Functional heterogeneity in resources within landscapes and herbivore population dynamics

Large mammalian herbivores are notorious for their propensity towards population irruptions and crashes, yet many herbivore populations remain relatively stable. I explore how resource heterogeneity within landscapes dampens population instability, using a metaphysiological modelling approach considering patch state distributions. Resource heterogeneity is functionally stabilizing through spreading consumption away from preferred resources before these become critically depleted. Lower-quality resources act as a buffer against starvation during critical periods of the seasonal cycle. Enriching resource quality is destabilizing, even if patch diversity is maintained, because food quantity then becomes the limitation. The potential consequences of landscape fragmentation are explored using the Serengeti ecosystem, characterised by broadscale resource gradients, as a hypothetical example. Further insights provided by the model are illustrated with specific examples concerning the effects of patch scales and waterpoint distribution. A metaphysiological modelling approach enables the basic consequences of landscape heterogeneity to be distinguished from further effects that may arise from specific patch scales and configurations, without the distracting detail of spatially explicit models.     

Fertilizer location modifies root zone salinity,root morphology,and water‐stress resistance of tree seedlings according to the watering regime in a dryland reforestation

There is a direct relationship between soil nutrient concentration in localized zones and root proliferation and elongation under well‐watered conditions. However, in field studies under semiarid conditions this relationship can change due to higher salt accumulation and soil dryness that affect root growth, water stress resistance, and seedling survival. We assessed the effect of different locations of fertilizer placement in the soil profile and water availability on root zone salinity, root development and ecophysiological responses of Quillaja saponaria Mol. after outplanting. A single dose (6 g L^?1) of controlled‐release nitrogen fertilizer (CRF_N) was placed at 0 cm (top layer), 15 cm (middle layer), or 30 cm (bottom layer) depth in the containers in a greenhouse, in addition to an unfertilized treatment (control). After 6 months, seedlings were transplanted to the field and subjected to weekly watering regimes (2 L plant^?1 and unwatered). Morphological and ecophysiological parameters were periodically measured on seedlings, as well as soil electrical conductivity (EC). After 1 year, the shoot : root ratio of unwatered seedlings decreased as a function of CRF_N placement depth, which was attributed to lower shoot growth and not to greater root growth. The root morphology of the bottom layer treatment was negatively affected by high EC in unwatered seedlings. Greater total root length and root volume of the middle layer treatment was found only when well‐watered; however, this did not contribute to improve physiological responses against water stress. The lowest EC and the highest photochemical efficiency, net photosynthesis, and stomatal conductance were shown by unfertilized seedlings, independent of water availability. Our findings suggest that varying depth of CRF_N placement does not contribute significantly to improve root growth under water restriction. Water supplements, independently of the CRF_N location in the substrate, contribute to decrease root zone salinity, and consequently, improve root volume growth.     

Herbicide savings and economic benefits of several strategies to control Sorghum halepense in maize crops

This study was conducted to assess the herbicide savings and the cost efficiency of site-specific herbicide application strategies in comparison with other strategies based on uniform application of herbicides throughout the whole field. The specific situation considered was Sorghum halepense infested maize fields in Spain. The results from a theoretical economic model were contrasted with the information derived from a S. halepense survey conducted in 37 commercial maize fields distributed over three Spanish maize production areas. Seven application strategies were simulated: 1) no herbicide; 2) overall full-rate; 3) overall half-rate; 4) and 5) site-specific spraying infested cells with full- and half-rate, respectively; 6) and 7) site-specific spraying infested cells plus adjacent buffer areas with full- and half-rate, respectively. The simulation results showed that site-specific weed management was the most profitable strategy when S. halepense infested area ranged between 6.5 and 18.7%. This scenario was present in 22% of the surveyed fields. In fields with less than 6.5% infestation (a situation present in 51% of the surveyed fields), yield losses were slight and the most profitable strategy was using no herbicide. When the infested area ranged between 18.7 and 40.8% (19% of the surveyed fields), no significant differences were observed between the net benefits of the various strategies. Full rate herbicide applications throughout the entire field resulted in the highest net returns in fields with more than 40.8% weed infestation (8% of the surveyed fields). In these cases, the added costs of weed detection, mapping and site-specific herbicide application were not justified by the herbicide savings obtained. Likewise, herbicide savings obtained with the various strategies depended on the proportion of the field infested. Considering the high difference between site-specific treatments and uniform application of herbicides when the infested area ranged between 18.7 and 40.8%, and that no significant differences in net returns were observed in this range, site-specific treatments should be preferred. Adding a buffer area to the site-specific treatments does not seem a suitable strategy due to the considerable increases in costs and reductions in herbicide savings.     

高校校园网内联网计算机的故障分析与维护方案

在高校校园网中,计算机的使用量和计算机的数量与日俱增,无论是负责维护计算机网络教室或者电子阅览室的工程技术人员还是接入校园网的个人用户都会遇到各种各样的计算机故障.本文通过对计算机产生故障现象的分析,对计算机硬件及系统软件的维护提出了解决方法.     

肠相关性淋巴样组织研究概况

系统论述了肠相关淋巴组织的组成结构及其在肠道黏膜免疫中的功能作用，同时，概述了肠相关淋巴组织在消化道感染中的可能作用，并指出从黏膜免疫的角度来研究畜禽消化道疾病的病原体与消化道粘膜感染的关系是揭示这类疾病发生机理的一个新的突破点，它将为策划新的防制战略以控制畜禽消化道疾病的发生和消化道感染提供重要的依据。     

Spatial distribution of weeds in arable crops: are current sampling and analytical methods appropriate?

