Showing papers by "Marcel Rejmánek published in 1999"

Small rodents as significant dispersers of tree seeds in a Neotropical forest

Abstract: Through seed dispersal and predation, terrestrial mammals should be an important component of the mecha- nisms that determine patterns of tree recruitment in tropical forests. Despite their great abundance and ubiquity in Neo- tropical forests, small rodents as seed predators and dispersers remain largely forgotten. To investigate the fates of seeds in a hunted primary forest in Belize, we tagged seeds ofAstrocaryum mexicanum (Palmae), Ampelocera hottlei (Ulmaceae), and Pouteria sapota (Sapotaceae) and placed them into open plots, exclosures accessible only to small mammals, and exclosures accessible to medium-sized and small mammals. The exclosure experiments and fates of the seeds show that the spiny pocket mouse, Heteromys desmarestianus (Heteromyidae), was the dominant handler of seeds of the first two species and also removed a significant proportion of the very large-seeded Pouteria. Most of the seeds were killed immediately upon removal, but many of the seeds (3 - 18 %) of the first two species were scatterhoarded (dispersed and buried in the soil) by Heteromys. Some of the scatterhoarded seeds (29 %) remain buried and therefore protected from predation by other animals. Agoutis (Dasyprocta punctata), a caviomorph rodent, buried 13 % of the seeds of Pouteria, and Heteromys consumed and dispersed but did not bury Pouteria seeds. Results of this study support predictions by some researchers that small rodents are dominant terrestrial granivores in Neotropical forests. The role of small rodents as seed dispersers, however, has never been fully appreciated.

Status and habitat preferences of Uganda cheetahs: an attempt to predict carnivore occurrence based on vegetation structure

Abstract: In this paper we examine whether the occurrence of cheetahs (Acinonyx jubatus) in Uganda can be predicted from habitat characteristics extracted from a vegetation map. We first established the status of the cheetah in Uganda through field-interviews that Gros conducted in 1990. Cheetahs occurred almost exclusively in the Karamoja region where we estimated 53–310 individuals. Based on 216 sightings, the average number of adults in all-adults sightings was 1.65 + SD 0.95 and the average number of cubs in family groups 2.5 + SD 1.65. Compared to Graham and Parker's 1965 East African survey, average adult group size was slightly smaller in 1990 and large family groups were rarer. Comparison with Gros 1990 survey showed considerably lower cub-to-adult ratio and percent of observations with cubs in Uganda than in Kenya. A Geographic Information System (GIS) analysis of vegetation structure in areas where cheetahs were observed and in those where none were reported suggested that cheetahs might favor habitats with 25–50% woody cover and grasses of medium height (50–100 cm). A discriminant analysis correctly classified 72.1% of ‘used’ habitats and 70.4% of ‘no-report’ habitats. A logistic regression analysis improved the correct allocation of ‘used’ habitats by 2.2%. Either the discriminant function or the logistic regression, which require only four easily obtainable vegetation characteristics, may help to pinpoint suitable cheetah habitats for conservation purposes. Our approach could be adapted for analyzing habitat suitability for other species of carnivores.

Resistance and resilience of subalpine wetlands with respect to prolonged drought

Abstract: Vegetation dynamics of subalpine wetlands in the Sierra Nevada, California, were studied from 1988 through 1996 During this period, the region experienced a drought lasting from 1988 to 1994 and reaching its extreme in 1992 Our intention was to analyze the changes in plant species diversity, composition, and biomass, and interpret them in terms of drought resistance and resilience Four plant communities, dominated respectively byCarex rostrata, Juncus balticus, Scirpus acutus orNuphar polysepalum were clearly discernible in the marsh along the water depth gradient Species diversity ofCarex rostrata, Scirpus acutus andNuphar polysepalum communities was the highest during the driest year, 1992, while biomass was lowest for all vegetation types in that year Dominance ofCarex andJuncus has not changed over the years, however,Scirpus almost totally disappeared from the marsh, and theNuphar zone become dominated by the rhizomatous perennial,Hippuris vulgaris, and terrestrial ruderals in dry years In terms of changes in species composition,Carex andJuncus communities were both resistant and resilient TheNuphar community seemed to be less resistant and more resilient, while theScirpus community was neither resistant nor resilient If we consider biomass per plot as a variable of interest, regardless of species composition, thenCarex andJuncus were to some extent resistant and all plots were resilient because they were able to recover quickly to their pre-drought biomass Life histories of dominant species were a more important determinant of community stability than species diversity