关键词： ant assemblages   ant-homoptera interaction   ant species turnover   cerrado savanna   daily foraging schedule   Didymopanax vinosum   Guayaquila xiphias   honeydew   mutualism   plant phenology  

摘要： We investigate the ant fauna associated with the Neotropical treehopper, Guayaquila xiphias, on shrubs of Didymopanax vinosum in the cerrado (savanna) of SE Brazil. Treehoppers infested plane at the border of the cerrado almost euclusively and preferably fed near the apical meristem. During the reproductive phase of the host plant, however, the vast majority of the treehoppers aggregated on the inflorescences. We found 21 ant species harvesting honeydew at G. xiphias aggregations, the most frequent being Camponotus rufipes, Ectatomma edentatum, C. crassus, and C. renggeri. Such a taxonomic diversity of ants tending G. xiphias aggregations in the cerrado is far greater than that reported for any other ant-homopteran system. Daily turnover of ant species at a given treehopper aggregation was observed on 29 percent (64 out of 222) of the G. xiphias aggregations recorded on D. vinosum shrubs. Species replacements probably reflect distinct humidity and temperature ranges tolerated by the species, and may ultimately reduce interspecific competition at homopteran aggregations. Since predation and parasitism on G. xiphias can be severe, and tending: ants protect the homopterans against predators and parasitoids, the round-the-clock activity by ants at G. xiphias aggregations is regarded as crucial for the survival of these treehoppers in the cerrado.

关键词： ants   anurans   associations   chemical defense   Kassina   Megaponera   Paltothyreus   Phrynomantis   savanna habitat  

摘要： We report observations and experiments on associations of the frogs, Phrynomantis microps and Kassina fusca, and the ponerine ante, Megaponera foetens and Paltothyreus tarsatus in a West African savanna. Several experiments show chat the tolerant behavior of the ants toward these frogs is due to chemical components of the frogs' skin. Comparisons to other syntopic hog species suggest that this enables these frogs to make use of burrows that otherwise are unavailable. This is advantageous because in the savanna, most hiding places that remain moist during the dry season are occupied by ants.

关键词： Forests   Trees   Ecosystem structure   Forest floor   Stand structure   Sweden   Soil fauna   Alces alces   Invertebrata  

摘要： The role of moose in structuring the boreal forest ecosystem has been studied extensively in recent pears. This research has focused mainly on the effects of moose on vegetation and soil dynamics. However, the extent to which these effects influence animal communities has received little attention. We studied how invertebrate assemblages on the forest floor of two Swedish pine forests were affected by the presence of moose. Our approach was to trap invertebrates using pitfall traps in unbrowsed exclosures and browsed reference plots, estimate the percentage cover of herb-layer plants, measure the height and density of tree species, and measure the accumulation of leaf litter. The effect of moose on tree strata differed between the two areas. In Sunnas, where the stand was a mixture of deciduous trees and pines, the main effect of moose was to eliminate most of the deciduous trees from the canopy. Here the amount of deciduous leaf litter was ten times higher in unbrowsed exclosures compared with browsed plots. The stands on the dry sites in the other area, Furudal, were almost pure pine. Here, moose browsing reduced the density of taller pines, but increased the number of pine saplings. At Sunnas, the herb-layer plant cover did not differ between browsed and unbrowsed plots. However, there was a difference between exclosures and browsed plots in terms of the ground-dwelling invertebrate assemblage. Number of individuals of most invertebrate groups that differed between exclosures and browsed plots were higher in unbrowsed plots and the total number of invertebrates was also higher in unbrowsed plots, but diversity of invertebrate fauna was higher in browsed plots. At Furudal both the vegetation and invertebrate assemblages differed between browsed and unbrowsed plots. Browsed plots had a higher cover of ground lichens;while unbrowsed plots had more mosses. Among the invertebrate taxa that differed between treatments, the number favored by browsing and the number disf

关键词： Tree cavities   Hardwood forests   Hardwood trees   Highlands   Pine trees   Ducks   Coniferous forests   Wetlands   Hardwoods   Wildlife refuges  

