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We examined the responses of a Spartina densiflora (Poaceae) marsh to an accidental summer fire taking place in the Otamendi Natural Reserve (Argentina). Green, standing dead, reproductive and litter fractions of S. densiflora, as well as forbs biomass were determined before and after burning using randomly selected quadrates. Fire reduced most of the aboveground biomass of the green, standing dead and litter fractions of S. densiflora. Green biomass attained higher values before the fire as compared to those from the first year after burning during the spring, while the second year spring attained intermediate values. The standing dead material represented the main fraction of S. densiflora biomass in the pre-fire period, and after burning, this fraction reappeared in autumn, 4 months after the fire. The litter showed zero values throughout the first 16 months after the fire, and reappeared at end of the second winter. The flower biomass was exclusively recorded on late summer and showed relatively low values. Seedling recruitment of S. densiflora was not observed throughout the study period. Forbs biomass showed large fluctuations both before and after fire and highest forbs biomass values were observed during the spring before fire, and at end of summer 1 year after the fire. Although the results of the present study showed a tendency towards the original structure, most of the biomass fractions did not reach values similar to those before the fire after 2 years of post-fire growing, suggesting that a longer recovering time is expected.

We employed a live trapping grid to analyse the effect of heterogeneity habitat on dispersion of Akodon azarae, Oxymycterus rutilans and Oryzomys delticola. The reproduction and survival of three species are associated with their residency in the grassland, avoided the use of woodland. A. azarae reproductive and non-reproductive individuals preferentially use the sites dominated by Ambrosia scabra (microhabitat 1). Their residency at those sites would present nutritional advantages due to higher availability of seeds. This species begins to use other microhabitats more frequently only after the population peak. O. delticola preferentially uses the sites dominated by Lonicera japonicaBaccharis spicata (microhabitat 3). As both species present similarities in their diets, the differential use of the grassland microhabitat would contributate to diminish competitive interactions between them. O. rutilans reproductive individuals do not show preference for either microhabitat. During autumn-winter, non-reproductive individuals are more frequently captured in microhabitat 1. The important diminution herbaceous cover among Cortaderia selloana bushes (microhabitat 2) and under the dominant shrubs of microhabitat 3, led as to think that predation could be an important factor regarding the habitat use pattern of A. azarae and O. rutilans during autumn and winter. The climbing ability of O. delticola would contribute to diminish the impact of predation it could suffer at microhabitat 3.Resume. -Les auteurs ont etudie ä Paide d'une grille de piegeage 1'effet d'un habitat heterogene sur la dispersion a'Akodon azarae, Oxymycterus rutilans et Oryzomys delticola. Les trois especes vivent dans la prairie et evitent la foret, ce qui assure leur reproduction et leur survie. A. azarae, qu'il soit ou non en reproduction, utilise de preference des sites ou domine Ambrosia scabra (microhabitat 1) oil il doit trouver des graines tres nourrissantes. Cette espece ne commence ä utiliser d'autres sites qu'apres le pic de population. O. delticola frequente de preference les sites ou dominent Lonicera japonica et Baccharis spicata (microhabitat 3). Les deux especes ayant des regimes alimentaires compa- Mammalia, t. 55, n° 3, 199L Brought to you by | University of Arizona Authenticated Download Date | 6/3/15 4:49 PM 340 MAMMALIA rabies, la frequentation par chacune de microhabitats differents dans la prairie peut contribuer ä diminuer la competition entre elles. O. rutilans en periode de reproduction ne montre pas de preference pour Tun ou Pautre microhabitat. En automne et hiver, les individus non reproducteurs sont plus frequemment captures dans le microhabitat 1. Une importante reduction du couvert herbace entre les buissons de Cortaderia selloana (microhabitat 2) et sous les plantes dominantes du microhabitat 3, conduit ä penser que la predation peut etre un facteur important d'utilisation de Phabitat a 1 A. azarae et de . rutilans en automne et en hiver. L'aptitude au grimper que presente O. delticola doit contribuer ä diminuer Pimpact...

