Pack rats (Neotoma spp.): Keystone ecological engineers?
Abstract
The potential role of two species of pack rats (Neotoma albigula and Neotoma micropus) as keystone ecological engineers was examined by estimating the species diversity of invertebrates living in the nest middens, and nitrogen mineralization rates in soils associated with the middens. Although pack-rat middens in tarbush (Flourensia cernua) shrublands were smaller than those in creostebush (Larrea tridentata) shrublands, they housed a higher abundance and diversity of arthropods. The Neotoma spp. 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 middens in the creosote-bush shrubland, and 12 arthropod taxa that occupied all middens in the tarbush shrubland. Soils associated with pack-rat middens had significantly higher soil organic-matter content than reference soils. Nitrogen mineralization was significantly higher in soils associated with pack-rat middens than in reference soils. Neotoma spp. create habitats with moderate microclimates that are 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.
Keywords
- Habitat;
- Mineralization;
- Species diversity;
- Stick nest;
- Woodrat
1. Introduction
Animals that modify the structure and/or function of ecosystems have been described as ecosystem engineers. Ecosystem engineers directly or indirectly affect the availability of resources to other species by changing some biotic and abiotic components of the environment. These changes modify, maintain, or create habitats for ecological engineers themselves or other species (Jones et al., 1994). In arid regions, animals that create soil disturbances have been identified as important ecosystems engineers. Soil modifications by animals include pits which serve as collection areas for seeds and organic debris (Whitford, 1998), soil tunnels and chambers that affect water infiltration (James et al., 2008), and burrows and mounds that affect water infiltration, soil nutrients, abundance, and diversity of plants and soil biota (Eldridge et al., 2009, Ginzburg et al., 2008, Ukabi et al., 2009, Whitford and Kay, 1999 and Whitford et al., 2008). Some ecosystem engineers, such as beavers building dams on streams, have pervasive, cascading effects on other organisms and physical/chemical processes that have earned those species designation as keystone ecosystem engineers (Brown, 1995). Mammals that collect plant parts and other materials to build structures within which the animals reside may be the terrestrial equivalent of beavers and may function as keystone ecosystem engineers.
In North American deserts, several species of rodents, Neotoma spp., build houses or middens of sticks plus other plant materials, dried dung, and frequently include shiny man-made objects in their middens ( Vaughan, 1990). This behavior has earned members of this genus the common name pack rats. Neotoma spp. are widely distributed in North American deserts and the nest midden structures have been shown to vary considerably depending upon the availability of suitable shelter of rock or vegetation for the location of the nests. Two pack-rat species inhabit the Chihuahuan Desert areas of southern New Mexico: the white-throated woodrat, Neotoma albigula, and the plains woodrat, Neotoma micropus. N. albigula has expanded its range from the rocky slopes of mountains and rocky drainages into the desert grasslands, which 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 and Whitford, 1976). N. micropus continues to occupy some of the shrublands that were grasslands in the past ( Whitford and Steinberger, 1989). N. micropus middens in shrubland habitats are similar to those of N. albigula, and are different from N. micropusnest structures in plains grassland habitats. The species composition and life form of the dominant vegetation constrain the materials that are available to pack rats for midden construction. In this study, the density and structural materials of pack-rat middens were estimated in several vegetation communities on two shrub-dominated watersheds.
The stick nests of Neotoma spp. are similar to those of the Stick-nest rats (Leporillusspp.) of Australia ( Copley, 1999 and Nelson et al., 1990) and nests of three Southern African rodents (Acomys spinosissimus, Thallomys nigricauda, and Acthomys namaquensis) ( Skinner and Smithers, 1990). It is plausible that the stick nests of rodents in other deserts would have similar effects on the structure and function of the ecosystems in which they occur. The stick nests of Neotoma spp. have been reported as an essential habitat for pseudoscorpions ( Francke and Villegas-Guzman, 2006) but there are no records of other arthropod inhabitants of pack-rat middens. Desert pack-rat middens have been reported to provide a moderate thermal environment that may make the stick nests a desirable habitat for other animals ( Thies et al., 1996). If pack-rat houses provide a moderate thermal environment, they may also moderate the relative humidity of the midden interior. If pack-rat middens do provide moderate microclimates in environments characterized by high temperatures and low relative humidity, Neotomaspp. houses would represent the activities of an ecosystem engineer that help maintain or enhance biodiversity. One part of this study was designed to test this hypothesis.
Since pack-rat middens are occupied by successive generations that add materials to the middens, the immediate surroundings of the middens should be fertilized by the feces and urine of the occupants. We measured rates of nitrogen mineralization in soils adjacent to middens in comparison to reference sites as a test of this hypothesis.
2. Methods
2.1. Site description
Studies were conducted on two watersheds of the Doña Ana Mountains located at the Chihuahuan Desert Rangeland Research Center and the USDA-ARS Jornada Experimental Range approximately 40 km NNE of Las Cruces, NM. The watersheds are composed of several plant communities occupying different topographic positions. The plant community of the montane toe slopes is a black-grama (Bouteloua eriopoda), Lehmann’s love grass (Eragrostis lehmanniana) grassland with abundant large platyopuntia, (Opuntia phaecantha). The piedmont (bajada) below the toe-slope grassland is dominated by creosote-bush (Larrea tridentata) on sandy or sand-gravel soils. The piedmont is dissected by numerous small drainage channels, the margins of which support fairly large multi-stemmed mesquite (Prosopis glandulosa), large patches of banana yucca (Yucca baccata), and apache plume (Fallugia paradoxa). One of the watersheds drains into a small ephemeral lake with a 30-m-wide dense thicket of mesquite along the lake margin. The other watershed has a mixed shrub community of creosote-bush and tarbush (Flourensia cernua) on the lower slopes of the piedmont. The mixed shrub community merges into tarbush shrubland on the lowest slopes of the watershed.
