Revista Chilena de Historia Natural 66: 357-368, 1993 Ecological relevance of subterranean herbivorous rodents in semiarid coastal Chile Importancia ecol6gica de roedores herbfvoros subterráneos en el desierto costero de Chile LUIS C. CONTRERAS1, JULIO R. GUTIERREZ1, VICTOR VALVERDE1 and GEORGE W. COX2 1 Departamento de Biologfa, Universidad de La Serena, Casilla 599, La Serena, Chile. 2 Biology Depanment, San Diego State University, San Diego, CA 91182-0057, USA. ABSTRACT We first call attention to the ecological relevance and characteristics of subterranean herbivorous rodent activities in different ecosystems. Second, we note that in arid coastal Chile the ecological impact of their activities is considerable, showing similarities, as well as important differences, to impacts in other areas of the world. The amount of excavated soil deposited on the surface is similar to that reported for pocket gophers in North America. These rodents also promote dominance of introduced mediterranean annual plants. In Chile, as in South Africa, however, they seem to greatly promote the growth of geophytes, which are their preferred food. Their activities promote large-scale soil homogeneity. We hypothesize that these aspects of their ecology in Chile reflect a close relationship between a fluctuating and unpredictable climate, a relatively constant availability of clumped geophytes, and the colonial social system of these rodents, all of which promote a slow but constant shifting of their foraging areas. Subterranean rodents certainly play an important role in determining the structure and dynamics of plants in the coastal arid ecosystem in Chile. However, very little is known about the mechanisms involved. Key words: Chile, desert ecology, soil disturbance, Spalacopus, subterranean rodents, geophytes. RESUMEN Primero destacamos la importancia ecol6gica de la actividad de roedores herbivoros subterráneos. Segundo, hacemos notar que la actividad de estos roedores tienen un impacto ecol6gico considerable en el ecosistema árido costero de Chile, presentando similiwdes pero también imponantes diferencias con sistemas investigados en otras partes del mundo. La cantidad de suelo depositado sobre la superficie del suelo es similar a la reportada para ge6midos en Norteamerica. Estos roedores tambien promueven la dominancia de plantas anuales mediterráneas introducidas. Sin embargo, en Chile como en Sudáfrica, ellos también parecen promover en gran medida el crecimiento de ge6fitas que constituyen su alimento preferido. Sus actividades promueven la heterogeneidad del suelo en el corto plazo en escalas pequeiias, pero en el largo plazo ellos promueven la homogeneidad del suelo en una escala mayor. Proponemos que estas caracterlsticas ecol6gicas y diferencias con otros sistemas reflejan una estrecha relaci6n entre un regimen climatico fluctuante e impredecible, la disponibilidad relativamente constante de ge6fitas distribuidas en forma agrupada, y el sistema social en colonias de estos roedores, todo lo cual promueve un desplazarniento lento pero constante de Ios tuneles de forrajeo. Ciertamente, Ios roedores herbivoros subterráneos juegan un rol importante en determinar la estructura y dinámica de plantas en el ecosistema árido costero de Chile; sin embargo, se conoce poco de Ios mecanismos involucrados. Palabras claves: Chile, ecosistemas deserticos, penurbaci6n del suelo, Spalacopus, roedores subterraneos, ge6fitas. IN1RODUCTION species (Grubb 1977, Connell 1978, Huston 1979, Armesto & Contreras 1981, Sousa 1984, Pickett & White 1985). These ideas are in The importance of disturbance as a source of constrat to the old long-held view of com- spatial and temporal variation in natural munities as equilibrium systems, with high communities has became increasingly species diversity resulting from prevention of recognized as an important process in competitive displacement by resource sub- ecological theory. Recent hypotheses on the division (MacArthur 1972, Schoener 1974, maintenance of species diversity have Pianka 1976). In addition to being an important emphasized the role of natural disturbance, source of environmental heterogeneity, which both prevents intra and interspecific disturbance may act as a strong selective competitive displacement and provides sites influence on the life history of organisms for colonization by competitively inferior (Harper 1977, Denslow 1980). (Received 13 July 1992; accepted 30 November 1992.) 