首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 0 毫秒
1.
景观异质性研究评述   总被引:41,自引:2,他引:41  
通过对国内外大量资料的整理分析,对景观异质性的产生机制、分类、 在景观生态学研究中的地位及其测度方法进行了评述,在对景观异质性与景观稳定性、景观规划和管理以及生态学上的干扰、尺度、生物多样性之间相互依存关系论述的基础上,指出了当前景观异质性研究的复杂性,测度方法的不系统性,景观异质性建模在理论与实践上的困难性与局限性,并对生态复杂性研究的理论和方法应用于景观异质性研究进行了展望。  相似文献   

2.
By varying the time spent searching for food, parasitoids modifytheir expected lifespan, and therefore their total lifetimereproductive success. Using a stochastic dynamic approach, wedefine the best choice between searching for food and searchingfor hosts as a function of the state of the parasitoid and theavailability of food when hosts and food are found in differentparts of the environment A first model deals with the influenceof food availability and survivorship conditions on the behaviorof a single parasitoid. Our results suggest that under conditionsof very low food availability, parasitoids should never searchfor food. When food availability is moderate, parasitoids shouldnot wait until their reserves are low before searching for food.When food is abundant and survivorship is independent of foodconsumption, parasitoids should search for food only when theirreserves are almost exhausted. They should not wait so longif survivorship depends on the energy reserves. By finding thestate-dependent ideal free distribution for a population ofparasitoids, we are able to predict their distribution betweenthe feeding area and the host living area at equilibrium. Theproportion of parasitoids in each area is altered by the numberof competitors and interference. Finally, the model predictsthat optimal time sharing between food searching and host searchingmay promote the stability of the host-parasitoid system.  相似文献   

3.
How 'hidden' reading frames are expressed   总被引:5,自引:0,他引:5  
Secondary reading frames, 'hidden' under other reading frames, are used for coordinated expression of proteins in several eukaryotic viruses. In some genes, ribosomal frameshifting and initiation or reinitiation of protein synthesis on internal AUG codons are translational mechanisms allowing access to such 'hidden' reading frames. In others, secondary reading frames are translated from alternatively spliced or edited mRNAs.  相似文献   

4.
Prediction approaches to sequentially searching for an optimal dose   总被引:1,自引:0,他引:1  
W J Shih 《Biometrics》1989,45(2):623-628
The search for an optimal dose of a drug has been investigated in a sequential setting to ensure the safety level by Eichhorn and Zacks (1973, Journal of the American Statistical Association 68, 594-598; 1981, Communications in Statistics--Theory and Methods A10, 931-953), Eichhorn (1974, Communications in Statistics 3, 263-271), and Robinson (1978, Biometrika 65, 75-78). In this paper the procedure proposed by Robinson is given a new interpretation in terms of mean coverage tolerance prediction, and a new procedure in terms of guaranteed coverage tolerance prediction is proposed. The new procedure has better properties and is more stringent, and hence more desirable for safety consideration.  相似文献   

5.
Forecasts of range dynamics now incorporate many of the mechanisms and interactions that drive species distributions. However, connectivity continues to be simulated using overly simple distance-based dispersal models with little consideration of how the individual behaviour of dispersing organisms interacts with landscape structure (functional connectivity). Here, we link an individual-based model to a niche-population model to test the implications of this omission. We apply this novel approach to a turtle species inhabiting wetlands which are patchily distributed across a tropical savannah, and whose persistence is threatened by two important synergistic drivers of global change: predation by invasive species and overexploitation. We show that projections of local range dynamics in this study system change substantially when functional connectivity is modelled explicitly. Accounting for functional connectivity in model simulations causes the estimate of extinction risk to increase, and predictions of range contraction to slow. We conclude that models of range dynamics that simulate functional connectivity can reduce an important source of bias in predictions of shifts in species distributions and abundances, especially for organisms whose dispersal behaviours are strongly affected by landscape structure.  相似文献   

