Interspecific relationships among cricetid rodents in agrarian ecosystems were studied. The principal rodent species found in these habitats are Akodon azarae, Calomys laucha, Calomys musculinus and Oligoryzomys flavescens. Fieldwork was carried out in D. Gaynor, Buenos Aires Province (Argentina), from 1979 to 1982. Spatial and numerical relations in a two-habitat system comprising cropfields and their borders were analyzed. Asymmetrical interspecific competition for space was found mainly in summer with A. azarae being the dominant species. Individual differences in competitive ability were found. Females of A. azarae were more dominant than males, and juveniles of C. laucha were more segregated than adults. Coexistence mechanisms of different species in borders are discussed.Resume. -Les relations interspecifiques entre une serie de rongeurs Cricetides ont ete etudiees dans des ecosystemes agraires de la province de Buenos-Aires de 1979 ä 1982. Leurs relations spatiales et numeriques ont ete analysees dans un Systeme de double habitat comprenant les champs cultives et leurs bordures. Une competition interspecifique asymetrique pour Pespace a ete mise en evidence, surtout pendant Fete chez 1'espece dominante Akodon azarae^ et on a constate des differences individuelles dans 1'aptitude ä la competition. Les femelles de cette espece etaient plus dominantes que les males, et les juveniles de Calomys laucha etaient plus disperses que les adultes. Les mecanismes de coexistence d'especes differentes dans les bordures sont discutees.

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The goal of this study was to analyze the variations in abundance of rodent communities over the last 24 years in a rural area, and their relation to possible changes in climatic variables and land use. The principal change in the area observed along the study period was an increase in the area covered by soybean. The habitats studied were crop fields and borders. The total abundance of rodents did not show a significant trend of variation over time in crop fields while in borders the abundance in autumn-winter (A-W) showed a significant trend to decrease over time. The different rodent species showed a differential response over time. While Calomys laucha and C. musculinus showed a decrease between the period before and after the soybean expansion, Akodon azarae did not change its abundance over time. The mean minimum temperature increased over time while the number of days with frost decreased. Total rodent abundance in A-W was positively associated with the cumulated precipitation of the previous spring-summer period and negatively with the cumulated precipitation of the same period. We conclude that rodent abundance variations in crop fields and borders of the study area are influenced by precipitation, but the observed trends of variation over time are better explained by changes in agricultural practices than by meteorological variables.

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We analyzed the effect of density-dependent habitat selection and interspecific competition on the use of space by small rodent species in agroecosystems. We studied rodent abundance in cropfields and their borders, weedy margins less disturbed by agricultural activities. We could distinguish two kind of species : those habitat intolerants, Akodon azarae and Oil· goryzomys flavescens, which select the borders, and those habitat tolerants, Calomys laucha and Calomys musculinus. The two most abundant species in the community, A. azarae and C. laucha, differ in their habitat use : while the first has a clear preference for borders, the other is more equally distributed between habitats, although it is more abundant in fields. Both species show a shift towards an increased use of fields via density-dependent habitat selection with increasing abundance at some moments of the year. C. musculinus does not show habitat preference in any season, overlaping with the other species in both habitats. There is an interspecific effect on habitat use between A. azarae and C. laucha: While the first species cause a shift in habitat preference of C. laucha towards fields, this latter species affects the density-dependent habitat use of A. azarae. It is concluded that the pattern of habitat use of the studied species is affected by their differential preferences as well as by intra and interspecific density-dependent processes that change seasonally.Resume. -Nous avons analyse Peffet de la selection de l'habitat densite-dependante, et de la competition interspecifique sur 1'utilisation de 1'espace par les especes de petits rongeurs qui vivent dans les agroecosystemes. Nous avons etudie l'abondance des rongeurs dans les champs cultives et leurs limites, des bordures couvertes d'herbe moins perturbees par les acti-vites agricoles. On distingue deux categories d'especes : celles qui ne tolerent pas de variation de leur habitat, Akodon azarae et Oligoryzomys flavescens qui choisissent les bordures, et celles qui sont tolerantes, Calomys laucha et Calomys musculinus. Les deux especes les plus importantes dans la communaute, A. azarae et C. laucha, different dans leur utilisation de l'habitat. La premiere a une nette preference pour les bordures, tandis que 1'autre est repartie plus egalement dans les differents habitats, bien qu'elle soit plus abondante dans les champs. Les deux especes manifestent une tendance ä un usage accru des champs par intermediate d'une selection de l'habitat densite -dependante, avec une abundance qui s'accroit ä certains moments de 1'annee. C. musculinus ne montre en aucune saison de preference d'habitat, se trouvant avec 1'autre espece dans les deux habitats. II y a un effet interspecifique sur ('utilisation de l'habitat entre A. azarae et C. laucha: tandis que la premiere espece cause un changement dans la preference d'habitat de C. laucha vers les champs, cette derniere espece affecte l'utilisation de l'habitat densite-dependante de A. azarae. On conclut que le schema d'utilisation de l'habitat par les espe...

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Woody plant invasion in grassland ecosystems is a worldwide phenomenon, and biotic interactions as competition and predation have been invoked as a possible barrier to woody encroachment in many ecosystems. We evaluated the role of rodents as seed predators in Pampean grasslands, and we assessed the differences in removal by rodents between one native species, Prosopis caldenia (Caldén) and one exotic species, Gleditisia triacanthos (Honey locust). The experiment was conducted at different phases of the rodent population cycle in two grassland communities, a remnant of a native grassland and a post agriculture grassland (old field). The amount of seed loss caused by predation was estimated by a bait‐removal experiment in foraging stations. We estimated the frequency of foraging stations with consumption, the overall amount of seed predation and the individual rate of seed predation. The total amount of seed removal and the individual rate of seed removal were higher for P. caldenia than for G. triacanthos, in the native grassland than in the old field, and in autumn when rodent density was maximum. Overall, the role of rodents on woody seed removal varied according to the plant species and depending on the local conditions that vary through time and space.