关键词： HERBIVORES   RESOURCE availability (Ecology)   FORAGE plants   BIOTIC communities   SAVANNAS   ANIMAL diversity   PREDATION (Biology)  

摘要： Herbivores are regulated by predation under certain environmental conditions, whereas under others they are limited by forage abundance and nutritional quality. Whether top-down or bottom-up regulation prevails depends both on abiotic constraints on forage availability and body size, because size simultaneously affects the risk of predation of herbivores and their nutritional demands. Consequently, ecosystems composed of similar species can have different dynamics if they differ in resource supply. Here, we use large herbivore assemblages in African savanna ecosystems to develop a framework that connects environmental gradients and disturbance patterns with body size and trophic structure. This framework provides a model for understanding the functioning and diversity of ecosystems in general, and unifies how top-down and bottom-up mechanisms depend on common underlying environmental gradients.

关键词： Acacia drepanolobium   ant symbionts   direct and indirect effects   ecosystem engineers   termites   top-down and bottom-up effects   vertebrate herbivores  

摘要： In African savannas, vertebrate herbivores are often identified as key determinants of plant growth, survivorship, and reproduction. However, plant reproduction is likely to be the product of responses to a suite of abiotic and biotic factors, including nutrient availability and interactions with antagonists and mutualists. In it relatively simple system, we examined the role of termites (which act as ecosystem engineers-modifying physical habitat and creating islands of high soil fertility), vertebrate herbivores, and symbiotic ants, on the fruiting success of a dominant plant, Acacia drepanolobium, in East African savannas. Using observational data, large-scale experimental manipulations, and analysis of foliar N, we found that Acacia drepanolobium trees growing at the edge of termite mounds were more likely to reproduce than those growing farther away, in off-mound soils. Although vertebrate herbivores preferentially used termite mounds as demonstrated by dung deposits, long-term exclusion of mammalian grazers did not significantly reduce A. drepanolobium fruit production. Leaf N was significantly greater in trees growing next to mounds than in those growing farther away, and this pattern Was unaffected by exclusion of vertebrates. Thus, soil enrichment by termites, rather than through dung and urine deposition by large herbivores, is of primary importance to fruit production near mounds. Across all mound-herbivore treatment combinations, trees that harbored Crematogaster, sjostedti were more likely to fruit than those that harbored one of the other three ant species. Although C. sjostedti is less aggressive than the other ants, it tends to inhabit large, old trees near termite mounds which are more likely to fruit than smaller ones. Termites play a key role in generating patches of nutrient-rich habitat important to the reproductive success of A. drepanolobium in East African savannas. Enhanced nutrient acquisition from termite mounds appears to allow plants to

关键词： Tropical rain forest   Non-equilibrium   Succession   Primates   Kibale National Park Uganda   Forest change  

摘要： In tropical forest systems tree community change after initial succession (approximately 50-100 years) is very difficult to detect because of the very slow pace of transformation. Since the mid 1980s, there has been an accumulation of evidence that many forests traditionally considered old growth or mature forests have been disturbed. Using 18 years of data on forest change in Kibale National Park, Uganda, we tested the following hypotheses. Species that frequently recruit only into areas of large-scale disturbance (e.g., conversion to agriculture) (1) have a more strongly negative annualized rate of population change (i.e., recruitment is less than mortality) than trees recruiting into the understory or canopy treefall gaps and (2) these species are declining in their average cumulative diameter at breast height (DBH). Both hypotheses were verified. We then examined relationships between forest change and diets of the five diurnal primates in Kibale. The emergent patterns suggest that forest change will lead to declines in some species, particularly the black-and-white colobus (Colobus guereza). We concluded that what was considered mature forest in Kibale has actually been disturbed in the recent past, and we discuss how potential sources of disturbance (dry-periods, elephant population fluctuations, and human disturbance) may affect both forest change and animal populations. We assess how such information might be useful in forest management. (C) 2009 Elsevier Ltd. All rights reserved.