6.
Tree diseases and landscape processes: the challenge of landscape pathology   总被引:2,自引:0,他引:2  
Forest pathology inherently involves a landscape perspective, because tree pathogens propagate according to heterogeneous spatial patterns of flow and isolation. Landscape pathology is a field that is now emerging from the transdisciplinary cooperation of forest pathologists with landscape ecologists. Here, we review recent broad-scale assessments of tree disease risk, investigations of site and host preferences for several root rot pathogens, and regional historical analyses of pathogen outbreak in plantations. Crucial topics include fragmentation effects on pathogen spread and geophysical features that predispose forest patches to disease expression. Recent methodological developments facilitate the spatially explicit analysis of reciprocal coarse-scale relationships among hosts and pathogens. Landscape pathology studies fill a significant research gap in the context of our understanding of sustainable forest management, the introduction of exotic organisms and how climate change might affect the spread of disease.  相似文献   

7.
关帝山森林景观异质性及其动态的研究   总被引:22,自引:4,他引:22  
以4期航片为基础资料,在ARC/INFO支持下,应用地理信息系统(GIS)技术,通过景观要素优势度指数、景观多样性指数、景观斑块密度、景观边缘密度、景观镶嵌度指数和景观聚集度指数5类指标,对关帝山林区50年代末以来的景观异质性及其动态特征进行了全面分析,并介绍了各指标的数据来源和计算方法,分析了各指标的实用性.在GIS支持下,上述各指标可以从不同侧面描述和反映景观异质结构特征,用于景观异质性动态变化规律的研究.研究表明,自50年代末以来,关帝山森林景观受环境异质性、植被自然演替和人为活动的共同控制,总体异质性发生了很大变化,3个时期表现出不同的动态特征.对人为活动频繁的次生林区的森林景观异质性动态变化的基本规律和控制因素进行了讨论  相似文献   

8.
Interactions between dispersal, competition, and landscape heterogeneity   总被引:4,自引:0,他引:4  
AceNorth  OtsoOvaskainen 《Oikos》2007,116(7):1106-1119
It is widely acknowledged that space has an important role in population regulation, yet more specific knowledge into how the relevant factors interact attains little consensus. We address this issue via a stochastic, individual based model of population dynamics, in a continuous space continuous time framework. We represent habitat quality as a continuously varying surface over the two-dimensional landscape, and assume that the quality affects either fecundity (rate of propagule production) or probability of propagule establishment. We control the properties of the landscape by two parameters, which we call the patch size (the characteristic length scale in quality variation), and the level of heterogeneity (the characteristic quality difference between poor quality and high quality areas). In addition to such exogenous variability, we also account for endogenous factors causing spatial variation by assuming localised dispersal and competition. We find that heterogeneity has a general positive effect on population density, and hence it is beneficial to improve best quality habitat at the expense of worst quality habitat. With regards to patch size, we find an intermediate optimum, due to a conflict between minimising the loss of propagules to low quality regions and maximising the benefits of heterogeneity. We address the consequences of regional stochasticity by allowing the environmental conditions change in time. The cost of having to continuously track where the favourable conditions have moved to ultimately reduces population size.  相似文献   

9.
Optimal drug regimens for cancer chemotherapy are determined when knowledge is only available on the behaviour of the tumour and the drugs used, over a population of patients. The case of two drugs is investigated where they are equivalent on average. Our calculations indicate that the optimal regimen has both drugs given initially but then sequences the two drugs. Our calculations also indicate that as tumour heterogeneity increases, the benefit to be gained from the optimal regimen can decrease in comparison to reasonable regimens. This has the effect of complicating the calculation of optimal regimens in a clinical setting, and may explain why results in experimental oncology fail to carry over to clinical oncology.  相似文献   