Plant cover availability has been mentioned as one of the most important resources determining cricetid habitat use. In this paper, we describe the effect of variations in the principal dietary items (identified through in situ dietary studies) on the use of low and uphabitats by Oxymycterus rutilans, Akodon azarae, Scapteromys iumidus and Olygoryzomys delticola, in the low delta from Buenos Aires. Our results show that A. azarae feed mainly on invertebrates and to a lesser degree, on plant remains. Invertebrates are also the principal item in 5. iumidus and O. rutilans diet, while O. delticola prefer green remnants. Both S. tumidus and O. delticola use low habitats all year round, while A. azarae and O. rutilans show seasonal variations in their habitat preference. In neither case, seasonal variations were associated to food availability. We suggest that the morphological adaptations to amphibious life of S. iumidus and the climbing habits of O. delticola made these species more efficient in low habitat (i.e., floodable and with an arboreal-shrub stratus), than A. azarae and O. rutilans. In the latter species, the seasonal variations of their habitat use seem to be a strategy used by populations to avoid the negative effects of extreme temperatures.Resume. -La disponibilite de la couverture vegetale a etc consideree comme Tune des ressources les plus importantes qui determinent l'utilisation de l'habitat par les Cricetines. Nous decrivons dans ce travail l'effet des variations des principaux items alimentaires (identifies sur place) sur utilisation des habitats par Oxymycterus rutilans, Akodon azarae, Scapteromys tumidus ct Oligoryzomys delticola dans le delta de Buenos Aires. Nos resultats montrent que A. azarae sc nourrit surtout d'invertebres, et ä un degre moindre de vegetaux. Les invertebres sont aussi item principal du regime de S. tumidus et O. rutilans, alors que O. delticola prefere la vegetation verte. 5. tumidus et O. delticola utilisent toute 1'annee des habitats bas tandis que A. azarae et O. rutilans montrent des variations saisonnieres dans leurs preferences d'habitat; mais ces variations n'etaient pas associes ä la disponibilite dc la nourriture. Nous suggerons que les adaptations morphologiques ä la vie amphibie de S. tumidus et les aptitudes au grimper de O. delticola rendent ces especes plus efficaces dans les habitats bas (c'est-a-dire inondables, avec une strate de buissons) que A. azarae et O. rutilans. Chez cette derniere espece les variations saisonnieres de l'utilisation de l'habitat paraissent etre une strategic utilisee par les populations pour eviter les effets negatifs des temperatures extremes.

Home range of Calomys musculinus was investigated during two and a half years in four enclosures of 8,400 m 2 and 7,500 m 2 , in alfalfa and sorghum fields in the western Pampa of Argentina. C. musculinus represents 91 % of the catch ; the other 9 % is made up of three other species. Sixty seven percent of the individuals of this species were captured only once, whereas only 3.2 % were captured five or more times. Recorded sex ratios show that males are more frequently captured than females.Maximum distance between points of capture (av.M), and the average distance (av.D) between successive points of capture present a seasonal variation and are greater in males than in females. The radius of the standard diameter (rH) was 22.5 m for males and 14.2 m for females ; it was found to vary with habitat, season, and age, especially in males. The radius of recapture (rC) was greater for males than for females ; the pattern of use of home range was uniform except in males in the spring of 1966 in the previously overgrazed alfalfa field. The pattern was circular for individuals captured in the sorghum field.Resume. -L'etendue du domaine de Calomys musculinus a fait 1'objet de recherches pendant deux ans et demi dans 4 enclos de 8 400 m 2 et 7 500 m 2 , dans des champs d'alfa et de sorgho, dans l'ouest de la pampa d* Argentine. C. musculinus y represente 91 % des captures ; trois autres especes constituent les 9 % restants. 67 % des individus de cette espece n Ont ete captures qu'une seule fois ; 3,2 % ont ete captures 5 fois ou plus. Les males ont ete plus souvent captures que les femelies.Les distances maximales entre points de capture (av.M) et la distance moyenne (av,D) entre points de capture successifs montrent des variations saisonnieres et sont plus importantes pour les males que pour les femelles.

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