358 CONTRERAS Ef AL. Disturbance phenomena are often referred Contreras & McNab 1990, Martinsen et al. to abiotic causes affecting large areas, such as 1990, Thome & Andersen 1990, Contreras & fire, and volcanic eruptions. At the same time, Guti~rrez 1991, Heth 1991, Cox et al. in press). in ecological studies much attention has been Members of all groups of subterranean rodents paid to animals as consumers of primary maintain and expand extensive gallery production, even though they only consume a systems throughout all or much of the year, small fraction of the live biomass (Hairston et resulting in deposition of soil heaps on the al. 1960), without noting their importance as surface (Hickman 1990, Reichman & Smith agents affecting the structure, properties and 1991). All are herbivores that locally reach processes of ecosystems (Whitford 1993). densities up to 207 individuals per hectare or Nevertheless, the importance of disturbances biomasses of 32.4 kg/ha (Nevo 1979, occurring at smaller scales and produced by Reichman & Smith 1991), values that are biotic agents has been increasingly recognized usually greater than for surface dwelling (Martinsen et al. 1990, Reichman & Smith mammals. Their burrowing activities exert 1991, Whitford 1993, Cox et al. in press). major impacts on soil structure and nitrogen Disturbance generated by animals, and availability; affecting both plant succession particularly those created by subterranean and abundance of their food plants (Tilman mammals, are unique for several reasons: 1) 1983, Cox & Gakahu 1985, Hobbs & Mooney they are caused by a biotic agent, whereas 1985, Andersen & MacMahon 1981, 1985, most forms of disturbance that have been Reichman & Smith 1985, Cox & Zedler 1986, studied are abiotic (e.g., wind, fire, wave Andersen 1987, Inouye et al. 1987, Koide et al. action), 2) they occur in small patches, in 1987, Hobbs et al. 1988, Huntly & Inouye constrat to large-scale disturbance such as tree 1988, Reichman 1988, Cantor & Whitman blow-downs, 3) they occur with a predictable 1989, Reichman & Jarvis 1989, Cox 1990, and constant frequency compared to more Thome & Andersen 1990, Heth 1991, Hobbs unpredictable events like fires and storms & Mooney 1991a, Reichman & Smith 1991). (Martinsen et al. 1990), and 4) as a con- Burrowing activity varies between different sequence of 2) and 3) they are of minor short- species and within species, mainly in relation term impact, but accumulate and cause to social and foraging systems (solitary vs. substantial long-term effects (Cox & Scheffer colonial species), food resources, soil moisture 1991, Reichman & Smith 1991). and temperature, hardness of the ground, and In this article we first review information reproductive activity (Arieli 1979, Reichman on the ecological role of subterranean et al. 1982, Hickman 1990, Heth 1991, herbivorous rodents. Second, we summarize Reichman & Smith 1991, Cox & Hunt 1992). the few available studies on their role in arid Subterranean rodents usually remove large coastal Chile, noting similarities and dif- amounts of soil. For the pocket gopher ferences to other ecosystems where subte- Thomomys bottae in sourthem California it rranean mammals have been studied. may be as large as 26.6 ton/ha • year (Cox 1990). The most obvious impact of this mining on soil structure results from the deposition of MAGNITUDE OF DISTURBANCE BY excavated soil on the surface of the ground. SUBTERRANEAN HERBIVOROUS MAMMALS Because mound formation is easily perceived, its rate is frequently used as an estimate of Soil disturbance and herbivory by subterranean burrowing activities (Miller 1957, Hickman rodents are now recognized as major influences & Brown 1973, Arieli 1979, Bandoli 1981, on the structure and dynamics of many Cantor & Whithan 1989, Cox & Hunt 1992). ecosystems, including deserts (Andersen & The mass of soil deposited on the surface MacMahon 1981, 1985,Reichmanetal. 1982, values ranges from 0.8 to 34.5 ton /ha o year Gettinger 1984, Hobbs & Mooney 1985, in natural conditions (Miller 1957, Downhower 199la, Cox & Gakahu 1986, Andersen 1987, & Hall 1966 in Cox 1990, Richens 1966, Cox & Alien 1987a, Inouye et al. 1987, Kodie Andersen & MacMahon 1981) and may reach & Mooney 1987, Huntly & Inouye 1988, 105 ton/ha o year in irrigated alfalfa fields Reichman 1988, Reichman & Jarvis 1989, (Miller 1957). A significant portion of the ECOLOGICAL RELEVANCE OF SUBTERRANEAN RODENTS 359 excavated soil may not be deposited on the 3 years after the burrows have been abandoned surface, but placed in abandoned tunnels below (Reichman & Smith 1985). A similar impact the surface of the ground. Soil may also be has been noted over burrows of three sympatric deposited in tunnels that have been formed in species of bathyergids rodents from South the snow pack, producing soil cores that remain Africa (Reichman & Jarvis 1989). Further- on the surface of the ground after snow melt more, the effects of burrows on the vegetation (Ingles 1952, Reichman & Smith 1991, can be significant beyond the area of actual Contreras pers. obser. for Ctenomys