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31.
Juan Carlos lvarez-Ypiz Angelina Martínez-Yrízar Alberto Búrquez Cynthia Lindquist 《Forest Ecology and Management》2008,256(3):355
The objective of this study was to compare the vegetation structure and soil properties among old-growth tropical dry forests representing three categories of grazing intensity by cattle (light, moderate and heavy grazing) and a category of 20–30-yr-old secondary forest experiencing occasional grazing in a locality in northwestern Mexico. Within each forest type, three 0.1 ha plots located in different grazing ranges (“potreros”) were used as replicates. All woody plants (stem ≥ 2.0 cm diameter at 1.30 m height, DBH) were identified and measured in each plot. Mean basal area and above-ground biomass (AGB) were significantly higher in the moderately grazed old-growth forest. Species density was significantly lower in the secondary forest, where a leguminous tree species was dominant. Accumulation of AGB after 20–30 yr of secondary forest recovery accounted for 43% of the old-growth forest AGB. Soil properties varied among forest categories but did not follow a consistent pattern: mean total N and organic matter content were highest in the old-growth forest with moderate grazing; cation exchange capacity (CEC) was similar among the three old-growth forests categories, but it was significantly lower in the secondary forest compared to the old-growth forest with low grazing. Canonical Correspondence Analysis showed that CEC was highly correlated with the actual species distribution in the study area, especially with Acacia cochliacantha the dominant species of the secondary forest category. Resprouting capacity of the persisting species in the old-growth forests experiencing chronic disturbance could have contributed to the maintenance of some of the structural characteristics of a mature forest. Tropical secondary forests seem to accumulate AGB relatively fast, reflecting their potential for carbon storage and provision for other ecosystem services; therefore, they deserve urgent protection measures. 相似文献
32.
In this study we investigate the effect of fragmentation and disturbance on the spatial genetic structure, heterozygosity and inbreeding in Tabebuia ochracea (Bignoniaceae) in a seasonally Neotropical dry forest in the medium São Francisco River basin, Centre-East Brazil, based on the polymorphism at seven microsatellite loci. Four populations with different histories of disturbance and fragmentation were sampled: two continuous population (CP1 and CP2), with no history of recent disturbance and two fragmented and isolated population (FP1 and FP2), with recent history of disturbance due to logging for pasture establishment. Fragmented and continuous populations did not differ in any estimated parameter. However, all populations showed low levels of polymorphism and genetic diversity and high levels of inbreeding. Also, no spatial genetic structure was detected for populations using SPAGeDI software and no differentiation between these four populations was detected by Bayesian analyses performed with STRUCTURE software (K = 1). Differentiation measure by Wright's θ (0.032) and Hedrick GST (0.032) were significant but low. Our results strongly suggest that continuous populations are seed sources for the fragmented populations and that fragmentation and disturbance have been affecting these populations of T. ochracea in the Centre-East Brazil, leading to low levels of polymorphism and genetic diversity, and high inbreeding. Therefore, conservation efforts should increase in this region, with a reduction of agriculture expansion and the remove of cultivated areas and cattle from the Mata Seca and Lagoa do Cajueiro State Parks. 相似文献
33.
Habitat fragmentation is thought to create a barrier to individual movements particularly for area-sensitive species which, by definition, prefer to breed in large tracts of forest. For two breeding seasons, we radio-tracked an area-sensitive species, the scarlet tanager Piranga olivacea, in a fragmented landscape in northeastern PA. We found that scarlet tanagers made extensive and frequent movements among fragments. Paired males were less likely to leave their capture fragment, and traveled shorter distances. Unpaired males in fragments had two distinct tactics we labeled: “Sedentary” and “Mobile”. Sedentary males stayed at one fragment and sang at high rates, while mobile males spent 58% of the total time tracked off their capture fragment and traveled over a kilometer away from the capture site over open fields and through forests. Mobile males were not floaters per se because they were territorial (i.e., singing) in multiple sites. Habitat structure of a fragment did not correlate with the percent time a male spent off the capture fragment. Males in fragments experienced lower pairing success and were more likely to be first time breeders compared to males in continuous forest. Our results suggest that movements by scarlet tanagers in fragmented landscapes are not restricted during the breeding season, and that these movements are related directly to pairing status and indirectly to population density. 相似文献
34.
Forest vertebrates are critical to the subsistence of many tropical forest dwellers enjoying little or no access to other sources of animal protein. Yet the ecological and socioeconomic value of forest wildlife is being undermined as many large vertebrate populations are driven to local extinction by unsustainable hunting practices. Although large mammals that are preferred by hunters are widely variable in their morphology and ecology, they share a set of life history traits, which make them particularly vulnerable to overhunting. In this paper we compile data on game harvest from 31 tribal and nontribal settlements in Neotropical forests to examine how mammal assemblages are affected by the history of hunting within settlement catchment areas. The structure of hunter-kill profiles is related to settlement age and size in an attempt to understand how changes in hunting pressure may affect prey selectivity and the structure of residual game assemblages. There was a predictable shift from a few large-bodied to several small-bodied species harvested by increasingly older villages. Settlement persistence thus explained a significant proportion of the variation in mean body mass and species richness of mammals harvested. We conclude that differences in prey species profiles obtained by subsistence hunters of different ethnic groups can be largely explained by the local depletion status of game stocks, particularly large mammals, rather than by cultural factors. 相似文献