关键词： Grazing   Nutrient cycling   Soils   Nitrogen mineralization   Foliage nutrients  

摘要： In African savannas, bottom-up soil nutritional factors and top-down herbivory have both been suggested to control the distribution patterns of bunch and lawn grasses. We tested the separate and combined roles of these factors on grassland distribution in a South African savanna by focusing on three main objectives, namely (1) are grazing patches and lawns restricted to specific soils or sites, (2) does herbivory alter rates of nutrient cycling which facilitates lawns and (3) are there any differences in foliage quality between lawn and bunch grass-dominated sites that might influence animals in choosing to feed on lawns? We set up ten sites along a grazing gradient in the Hluhluwe-iMfolozi Park in KwaZulu-Natal, South Africa. At these sites, we measured total soil nutrients, soil nutrient turnover rates and grass foliage nutrient concentrations. We found that in the Hluhluwe-iMfolozi Park the spatial distribution of lawn and bunch grassland types appears to be an animal-driven phenomenon and not dependent on specific soil properties. The short-structured and distinct species found in lawns do not appear to be restricted to nutrient-enriched patches. However, the grasses of these lawns had significantly higher nutrient concentrations, in their foliage, which might explain the high attraction to these patches by herbivores. We also did not find any animal-induced stimulation of nutrient cycling rates that are often associated with lawn grass species.

关键词： bird predation   bottom-up   community effects   foundation species   herbivory   limiting factor   Populus   predator removal   strong interactors   top-down   trophic cascade  

摘要： Terrestrial systems are thought to be organized predominantly from the bottom-up, but there is a growing literature documenting top-clown trophic cascades under certain ecological conditions. We conducted an experiment to examine how arthropod community structure on a foundation riparian tree mediates the ability of insectivorous birds to influence tree growth. We built whole-tree bird exclosures around 35 mature cottonwood (Populus spp.) trees at two sites in northern Utah, USA, to measure the effect of bird predation on arthropod herbivore and predator species richness, abundance, and biomass, and oil tree performance. We maintained bird exclosures over two growing seasons and conducted nondestructive arthropod surveys that recorded 63 652 arthropods of 689 morphospecies representing 19 orders. Five major patterns emerged: (1) We found a significant trophic cascade (18% reduction ill trunk growth when birds were excluded) only at one site in one year. (2) The significant trophic cascade was associated with higher precipitation, tree growth, and arthropod abundance, richness, and biomass than other site-year combinations. (3) The trophic cascade was weak or not evident when tree growth and insect populations Were low apparently due to drought. (4) Concurrent with the stronger trophic cascade, bird predation significantly reduced total arthropod abundance, richness, and biomass. Arthropod biomass was 67% greater on trees without bird predation. This pattern was driven largely by two herbivore groups (folivores and non-aphid sari-feeders) Suggesting that birds targeted these groups. (5) Three species of folivores (Orthoptera: Melanoplus spp.) were strong links between birds and trees and were only present in the site and the year in which the stronger trophic cascade occurred. Our results Suggest that this trophic system is predominately bottom-up driven, but under certain conditions the influence of top predators can stimulate whole tree growth. When the most limiti

关键词： Competition for light   Grazing   Natural disturbances   Tree recruitment and establishment   Trampling   Vegetation gap  

摘要： Tree establishment in grazed vegetation mosaics involves a series of early bottlenecks, including seed dispersal, germination, seedling emergence, survival and growth. In a field experiment, we studied seedling emergence of two species with contrasting recruitment strategies, Fraxinus excelsior and Quercus robur, in five structurally different vegetations: grazed and ungrazed grassland, ruderal pioneer vegetation, soft rush tussocks, tall sedge mats and bramble scrub. In a simulation experiment, we studied the interaction effects of pre-emergence flooding (3 weeks of inundation), trampling and grazing (simulated by clipping) of grassland vegetation on the emergence and early growth of both tree species in grass swards. Seedling emergence was enhanced in low swards and sparse vegetation types. Despite different recruitment strategies, the interaction of flooding and trampling of swards enhanced seedling emergence of both species. Grazing of soft rush and tall sedges enhanced emergence of F. excelsior. Clipping grass swards increased early growth of emerging Q. robur. Our results support the hypothesis that natural disturbances of soil and vegetation create microsites for seedling emergence and reduce above-ground competition. In grazed systems however, these results suggest a discordant relationship between successful seedling emergence and subsequent seedling growth/survival during the establishment process in structurally different vegetations.

