Pack Rats (Neotoma Spp.): Keystone Ecological Engineers?
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Journal of Arid Environments 74 (2010) 1450e1455 Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv Pack rats (Neotoma spp.): Keystone ecological engineers? Walter G. Whitford a, Yosef Steinberger b,* a USDA-ARS Jornada Experimental Range, MSC 3JER, New Mexico State University, Las Cruces, NM 88003, USA b The Mina & Everard Goodman, The Mina & Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel article info abstract Article history: The potential role of two species of pack rats (Neotoma albigula and Neotoma micropus) as keystone Received 8 December 2009 ecological engineers was examined by estimating the species diversity of invertebrates living in the nest Received in revised form middens, and nitrogen mineralization rates in soils associated with the middens. Although pack-rat 15 April 2010 middens in tarbush (Flourensia cernua) shrublands were smaller than those in creostebush (Larrea tri- Accepted 21 May 2010 dentata) shrublands, they housed a higher abundance and diversity of arthropods. The Neotoma spp. Available online 15 June 2010 middens were an important microhabitat for crickets (Gryllus sp.), wolf spiders (Lycosa spp.), and lycid beetle larvae (Lycidae) in all of the shrub habitats. There were five arthropod taxa that occupied all Keywords: Habitat middens in the creosote-bush shrubland, and 12 arthropod taxa that occupied all middens in the tarbush fi Mineralization shrubland. Soils associated with pack-rat middens had signi cantly higher soil organic-matter content Species diversity than reference soils. Nitrogen mineralization was significantly higher in soils associated with pack-rat Stick nest middens than in reference soils. Neotoma spp. create habitats with moderate microclimates that are Woodrat essential for several invertebrates, thus contributing to maintenance of biodiversity. The effects of middens on soil organic matter and nitrogen mineralization create nutrient-rich patches. Neotoma spp. affect biodiversity and critical ecosystem processes, thus supporting the designation of keystone ecological engineers. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction have earned those species designation as keystone ecosystem engineers (Brown, 1995). Mammals that collect plant parts and Animals that modify the structure and/or function of ecosys- other materials to build structures within which the animals reside tems have been described as ecosystem engineers. Ecosystem may be the terrestrial equivalent of beavers and may function as engineers directly or indirectly affect the availability of resources to keystone ecosystem engineers. other species by changing some biotic and abiotic components of In North American deserts, several species of rodents, Neotoma the environment. These changes modify, maintain, or create habi- spp., build houses or middens of sticks plus other plant materials, tats for ecological engineers themselves or other species (Jones dried dung, and frequently include shiny man-made objects in their et al., 1994). In arid regions, animals that create soil disturbances middens (Vaughan, 1990). This behavior has earned members of have been identified as important ecosystems engineers. Soil this genus the common name pack rats. Neotoma spp. are widely modifications by animals include pits which serve as collection distributed in North American deserts and the nest midden struc- areas for seeds and organic debris (Whitford,1998), soil tunnels and tures have been shown to vary considerably depending upon the chambers that affect water infiltration (James et al., 2008), and availability of suitable shelter of rock or vegetation for the location burrows and mounds that affect water infiltration, soil nutrients, of the nests. Two pack-rat species inhabit the Chihuahuan Desert abundance, and diversity of plants and soil biota (Eldridge et al., areas of southern New Mexico: the white-throated woodrat, 2009; Ginzburg et al., 2008; Ukabi et al., 2009; Whitford and Kay, Neotoma albigula, and the plains woodrat, Neotoma micropus. 