This paper reviews the literature concerning the spatial distribution of weeds; highlighting the limitations of our current sampling and analytical methodologies, and suggesting how these inadequacies can be addressed. Most research studies have used discrete sampling, i.e. weeds are counted within a quadrat, on a grid basis. Few have mapped weeds at a whole-field scale, either with a resolution appropriate to spraying operations or key ecological processes. Statistical analyses used to describe the data can be divided into two main types, spatially implicit (also at the scale of the sampling unit) or spatially explicit, in which the location of individuals is included in the analyses. Spatially implicit methods can be strongly affected by quadrat size and mean density and are of doubtful benefit. More attention is required to address sampling resolution issues for spatially explicit methods. Our understanding of the formation and dynamics of spatial pattern, as well as predicting the consequences of site-specific management, can be improved with models. Unfortunately, most models consider only newly expanding patches and appear incapable of predicting spatial distributions when an area has been fully invaded. More detailed biological information is required if models are to become more realistic and informative. We also need to ensure that we understand the spatial processes in the context of the whole field environment, to optimize the success of site-specific weed management in the longer term.     

间作植物根系对Pb斑块胁迫适应机制

为研究间作体系植物根系受Pb胁迫的形态学适应机制,利用根箱试验设计Pb斑块胁迫下玉米、小花南芥单作和玉米与小花南芥间作处理。结果表明:在Pb斑块(0、1000 mg·kg~(-1))处理根箱条件下,间作小花南芥根生物量的相对关系指数显著增加,小花南芥根趋向有Pb斑块生长,根系表面积显著增加,间作模式极显著影响小花南芥根系形态;间作玉米根生物量的相对关系指数显著降低,玉米根趋向无Pb斑块生长,Pb处理极显著影响玉米根系形态。径级在1.5 mmd3≤2.5 mm根表面积显著影响间作小花南芥与玉米对Pb的响应。研究表明,根间相互作用激发小花南芥根系趋向Pb斑块,玉米根系趋向无Pb斑块,揭示了超富集植物与作物间作体系促进两种植物根系趋向性适应的重要原因。     

Filamentous fungi in wrapped forages determined with different sampling and culturing methods

The use of wrapped forage bales with high dry‐matter (DM) content implies risk of fungal growth inside the wrapping, and impaired feed quality. Since fungi may be unevenly distributed in bales, the method of sampling can influence the outcome of the analysis. Three common sampling methods for detection of fungi in wrapped forage were compared: direct plating of visible mouldy patches on bale surfaces (Method I), direct plating of pieces of foliage from drilled core samples (Method II) and quantitative analysis dilution series of drilled core samples (Method III). All samples were cultured on two media at two temperatures. Samples were collected from 124 farms in Sweden and Norway. Using Methods I, II and III, fungi were detected on 52%, 77% and 56% of the farms respectively. Fifty‐two fungal species were identified using a combination of culturing and molecular methods. The most predominant species were Arthrinium spp., found on 55% of the farms, followed by Penicillium roqueforti at 48% of the farms. Incubation at 25°C resulted in higher numbers of all genera, except Aspergillus, compared to incubation at 37°C. The different methods of sampling and culturing did not identify the same fungal species within the same bale. Analysing one bale per farm resulted in detection of less than half the number of species compared to analysing three bales. Of the sampling and culturing methods compared, direct plating of drilled core samples cultivated at malt extract agar at 25°C performed best in terms of qualitative analysis of fungi species.     

Environmental values and forest patch conservation in a rural Costa Rican community

Although conservation attention has generally focused on large forest tracts, there is increasing evidence that smaller forest patches are important for both conservation and rural development. A study of forest patch conservation in a rural Costa Rican community found that, although forest patch conservation was influenced by landholding size, material factors did not account for all the variation in forest patches conservation behavior or conservation orientations of farmers. A qualitative interpretive approach, using semi-structured interviews, found that environmental values were influenced by at least three factors. First, colonist farmers with direct experience with environmental degradation in other regions of Costa Rica were more likely to be concerned about environmental degradation. Second, childhood exposure to conservation played an important role in influencing conservation orientations. Third, the environment was frequently discussed in relation to the moral and social values present in religious doctrines. The study also found general support for forest conservation laws, and a clear indication that interest in forest conservation has grown in recent years. The study highlights the importance of environmental values, in conjunction with material factors, in influencing forest conservation in rural communities.     

Exposure assessment of professional pesticide users during treatment of potato fields

In this study four different mixing/loading and application practices in potato fields were monitored for exposure of operators to pesticides. Each operation – mixing, loading, and application – was measured individually in order to assess its relative contribution to the total exposure value. Inhalation exposure was measured by trapping the pesticides with a sorbent tube while sampling the air around the operator's face. Dermal deposition, which was measured by means of cotton gloves on the hands and by attaching patches to the operator's clothing, was the main contributor to the total exposure. Dermal deposition on the hands during mixing and loading exceeded all other dermal values. The experimental results are compared with the results obtained by the exposure assessment model PHED V1.1. This model gives an underestimation of the levels of operator exposure during mixing, loading and application. © 1999 Society of Chemical Industry