关键词： boreal forest   forest bird community   irregular structure stands   non-parametric multivariate analysis   selection cutting   small mammal community  

摘要： The effects of logging on wildlife have been evaluated mainly in landscapes dominated by even-aged forest stands, leaving the impacts of harvesting on wildlife in irregularly structured stands less documented. We assessed the response of small mammals and birds to 4 silvicultural treatments with different levels of tree retention (from < 10% to 73%) in old-growth boreal forests of eastern Canada, where approximately 70% of stands have irregular structure. The experimental design controlled for local variation in the abundance of forest birds and small mammals by pairing each experimental harvested stand with an uncut stand. We found that species richness of small mammals and birds did not vary according to tree retention. Moss cover and vertical cover influenced small mammal community composition, whereas forest bird assemblages responded to conifer basal area, vertical cover, and snag availability. Species associated with silvicultural practices entailing retention levels > 55% included those that prefer closed canopy, such as the brown creeper (Certhia americana), ruby-crowned kinglet (Regulus calendula), and red-backed vole (Myodes gapperi). Our study demonstrates that silvicultural practices with retention levels > 55% are the most suitable for mitigating the impact of logging on animal communities in old-growth irregularly structured stands.

关键词： Ecosystem process   Plant-animal interaction Feeding trials Camera traps   Landscape structure   Long-distance dispersal Gut passage Home range  

摘要： The role of terrestrial mammals as seed dispersers of fleshy-fruited plants has only rarely been investigated in temperate regions although recent studies underline the importance of these animals for long-distance seed dispersal Here we examine the potential role of mammals as seed dispersers of wild cherry (Prunus avium L) along a gradient of human land-use intensity We placed camera traps at 21 wild cherry trees to identify the mammal species that visited the trees We conducted feeding trials to test if the recorded species were legitimate seed dispersers or seed predators and to assess gut passage times We tested the influence of human land-use intensity by quantifying habitat and landscape structure around the study trees at different spatial scales and analyzing its influence on visitation rates of tree visitors Red fox (Vulpes vulpes), roe deer (Capreolus capreolus), wild boar (Sus scrofa), marten (Martes spp) and badger (Meles meles) were identified as seed dispersers of wild cherry, of which wild boar was largely a seed predator Habitat and landscape structure at local spatial scales (70 m, 500 m radius) had no effect on the total visitation rates of mammals At larger spatial scales (1 0-10 0 km radius) total visitation rates increased with increasing proportion of extensively used farmland and seminatural habitat in the area The proportion of forest had no influence on visitation rates The results suggest that high proportions of extensively used farmland and seminatural habitat increase the visitation and seed dispersal rates of large mammals Comparing gut passage times with home range sizes and daily travel distances suggests that these mammals have the potential to disperse seeds over long distances and can provide gene flow in fragmented agricultural landscapes

