Mastozoología Neotropical ISSN: 0327-9383 [email protected] Sociedad Argentina para el Estudio de los Mamíferos Argentina Yahnke, Christopher J. Habitat use and natural history of small mammals in the central Paraguayan chaco Mastozoología Neotropical, vol. 13, núm. 1, enero-junio, 2006, pp. 103-116 Sociedad Argentina para el Estudio de los Mamíferos Tucumán, Argentina Available in: http://www.redalyc.org/articulo.oa?id=45713108 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Mastozoología Neotropical, 13(1):103-116, Mendoza, 2006 ISSN 0327-9383 ©SAREM, 2006 Versión on-line ISSN 1666-0536 www.cricyt.edu.ar/mn.htm HABITAT USE AND NATURAL HISTORY OF SMALL MAMMALS IN THE CENTRAL PARAGUAYAN CHACO Christopher J. Yahnke Department of Biology and Museum of Natural History, University of Wisconsin – Stevens Point, 800 Reserve Street, CNR 167, Stevens Point, WI 54481 USA, Telephone: 715-346-2455. <[email protected]> ABSTRACT: I conducted a small mammal trapping study in the central Paraguayan Chaco region of South America to investigate habitat selection by small mammals at different spatial scales. Small mammals were collected in forest, successional thorn scrub, pasture, and crop fields representing both relatively undisturbed habitats and agroecosystems. A total of 1089 small mammals representing 13 species were captured during 23 296 trap nights. Pastures had the highest species richness as well as the highest number of cap- tures. Some small mammal species such as Calomys laucha and Akodon toba were cap- tured in a variety of habitats whereas others like Holochilus chacarius and Bolomys lasiurus were captured almost exclusively in pastures. Principal components analysis distinguished small mammal species primarily associated with agricultural habitats (e.g. Calomys spp.) from those associated with more wooded habitats (e.g. Graomys griseoflavus and Oligoryzomys chacoensis). These results corroborate other studies on habitat use by small mammals in this region, but with some notable exceptions, such as the first documentation of high densities of Calomys musculinus in western Paraguay. Key words. Chaco. Habitat selection. Natural history. Paraguay. Small mammals. INTRODUCTION voirs of infectious diseases. For example, a number of studies have been conducted in A striking diversity of small mammals can be agroecosystems in central Argentina in the found in the variety of habitats scattered endemic region of Argentine Hemorrhagic throughout the central Paraguayan Chaco. The Fever (Mills et al., 1991a; Mills et al., 1992; central Chaco presents a broad array of habi- Ellis et al., 1997; Ellis et al., 1998), whereas tats ranging from undisturbed forest to grass- other studies have looked at agroecosystems lands, riparian habitats, and agroecosystems. associated with hantavirus reservoirs (Calderon Although studies of wildlife-habitat relation- et al., 1999; Yahnke et al., 2001; Carroll et ships in South America historically have been al., 2005). conducted in natural, relatively undisturbed Habitat selection, which affects nearly all of ecosystems (Meserve et al., 1988; Patterson et an individual’s subsequent choices concerning al., 1990; Kelt et al., 1994), interest is increas- food, shelter, risk of predation, and mating ing regarding studying small mammal com- opportunities, can be viewed as a hierarchical munities in agroecosystems. Much of this re- process in which organisms first choose a search has focused on small mammal reser- general place in which to live (a habitat) and Recibido 7 julio 2005. Aceptación final 7 abril 2006. 104 Mastozoología Neotropical, 13(1):103-116, Mendoza, 2006 C. J. Yahnke www.cricyt.edu.ar/mn.htm then make subsequent decisions about the use tion of the central Chaco by the Mennonites in the of different patches or microhabitats (Orians 1930s, most of this area consisted of low, dry thorn and Wittenberger, 1991). The spatial distribu- forest and native grassland. Remnants of these habi- tion of patches within a habitat affects the tat types, together with agricultural habitats and successional communities, have created a landscape ability of different species to coexist in a re- mosaic. gion. A necessary requirement for coexistence The central Chaco is subtropical and semixeric, is that each species possesses a habitat or patch with a pronounced seasonality in precipitation. in which it is the most efficient forager (Kotler Annual rainfall averages 850 mm, and tempera- and Brown, 1988). tures range from a mean maximum during the Most natural assemblages comprise compet- hottest month, January, of ca. 35°C, to a mean ing species that range from those with narrow minimum