10.
11.
Feller IC  Chamberlain A 《Oecologia》2007,153(3):607-616
Complex gradients in forest structure across the landscape of offshore mangrove islands in Belize are associated with nutrient deficiency and flooding. While nutrient availability can affect many ecological processes, here we investigate how N and P enrichment interact with forest structure in three distinct zones (fringe, transition, dwarf) to alter patterns of herbivory as a function of folivory, loss of yield, and tissue mining. The effects of nutrient addition and zone varied by functional feeding group or specific herbivore. Folivory ranged from 0 to 0.4% leaf area damaged per month, but rates did not vary by either nutrient enrichment or zone. Leaf lifetime damage ranged from 3 to 10% of the total leaf area and was caused primarily by the omnivorous tree crab Aratus pisonii. We detected two distinct spatial scales of response by A. pisonii that were unrelated to nutrient treatment, i.e., most feeding damage occurred in the fringe zone and crabs fed primarily on the oldest leaves in the canopy. Loss of yield caused by the bud moth Ecdytolopha sp. varied by zone but not by nutrient treatment. A periderm-mining Marmara sp. responded positively to nutrient enrichment and closely mirrored the growth response by Rhizophora mangle across the tree height gradient. In contrast, a leaf-mining Marmara sp. was controlled by parasitoids and predators that killed >89% of its larvae. Thus, nutrient availability altered patterns of herbivory of some but not all mangrove herbivores. These findings support the hypothesis that landscape heterogeneity of the biotic and abiotic environment has species-specific effects on community structure and trophic interactions. Predicting how herbivores respond to nutrient over-enrichment in mangrove ecosystems also requires an assessment of habitat heterogeneity coupled with feeding strategies and species-specific behavior measured on multiple scales of response.  相似文献   

12.
13.
Landscape heterogeneity, namely the variation of a landscape property across space and time, can influence the distribution of a species and its abundance. Quantifying landscape heterogeneity is important for the management of semi‐natural areas through predicting species response to landscape changes, such as habitat fragmentation. In this paper, we tested whether the change in spatial heterogeneity of the vegetation cover due to farming expansion affected the distribution of the African elephant in the Tarangire‐Manyara ecosystem, northern Tanzania. Spatial heterogeneity (based on the normalized difference vegetation index) was characterized at multiple spatial scales using the wavelet transform and the intensity‐dominant scale method. Elephant distribution was estimated from time‐series aerial surveys using a kernel density function. The intensity, which relates to the contrast in vegetation cover, quantified the maximum variation in NDVI across multiple spatial scales, whereas the dominant scale, which represents the scale at which this maximum variation occurs, identified the dominant inter‐patches distance, i.e. the size of dominant landscape features. We related the dominant scale of spatial heterogeneity to the probability of elephant occurrence in order to identify: 1) the scale that maximizes elephant occurrence, and 2) its change between 1988 and 2001. Neither the dominant scale and intensity of spatial heterogeneity, nor the probability of the elephant occurrence changed significantly between 1988 and 2001. The spatial scale maximizing elephant occurrence remained constant at 7000 to 8000 m during each wet season. Compared to the findings of a recent, similar study in Zimbabwe, our results suggest that the change in the dominant scale was relatively small in Tarangire‐Manyara ecosystem and well within the critical threshold for elephant persistence. The method is a useful tool for monitoring ecosystems and their properties.  相似文献   

14.
Resource distribution can vary greatly in space and time. Consequently, animals should adjust their searching tactics to such spatio–temporal patterns in accordance with their innate capabilities, or alternatively, they should use a genetically fixed searching tactic that has been evolved in response to the specific pattern of the food they experience. Using a simulation model and a genetic algorithm, we show how optimal searching tactics change as a function of food spatial pattern. Searching tactics for hidden prey can be approximated using the following three components: (1) Extensive search mode (ESM), the type of movement before encountering a food item; (2) Intensive search mode (ISM), the type of movement after encountering a food item; and (3) ISM duration. Both ESM and ISM are characterized by movement tortuosity. We show that searching behavior adaptively changes as a function of food pattern. When food is distributed in a regular pattern, ISM is more directional than ESM, but under a clumped food pattern, ISM is much more tortuous than ESM. It may suggest that animals with larger spectra of searching tactics should experience greater variance or seasonal changes in their food pattern than animals with narrow spectra of searching tactics. Increased forager attack radius diminishes the differences between ESM and ISM, and thus the use of these three components to model searching in animals with higher attack radii is not appropriate. Increased handling time, which is a surrogate of reducing habitat profitability results in longer patch residency time as expected by optimal foraging theory. To conclude, we suggest that using such a combined approach of simulation models and genetic algorithms may improve our understanding of how extrinsic and intrinsic factors interact to influence searching behavior.  相似文献   