1999; Whitford et al., 2008). Some ecosystem engineers, such as N. albigula has expanded its range from the rocky slopes of beavers building dams on streams, have pervasive, cascading mountains and rocky drainages into the desert grasslands, which effects on other organisms and physical/chemical processes that are the historical habitats of N. micropus, and into the shrub- dominated plant communities that have replaced desert grasslands over the past one and a half centuries (Wright, 1973; Whitford, 1976). N. micropus continues to occupy some of the shrublands * Corresponding author. Tel.: þ972 3 5318571; fax: þ972 3 7384058. E-mail addresses: [email protected] (W.G. Whitford), [email protected] that were grasslands in the past (Whitford and Steinberger, 1989). (Y. Steinberger). N. micropus middens in shrubland habitats are similar to those of 0140-1963/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2010.05.025 W.G. Whitford, Y. Steinberger / Journal of Arid Environments 74 (2010) 1450e1455 1451 N. albigula, and are different from N. micropus nest structures in m intervals. Plots were selected on the right or left of the center line plains grassland habitats. The species composition and life form of by coin toss and the 50 m  100 m plots flagged. Pack-rat middens the dominant vegetation constrain the materials that are available were identified by systematic surveys of each plot. The intensive to pack rats for midden construction. In this study, the density and surveys provided absolute numbers of pack-rat middens in each structural materials of pack-rat middens were estimated in several 0.5 ha plot. vegetation communities on two shrub-dominated watersheds. Shrub cover and composition were estimated for each plot by The stick nests of Neotoma spp. are similar to those of the Stick- the line intercept method. Thirty-meter lines were stretched from nest rats (Leporillus spp.) of Australia (Copley, 1999; Nelson et al., three randomly selected meter points on a tape stretched between 1990) and nests of three Southern African rodents (Acomys spino- the center line stakes. sissimus, Thallomys nigricauda, and Acthomys namaquensis) (Skinner and Smithers, 1990). It is plausible that the stick nests of 2.3. Size and structure of middens rodents in other deserts would have similar effects on the structure and function of the ecosystems in which they occur. The stick nests The size of nine middens, selected at random, in each plant of Neotoma spp. have been reported as an essential habitat for community was determined by measuring two diameters of the pseudoscorpions (Francke and Villegas-Guzman, 2006) but there midden and the height above the soil surface of the nest mound. are no records of other arthropod inhabitants of pack-rat middens. The volume of each midden was calculated as one half the volume Desert pack-rat middens have been reported to provide a moderate of a prolate spheroid. The materials incorporated into the structure thermal environment that may make the stick nests a desirable of middens in the toe-slope grassland and tarbush shrubland were habitat for other animals (Thies et al., 1996). If pack-rat houses sampled by dismantling the structure, sorting the materials into provide a moderate thermal environment, they may also moderate general classes, i.e., sticks, grass, leaves, dung, Opuntia spp. clad- the relative humidity of the midden interior. If pack-rat middens do odes, and unidentifiable objects. The mass of materials in a midden provide moderate microclimates in environments characterized by was measured using a large capacity scale on nine middens in the high temperatures and low relative humidity, Neotoma spp. houses tarbush shrubland, creosote-bush shrubland, and the toe-slope would represent the activities of an ecosystem engineer that help grassland, and two middens in the playa mesquite fringe. Only two maintain or enhance biodiversity. One part of this study was middens were dismantled in the playa mesquite fringe in order to designed to test this hypothesis. minimize the effect on other on-going studies in that area. Before Since pack-rat middens are occupied by successive generations the middens were dismantled, 20 sticks were removed from the that add materials to the middens, the immediate surroundings of surface layer and the mid-length diameter was measured with the middens should be fertilized by the feces and urine of the calipers. When half of the original height of the midden was occupants. We measured rates of nitrogen mineralization in soils reached, 20 sticks were removed from that layer and the mid- adjacent to middens in comparison to reference sites as a test of length diameter measured. this hypothesis. 2.4. Midden microclimate and soil 2. Methods Air relative humidity and temperature were measured in five 2.1. Site description middens on the creosote-bush bajada, five middens in the tarbush community, and two middens in the playa fringe mesquite over Studies were conducted on two watersheds of the Doña Ana a two-week period in mid-October. After the outer midden material Mountains located at the Chihuahuan Desert Rangeland Research was removed, the relative humidity and temperature of the inner Center and the USDA-ARS Jornada Experimental Range approxi- nest were measured with a digital hygrometer and thermometer, mately 40 km NNE of Las Cruces, NM. The watersheds are respectively. Ambient