.5 times more browsing on prairie dog colonies, when compared to undis-turbed areas. For most metrics, disturbance history did not modify the effects of contemporary herbivoryon vegetation structure. However, shrubs on prairie dog colonies experienced significantly more leaderbrowsing in the combined presence of livestock, native ungulates, and small mammals than in treatmentswhere livestock were excluded. Our research has direct implications for wildlife conservation and range-land management by demonstrating that short-term (12 yr) rest from large ungulate grazing may notsubstantially alter vegetation structure in this system. Instead, structural variation is strongly driven byblack-tailed prairie dog colonization and historical wildfire. Understanding and managing multiple, poten-tially interacting disturbances is critical for maximizing wildlife conservation and livestock production inheterogeneous landscapes.
Key words: black-tailed prairie dogs; ecosystem engineer; ecotone; habitat mosaics; habitat structure; interactiveeffects; keystone species; livestock; northern mixed-grass prairie; sagebrush steppe.
Received 6 April 2018; revised 4 June 2018; accepted 25 June 2018. Corresponding Editor: Rebecca J. Rowe.Copyright: 2018 The Authors. This is an open access article under the terms of the Creative Commons AttributionLicense, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. E-mail: email@example.com
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Vegetation structure, or the way vegetation isspatio-temporally arranged in three-dimensionalspace (including vegetation height, visual obstruc-tion, shrub density, and shrub canopy), serves asa fundamental link between pattern and processin landscape ecology and is often sensitive to dis-turbance (Turner 1989). Habitat patch type, scale,and structure influence wildlife utilization, move-ment, and forage intake (MacArthur and Pianka1966, Simpson et al. 2004, van Beest et al. 2010).Vegetation structure provides microhabitats forsmall mammals and spiders (Brown and Kotler2004, Warui et al. 2005, Doherty et al. 2015, Cera-dini and Chalfoun 2017), influences the naviga-tion and utilization of a landscape by livestock(Clark et al. 2017) and wild ungulates (Van Dykeand Darragh 2006, Allen et al. 2014, Riginos2015), and provides a suite of habitat resourcesfor birds ranging from nesting to screening cover(Chalfoun and Martin 2009, Doherty et al. 2014).Moreover, natural disturbance can alter vegeta-tion structure and lead to shifts in foragingspace-use patterns in response to newly availableresources (Thompson et al. 2008, Nkwabi et al.2011, Augustine and Derner 2015). Natural distur-bance can also elicit settlement or nesting innewly disturbed areas (Augustine et al. 2007, Der-ner et al. 2009, Augustine and Derner 2015).Despite the importance of vegetation structure forthe conservation of biodiversity and ecosystemfunction, relatively little is known about how mul-tiple forms of disturbance operate and interactwithin the same ecosystem to shape vegetationstructure within variable temporal and spatialscales.
Globally, vegetation structure and patch vari-ability in grasslands and savannas are stronglydriven by natural disturbance regimes (Fuhlen-dorf et al. 2006, McGranahan et al. 2012, Ondeiet al. 2017). These disturbances influence heightand cover of herbaceous and woody plants, oftenwithin variable spatio-temporal regimes thatresult in heterogeneous landscapes (Fuhlendorfand Engle 2001). In North America, semi-aridrangelands include deserts, shrublands, grass-lands, and forests that evolved with spatially andtemporally variable disturbance regimes of wild-fire, large ungulate herbivory, and the ecosystemengineering activities of colonial burrowing
mammals (Beetle 1960, Stebbins 1981, Anderson2006). Moreover, interactions among multipledisturbances, including wildfire, herbivory bynative megafauna, and colonial burrowing mam-mals, are driving forces of plant communitystructure and composition in these ecosystems(Knapp et al. 1999, Fuhlendorf and Engle 2004,Gordon et al. 2004). The interactive effects of dis-turbance across space and time can serve tobroaden the suite of plant functional groups andtheir structural features on the landscape (Fahne-stock and Detling 2002, Fuhlendorf and Engle2004, Augustine et al. 2007, Augustine and Der-ner 2015). Ecological disturbances can also pro-mote diversity in vegetation communities andtheir structure (West 1993, Baker et al. 2013), thusoptimizing wildlife habitat and forage qualityand quantity (Derner et al. 2009, Augustine andBaker 2013, Hovick et al. 2015), ecosystem func-tion (Sousa 1984), and associated services (Fuh-lendorf and Engle 2001).Not all rangelands, however, have similar dis-
turbance dependence or function, and intention-ally managing for optimal disturbance regimes,especially in disturbance-sensitive plant commu-nities such as big sagebrush (Artemisia tridentata[Beetle & Young])-dominated shrublands, contin-ues to be challenging (Beck et al. 2009, Davieset al. 2011, Chambers et al. 2014). Much researchon vegetation community response to multipledisturbances has focused on the interactive effectsof two drivers, such as fire and herbivory by live-stock (Fuhlendorf and Engle 2004, Limb et al.2011, Scasta et al. 2016b), herbivory by native colo-nial mammals and livestock (Sierra-Corona et al.2015, Ponce-Guevara et al. 2016), fire and nativecolonial mammals (Augustine et al. 2007), fireand native ungulates (Larson et al. 2013, Brelandet al. 2014, Augustine and Derner 2015), andnative ungulates and livestock (Veblen et al.2016). These studies convincingly demonstratethat the interactive effects of two disturbancesoften differ both quantitatively and qualitativelyfrom the effects of isolated disturbances. Forexample, Ponce-Guevara et al. (2016) demon-strated interactive herbivory effects by cattle andblack-tailed prairie dogs decreased height of mes-quite (Prosopis glandulosa) bushes twice as much,when compared to areas where cattle and prairiedogs occurred alone or were absent. Despitethis recognition that multiple disturbances can
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interact, ecological research has only recentlybegun to investigate the interactive effects of morethan two distinct disturbance drivers operating inthe same system (e.g., Royo et al. 2010, Odadiet al. 2017). Nonetheless, most grasslands andshrublands frequently experience three or moresimultaneous and interactive disturbances (e.g.,fire, drought, grazing by livestock, grazing bynative ungulates, small mammal disturbances,and insect and disease outbreaks).
A better understanding of the interactive effectsof multiple disturbances on vegetation structure iscritical for effective management of complex land-scapes. In western North America, land managersare often responsible for meeting multiple, diverserangeland management objectives including pro-ductive and sustainable livestock grazing, energydevelopment, and the maintenance or restorationof ecosystem biodiversity and wildlife habitat(West 1993, Davies et a