15.
Vegetation gradients developing around water sources (i.e. piospheres) are important features of arid and semi-arid ecosystems. Studied extensively in pastoral areas, piospheres have rarely been investigated in areas hosting rich herbivore diversity. We studied piospheres in woody cover assessed through remote-sensing in Hwange National Park, Zimbabwe, which has one of the world's largest elephant populations. As no preferred statistical model for piosphere studies has emerged, we first contrasted results from ordinary least-square (OLS) regressions on raw data with those from other statistical models (OLS on aggregated data and simultaneous autoregressive models on raw data), and selected the most parsimonious, unbiased, model to study the influence of artificial and natural waterholes, and the abundance of elephants, others browsers, and grazers on piospheres. OLS models provided unbiased parameter estimates, despite the strong spatial autocorrelation present in woody cover data, whereas other statistical models had important drawbacks. Using an OLS framework we showed that despite an important negative non-linear mean effect, distance-to-water was a poor predictor of woody cover at any location. Woody cover was on average more reduced in the vicinity of water at artificial waterholes than at natural waterholes. Elephant abundance was not consistently associated with lower woody cover, and poorly explained woody cover heterogeneity, as did all other herbivore-related variables. Our study indicates that piospheres may develop differently in pastoral and protected areas, suggesting the importance of herbivore diversity in ecosystem functioning. Our results also show that heterogeneity in woody cover persists within piospheres, calling for further investigation on the origin and role of this heterogeneity in the maintenance of ecological processes and biodiversity within these key-areas of the landscape.  相似文献   

16.
John E. Banks 《Oecologia》1998,117(1-2):239-246
Using alternating bands of weeds and broccoli I experimentally manipulated vegetation composition and the spatial scale at which the landscape was fragmented in a factorial design. This experimental approach allowed me to distinguish the effect of spatial scale from that of simple crop heterogeneity on crop herbivores. The importance of scale depended on which insect species were examined. Cabbage aphids (Brevicoryne brassicae) were influenced by vegetation composition at all tested scales of fragmentation; cabbage butterflies (Pieris rapae) were not affected by scale or by composition and flea beetles (Phyllotreta cruciferae) revealed a striking dependence on scale of fragmentation as well as an interaction between scale and composition. This approach shows the importance of dissecting out the effects of scale from other aspects of landscape manipulation, and emphasizes the challenge of developing a theory that will enable prediction of species–specific responses to scale. Received: 15 February 1998 / Accepted: 10 June 1998  相似文献   

17.
Carabid beetle assemblages were studied to assess how diversity and community structure varied along a gradient of land-use. This gradient was composed of six 1 km2 quadrats with an increasing proportion of agricultural land reflecting the anthropogenic fragmentation and intensification of landscapes. Carabid species richness and abundance was predicted to peak in the most heterogeneous landscape, in accord with the intermediate disturbance hypothesis (IDH), and then decline as agricultural intensification increased. It was also predicted that the different landscapes would support beetle communities distinct from each other. The IDH was unsupported-in both years of this study carabid species richness and abundance was greatest in the most intensively managed, agricultural sites. Detrended correspondence analysis revealed a clear separation in beetle community structure between forested and open habitats and between different forest types. Canonical correspondence analysis revealed a significant correlation between beetle community structure and the environment, showing distinct beetle assemblages to be significantly associated with specific edaphic and botanical features of the land-use gradient. This study adds to increasing evidence that landscape-scale patterns in land-use significantly affect beetle community structure producing distinct assemblages.  相似文献   