temperature of ca. 23°C during the cold- habitat requirements (specialists) to others with est month, July (Fig. 1). A predictable dry season wider habitat requirements (generalists), with occurs from April through September, with most of the rain occurring during the months of Novem- specialists competitively excluding less well- ber to February (Fig. 1). Precipitation also can be adapted species (Morris, 1996). Brown (1996) patchy in distribution. For example, one of the demonstrated that widespread habitat general- trapping sessions during the wet season was re- ists might coexist with competing habitat spe- scheduled because the site received over 300 mm cialists if they exploit the shared environment of rain during a two-day period, whereas most other at a larger spatial scale. The generalist’s strat- sites received between 30-60 mm during the same egy is to exploit whichever habitats are un- two days. used or underused by more specialized spe- Gross habitat utilization cies. Because habitat selection can occur any- where along a continuum of spatial scales from Four habitats common to the central Chaco were the level of microhabitat to macrohabitat chosen for trapping of small mammals. Two of (Kotler and Brown, 1988), attempts were made these habitats (pastureland and cropland) repre- to sample small mammals at multiple spatial sented the predominant agricultural land use, whereas the other two habitats (thorn forest and scales for this study. I sought to investigate thorn scrub) represented the dominant undisturbed habitat selection in a variety of habitats, in- and successional habitats, respectively. Three sites cluding agroecosystems, at different spatial for each habitat type were chosen for a total of 12 scales as a mechanism of coexistence among sites (Fig. 2). The elevation of the 12 sites ranged small mammals in the central Paraguayan from 120 to 149 meters above sea level. Trapping Chaco. was concentrated in Fortin Toledo (including Corrales), Filadelfia (Chacra Experimental MATERIALS AND METHODS Filidelfia), and Cruce de Los Pioneros (Chacra Experimental Chaco Central, Isla Poi). Because thorn scrub represented a successional habitat, The Chaco of Paraguay covers an area of 246 925 identifying a uniform patch of similar age and km2 (61% of the land in Paraguay) and has about sufficient size in each of the trapping centers was 3% of the human population in Paraguay (Raidán, difficult. To maximize similarity of sites represent- 1985). The 2002 census listed 142 501 people liv- ing this habitat type, all were located near Estancia ing in the three departments of the Chaco (DGEEC, Toledo, a large cattle ranch that was abandoned 2002). Many of these are Mennonites, who live in about fifteen years ago. three large colonies in the central Paraguayan Sorghum was chosen to represent the common Chaco. The Mennonites are ardent farmers with cropland habitat in the central Chaco. Bland or large plantations of peanuts, castor beans, sorghum unstructured soil and a lack of trees and shrubs and cotton (Benirschke et al., 1989). This area of characterized cropland habitats. These habitats were the central Chaco is dominated by extensive agri- disturbed seasonally when crops were harvested. culture and cattle production. Although a large Generally, two to three crops are harvested during number of cattle ranches exist throughout the the wet season. If the last crop is not productive, Chaco, it is only in the vicinity of Mennonite the field usually will be left uncut until it is pre- colonies that dry farming of crops exists on a large pared for planting in spring (about mid-Septem- scale (Kleinpenning, 1984). Prior to the coloniza- NATURAL HISTORY OF PARAGUAYAN MAMMALS 105 Fig. 1. Mean monthly maximum and minimum temperatures, and total monthly rainfall, during the study period. Bars represent standard deviation about the mean. Measurements were taken at Fortin Toledo (S 22o20’23”, W 60o20’22”). Fig. 2. Map of the central Paraguayan Chaco showing trapping centers near Fortin Toledo, Filadelfia, and Cruce Los Pioneros. Boxes represent trap sites. Letters in boxes represent habitat (P = Pasture, C = Cropland, T = Thorn scrub, F = Forest), and numbers correspond to sites in Table I. 106 Mastozoología Neotropical, 13(1):103-116, Mendoza, 2006 C. J. Yahnke www.cricyt.edu.ar/mn.htm ber). The typical growing season in the central (one in each habitat type) were trapped in each of Chaco is from October through March. two consecutive dry seasons, yielding a total of 28 Pasture is the most common land use in the trapping sessions (Fig. 2). Trapping duration was central Chaco and extensive variation occurs