摘要： The availability of natural cavities for cavity-nesting waterfowl, especially wood ducks (Aix sponsa), was unknown for the regenerating forest of Sunkhaze Meadows National Wildlife Refuge, ME. An assessment of cavity availability was needed to determine if the existing nesting box program should be increased. During November to March, 1996-97 and 1997-98, I sampled 56 one-half ha random plots, stratified into 5 types (upland hardwood, upland conifer, upland mixwood, wetland conifer, and wetland hardwood) to assess availability of trees with cavities. The predominant tree species with cavities were red maple (Acer rubrum; 39%) and aspen (Populus sp.; 31%); 72% of all trees with cavities were alive. Density of nest-cavity trees/plot averaged from 1.0 plus or minus 0.4 (x plus or minus SE) in wetland softwoods to 1.9 plus or minus 0.4 in upland hardwoods. This low density of potential cavity trees and the small mean dbh (39.4 plus or minus 1.6 cm ) indicate a young forest with few suitable cavities. Forested areas, especially hardwoods near canopy openings, need to be allowed to mature to increase the number and quality of future cavities. An expanded nest box program seems justified.

关键词： congo   eucalyptus   forest plantations   soil macrofauna  

摘要： Many forest species can be found in understory vegetation of old plantation plots, despite the fact that the native vegetation was a poor savanna growing on highly nonfertile sandy soils. The aim of the present paper is to describe the changes that occur in the environmental conditions when savanna is planted with fast-growing trees, and is particularly concerned with vegetation and soil macrofauna. The study was carried out in industrial eucalyptus plantations, and in experimental Acacia and pine plantations. Most plots were located on sandy soil, but some measurements were also carried out on clay soil planted with the same species in order to assess the influence of soil type. A strong correlation was shown between the age of the eucalyptus trees and the percentage of forest species in undergrowth, emphasizing the progressive change from savanna vegetation towards forest vegetation. Biomass and density of macrofauna were very low in both sandy and clayey savanna soils, total biomass being 3.3 and 5.8 g/m(2) respectively. Soil macrofauna became more important as the age of plantations increased, and biomass reached 29 g/m(2) in the 20-year-old eucalyptus plot on sandy soil, and 74 g/m(2) in 26-year-old eucalyptus plantation on clay soil, compared to 33 g/m(2) in the natural forest plot on sandy soil;however, frequency of occurrence and number of taxa were lower in old eucalyptus plot as compared to forest. Large differences in the abundance of macrofauna were observed in relation to planted species. Acacia was most favourable to soil macrofauna, with a total biomass of 60 g/m(2) on sandy soil and many taxa present. Pine plantations had a poor macrofauna and several taxa were lacking, particularly in the sandy soil. Total macrofauna frequency was significantly correlated with the percentage of forest species in understory vegetation. Both were correlated with soil pH and soil organic-matter content. The results suggest that soil organic matter and litter quality ar

关键词： Seedlings   Grasses   Seeds   Feces   Rain   Cattle   Deer   Soil water   Growing seasons  

摘要： This study examined the potential of animals as vectors for mesquite seedling establishment via fecal deposition. Cattle, deer, and coyote fecal material containing emerging mesquite was located in May 1994 on a clay loam site in north Texas and assigned to two grass competition treatments (clipped to 3-cm height, or unclipped) and three moisture treatments (rainfall, rainfall + four 1.25-cm irrigations, or rainfall + one 10-cm irrigation). Except for a 2-week period in August 1993 when feces were deposited, cattle grazing was excluded from the site during the study and for several years prior to study initiation. Seedling emergence and survival were quantified throughout 1994, a dry year, and 1995, which had precipitation well above normal. By spring 1995, survival of 1994 seedlings (all treatments pooled) was 16.6, 9.3, and 1.3% at deer, cattle, and coyote fecal sites, respectively. Survival of 1994 seedlings was not affected by moisture or clipping treatments. New cohorts of emerging seedlings occurred from the same fecal sites during 1995. High rainfall during summer 1995 increased percent survival of 1995 seedlings when compared to survival of 1994 seedlings, even though grass growth was much greater in 1995 than in 1994. A strong trend emerged in which survival of 1995 seedlings appeared to be greater when surrounding perennial grasses were clipped (37.0% in clipped plots vs. 9.4% in unclipped plots), but this was not statistically significant (P less than or equal to 0.14). By spring 1996, average number of seedlings emerged per site (1994 and 1995 cohorts combined) was 7.8, 5.2, and 4.5;number of established seedlings per site was 0.8, 0.6, and 0.3;and percent survival was 12.1, 13.5, and 6.5% at cattle, deer, and coyote sites, respectively. Mesquite seedlings were capable of establishing from feces of all three animal vectors within dense, ungrazed stands of perennial grass.