18.
Soil cover, which is one of the most informative and integrative landscape factors, can be used for the analysis of landscape patterns. We studied the spatial autocorrelation (Moran's I) of raster format soil maps (1:10,000; 10 m pixel size) in 35 study areas representing all landscape regions in Estonia. The carbonate concentration of soils, volumetric soil moisture (%) and the depth of the groundwater table were taken into consideration in compiling a scale of contrast of 17 soil groups. We introduce a simple characteristic based on spatial correlograms: a half-value distance lag, hI = 0.5—a distance where Moran's I drops below 0.5. Spatial autocorrelation decreased very rapidly in the case of heights with a very heterogeneous landscape composition, showing low values of hI = 0.5 (<100 m in all 6 study areas). In uplands and depressions, the spatial autocorrelation also decreased relatively rapidly (hI = 0.5 < 200 m). In most of the plains, coastal lowlands, sea islands and inland paludified lowlands, the values of Moran's I did decrease slowly with increasing lag, being >200 m in all forest and bog areas with complex topographical conditions due to the variety of glacial landforms and peatlands. All of the eight FRAGSTATS landscape metrics studied demonstrated significant correlations with hI = 0.5, whereas five of them – Contrast Weighted Edge Density (CWED); Percentage of Like Adjacencies (PLADJ), Edge Density (ED), Patch Density (PD) and Mean Patch Area Distribution (AREA_MN) – had Spearman Rank Order Correlation values higher than 0.8. Landscapes with high ED, PD, and CWED values have a low autocorrelation: PD, ED, and CWED correlated negatively with hI = 0.5. PD, ED, and CWED decreased and PLADJ increased with the power-law relationship with increasing hI = 0.5. Spatial autocorrelation is lower in landscapes with complex structure and high contrast. The positive relationship with PLADJ indicates the same. Thus, spatial correlograms of potential landscape structure based on soil cover analysis can be used for the characterization of human-influenced landscape (land use) structure.  相似文献   

19.
Lee Sang-Hee 《Journal of Asia》2018,21(4):1396-1401
Although the environment in which termites live is very heterogeneous, most experimental studies on the termite tunnel patterns have been conducted on homogeneous sand substrates. In order to explore how the heterogeneity affects tunnel patterns, I developed an agent-based model to simulate termite tunneling behavior at the individual level. In this model, grid space consists of easy and difficult areas for tunneling. Heterogeneity, H, was defined as the degree of the mixture of the two areas. The tunnel patterns formed by changing the number of termites, N, and H were quantitatively characterized by territory circularity and the territory area. These patterns were categorized into two groups, one with a small territory area and high circularity (group 1) and the other with a large area and low circularity (group 2). Considering the previous study that the termite populations with high N values have high territorial scalability, it can be said that the territories belonging to group 2 have higher foraging abilities and viability than those belonging to group 1. The simulation results showed that the tunnel patterns generated for small N and high H belonged to group 2. This implies that the heterogeneity can make a positive contribution to the expansion of the foraging area by effectively focusing the foraging energy of a termite population. I briefly discussed the mechanism of this positive role and the limitations of this simulation study. In addition, I discussed issues that need to be resolved in the near future to overcome the limitations.  相似文献   

20.
The widespread conversion of natural habitats to agricultural land has created a need to integrate intensively managed landscapes into conservation management priorities. However, there are no clearly defined methods for assessing the conservation value of managed landscapes at the local scale. We used remotely sensed landscape heterogeneity as a rapid practical tool for the assessment of local biodiversity value within a predominantly agricultural landscape in Canterbury, New Zealand. Bird diversity was highly significantly correlated with landscape heterogeneity, distance from rivers and the Christchurch central business district, altitude and average annual household income, indicating that remotely sensed landscape heterogeneity is a good predictor of local biodiversity patterns. We discuss the advantages and limitations of using geographic information systems to determine local areas of high conservation value.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号