关键词： 树鼩   地塞米松   链脲佐菌素   2型糖尿病  

摘要： 本实验分为两部分,第一部分为高脂高糖饲料联合地塞米松诱导树鼩糖脂代谢异常动物模型；第二部分为小剂量链脲佐菌素多次注射诱导树嗣2型糖尿病动物模型。 第一部分目的用高糖高脂饲料+地塞米松(dexamethasone, DEX)联合诱导树鼩糖代谢和脂代谢紊乱,为建立实验性2型糖尿病树鼩模型提供前期基础。方法14只雄性中缅树鼩,随机分为高糖高脂组(HGF,n=6)和高糖高脂+DEX组(HGF+DEX,3mg/kg, h.i, n=8),于给药前后分别测定空腹血糖(FBG)、体重、甘油三酯(TG),总胆固醇(TC),糖化血红蛋白(HbAlc);成模实验动物取胰腺,肾脏,肝脏等组织做常规HE染色,光镜下观察其病理变化。结果HGF组无一成膜,HGF+DEX组给药后第3周空腹血糖(FBG)就显著升高,维持两周后下降,但第7周又升高。同时发生脂代谢紊乱,糖耐量异常且两组体重都下降。HGF+DEX组的树鼩胰腺常规HE染色发现胰岛数量明显减少,体积变小,胰岛β细胞数量减少,肿胀；部分肾小管上皮细胞萎缩、脂肪变性,肾小管管腔扩张,钙化,并可见蛋白管型,肾间质充血；肝细胞不同程度脂肪变性。结论高糖高脂+地塞米松连续多次腹腔注射可诱发树鼩糖代谢、脂代谢紊乱。 第二部分目的探索链脲佐菌素(Streptozotocin, STZ)诱导树鼩2型糖尿病动物模型的方法。方法22只雄性中缅树鼩,随机分为正常对照组(NC, n=6)、STZ组(120mg/kg, i.p,n=8)和DEX+STZ组(n=8)。连续观察15周,测定FBG、体重、TG和TC；在血糖明显升高并维持两周以上时做口服葡萄糖耐量试验(OGTT)。将FBG≥10.0mmol/L并维持两周以上且血脂与本组给药前相比有显著差异者视为成膜动物。并于实验结束时取成模实验动物胰腺,肾脏,肝脏等组织做常规HE染色,光镜下观察其病理变化。结果正常对照组(NC)组无一成膜；STZ组成膜率为100%,一只动物在OGTT实验中由于血糖过高而死亡(第7周),死亡率为12.5%(1／8)。成模树鼩具有胰岛素抵抗、高血糖、血脂紊乱的T2DM临床特征,高血糖持续时间最长、最稳定(到实验结束模型稳定至少80 d)。光镜下观察病理切片,FBG≥10.0mmol/L的树鼩胰腺常规HE染色中胰岛数量明显减少,胰岛萎缩,体积变小,胰岛界限不清晰；肾脏组织切片可见肾间质充血,肾小管上皮细胞水肿变性,有淋巴细胞浸润；肝脏最典型的变化是脂肪变性。结论以120mg/kg剂量腹腔注射STZ多次后能够快速诱导树鼩形成T2DM,是一种较好的制备树鼩T2DM的方法。

关键词： Herbivores   Pathogens   Phenolics   Coniferous trees   Chemical patterns   Plant productivity  

摘要： Coniferous trees of different species, or of the same species growing at different locations, vary in the extent to which they are attacked by various herbivores and pathogens. Plant secondary metabolites might be a key to understanding some of this variation. At the site level, we investigated if there was an intra- or interspecies pattern for individual compounds (or for groups of compounds) and their relationship to indices of plant nitrogen and plant productivity. For example, do plants exhibit similar covariance in defence compounds when evaluated across a number of sites varying in productivity? Here, we concentrated on the phenolic profile of Pinus sylvestris, Picea abies, Juniperus communis and Pinus contorta. Our results indicate striking differences in secondary chemistry profiles of the twigs including needles of the trees and in the inter-relationships amongst individual compounds and groups of compounds. Flavonols occurred in high variety in P. sylvestris and were highly correlated with each other, differing from P. contorta. But the results of the factor analyses indicate an underlying pattern for flavonols of the coumaroyl type for P. contorta. In contrast, the compounds of the other tree species showed a low degree of inter-correlation. Co-occurring phenolics of different tree species were not correlated. Overall, our analysis of site indices indicated that plant productivity was not a useful predictor for the concentration of specific phenolics. The relationship amongst plant nitrogen and specific phenolics might be the result of two defence strategies (one related and the other not related to nitrogen content). This might enable the plant to shift its defences against attacks with a high degree of flexibility.