关键词： competition   facilitation   functional response   herbivory   plant-herbivore model   savanna ecosystems  

摘要： Current hypotheses to explain dynamic transitions between savanna grasslands and woodlands in Africa focus on grazing by elephant or the influence of fire. Using a simple mathematical model, this paper argues that interactions between small herbivores such as impala or buffalo and large herbivores such as elephant or giraffe may provide a plausible alternative hypothesis. The interplay of competition and facilitation between these types of herbivores could explain transitions between grassland and woodland and vice versa. A review of the literature is presented in support of this hypothesis.

关键词： eutheria   mammalian evolution   mitochondrial genomes   molecular phylogeny   phylogeny   evolutionary trees   paralogy   immunological distances  

摘要： DNA sequences provide a direct record of the genealogy of extant species. This tremendous reservoir of phylogenetic information is only beginning to be exploited. If progress in molecular phylogeny is being made, it should be most conspicuous amongst mammals, as their evolution is probably the best-studied. Indeed, surprising changes have recently been proposed for the tree of mammalian orders. These range from grouping whales with hippos, to placing African golden moles closer to elephants than to their fellow insectivores.

关键词： production   copepoda   flooded savanna  

摘要： This is the first contribution to the copepod production in Venezuelan tropical savannas. Total abundance, biomass, production and mean P/B ratio of nauplii, copepodids and adults were determined in interdaily samples from a flooded, embanked savanna during February and March (end of dry season), and May (beginning of rainy season). Highest values of biomass and total production were recorded during dry season (61.5 mg m(-3), 153.8 mg m(-3) day(-1), respectively), compared to the rainy season (5.6 mg m(-3), 45.9 mg m(-3) day(-1)). The last values are related to a low population density found during rainy season. Highest production were observed in copepodids at the end of the dry season. Significant differences of production between nauplii and copepodid stages, as well as between nauplii and adults, were also found. (C) 1998 Elsevier Science B.V. All rights reserved.

关键词： grasses   forbs   dwarf shrubs   offtake   herbivores   desertification   nomads  

摘要： A three-year study was done to determine herbaceous biomass production and its utilization by herbivores in an arid zone inhabited by a largely nomadic population in northern Kenya. The indicator selected for study was aboveground live standing biomass of grasses, forbs, dwarf shrubs and the total sum biomass of these vegetation categories (total herbaceous layer). Sampling was done along grazing gradients in order to estimate the utilization levels in the arid zone 7 and the semi-arid zone 6. Mean peak standing biomass for grasses, forbs, dwarf shrubs and total herbaceous layer under non-grazed conditions was 184.4, 374.2, 1094.4 and 1504.0 kg/ha in eco-zone 7, respectively. in the zone 6, mean peak standing biomass was 55.3, 98.8, 4259.1 and 4320.1 kg/ha under non-grazed conditions, respectively. Results indicated graminoid removal to be respectively 57.1-99.8% and 24.2-87.2% of mean peak standing aboveground live biomass in the zone 7 and 6. Forb utilization in zone 7 was estimated to be 51.5-99.3%. Mean peak forb standing biomass however, showed a general increase on grazed sites compared to the non-grazed plot in zone 6. The corresponding utilization of the dwarf shrub layer was estimated to be in the range 40.5-80.0% and 76.6-92.3% in zone 7 and 6 respectively. Total herbaceous layer consumption of 39.3-85.3% and 74.0-90% was estimated for zone 7 and 6 respectively. These findings suggest that herbivores exert considerable control over biomass dynamics of the herbaceous layer in this zone and contribute to degradation in heavily utilised areas. Efforts to understand and tackle the desertification problem in this area must therefore take this fact into account. (C) 1998 Elsevier Science B.V. All rights reserved.