排序方式： 共有47条查询结果，搜索用时 421 毫秒
41.
How should we measure landscape connectivity? 总被引：9，自引：0，他引：9
The methods for measuring landscape connectivity have never been compared or tested for their responses to habitat fragmentation. We simulated movement, mortality and boundary reactions across a wide range of landscape structures to analyze the response of landscape connectivity measures to habitat fragmentation. Landscape connectivity was measured as either dispersal success or search time, based on immigration into all habitat patches in the landscape. Both measures indicated higher connectivity in more fragmented landscapes, a potential for problematic conclusions for conservation plans. We introduce cell immigration as a new measure for landscape connectivity. Cell immigration is the rate of immigration into equal-sized habitat cells in the landscape. It includes both within- and between-patch movement, and shows a negative response to habitat fragmentation. This complies with intuition and existing theoretical work. This method for measuring connectivity is highly robust to reductions in sample size (i.e., number of habitat cells included in the estimate), and we hypothesize that it therefore should be amenable to use in empirical studies. The connectivity measures were weakly correlated to each other and are therefore generally not comparable. We also tested immigration into a single patch as an index of connectivity by comparing it to cell immigration over the landscape. This is essentially a comparison between patch-scale and landscape-scale measurement, and revealed some potential for patch immigration to predict connectivity at the landscape scale. However, this relationship depends on the size of the single patch, the dispersal characteristics of the species, and the amount of habitat in the landscape. We conclude that the response of connectivity measures to habitat fragmentation should be understood before deriving conclusions for conservation management. 相似文献
42.
Landscape Ecology - Agricultural intensification has led to widespread declines in biodiversity. An important component of agricultural intensification in many regions, including eastern Ontario,... 相似文献
43.
Barrak M. Pressler DVM Lenore A. Mohammadian DVM Erning Li PhD Shelly L. Vaden DVM PhD Jay F. Levine DVM MPH Kyle G. Mathews DVM MS Ian D. Robertson BVSc 《Veterinary radiology & ultrasound》2004,45(3):189-197
Determination of urolith mineral composition is critical for management of urolithiasis in dogs and cats. Using computed tomography, urolith physical density, and hence chemical composition, can be quantified using mean beam attenuation measurements (Hounsfield units; HU). This study was designed to establish in vitro reference ranges for three types of compositionally pure uroliths retrieved from dogs. Sixty-six canine uroliths (22 uric acid, 21 calcium oxalate, 14 struvite, nine mixed or compound) were placed in a phantom array. Uroliths were scanned at 120 kVp, 200 mA, and 80 kVp, 200 mA. The region of interest (ROI) for mean HU calculation was determined using two techniques, and reference ranges were calculated for each kVp using either ROI technique. HU for urolith types of pure composition were statistically different (Wilcoxon's two-sample test, P < 0.0083 [Bonferonni correction with six comparisons for total P < 0.05]) using both ROI techniques at either kVp. Struvite uroliths were not statistically different from mixed or compound uroliths. The accuracy for determination of composition of pure uroliths ranged from 86% to 93%; the prediction accuracy for each urolith mineral type and for all uroliths in general was highest when the ROI was hand-drawn just within the visible urolith border at 80 kVp. Technique of ROI determination and kVp that yielded the highest sensitivity, specificity, and positive and negative predictive values varied for each urolith type. Therefore, in this study, HU could be used to differentiate three types of uroliths of pure mineral composition in vitro. Further studies are needed to determine the predictive value of HU in vivo. 相似文献
44.
We examined the effects of matrix structure and movement responses of organisms on the relationships between 7 patch isolation metrics and patch immigration. Our analysis was based on simulating movement behaviour of two generic disperser types (specialist and generalist) across mosaic landscapes containing three landcover types: habitat, hospitable matrix and inhospitable matrix. Movement, mortality and boundary crossing probabilities of simulated individuals were linked to the landcover and boundary types in the landscapes. The results indicated that area-based isolation metrics generally predict patch immigration more reliably than distance-based isolation metrics. Relationships between patch isolation metrics and patch immigration varied between the two generic disperser types and were affected by matrix composition or matrix fragmentation. Patch immigration was always affected by matrix composition but not by matrix fragmentation. Our results do not encourage the generic use of patch isolation metrics as a substitute for patch immigration, in particular in metapopulation models where generic use may result in wrong projections of the survival probability of metapopulations. However, our examination of the factors affecting the predictive potential of patch isolation metrics should facilitate interpretation and comparison of existing patch isolation studies. Future patch isolation studies should include information about landscape structure and the dispersal distance and dispersal behaviour of the organism of interest.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
45.
The purpose of our study was to compare the number, proportion, and species composition of introduced plant species in forest
patches situated within predominantly forested, agricultural, and urban landscapes. A previous study suggested that agricultural
landscape context does not have a large effect on the proportion of introduced species in forest patches. Therefore, our main
goal was to test the hypothesis that forest patches in an urban landscape context contain larger numbers and proportions of
non-native plant species. We surveyed the vegetation in 44 small remnant forest fragments (3–7.5 ha) in the Ottawa region;
15 were situated within forested landscapes, 18 within agricultural landscapes, and 11 within urban landscapes. Forest fragments
in urban landscapes had about 40% more introduced plant species and a 50% greater proportion of introduced plant species than
fragments found in the other two types of landscape. There was no significant difference in the number or proportion of introduced
species in forest fragments within forested vs. agricultural landscapes. However, the species composition of introduced species
differed among the forest patches in the three landscape types. Our results support the hypothesis that urban and suburban
areas are important foci for spread of introduced plant species. 相似文献
46.
Kaishan Song Lin Li Shuai Li Lenore Tedesco Bob Hall Linhai Li 《Water, air, and soil pollution》2012,223(4):1481-1502
The connection between nutrient input and algal blooms for inland water productivity is well known but not the spatial pattern
of water nutrient loading and algae concentration. Remote sensing provides an effective tool to monitor nutrient abundances
via the association with algae concentration. Twenty-one field campaigns have been conducted with samples collected under
a diverse range of algal bloom conditions for three central Indiana drinking water bodies, e.g., Eagle Creek Reservoir (ECR),
Geist Reservoir (GR), and Morse Reservoir (MR) in 2005, 2006, and 2008, which are strongly influenced anthropogenic activities.
Total phosphorus (TP) was estimated through hyperspectral remote sensing due to its close association with chlorophyll a (Chl-a),
total suspended matter, Secchi disk transparency (SDT), and turbidity. Correlation analysis was performed to determine sensitive
spectral variables for TP, Chl-a, and SDT. A hybrid model combining genetic algorithms and partial least square (GA-PLS) was
established for remote estimation of TP, Chl-a, and SDT with selected sensitive spectral variables. The result indicates that
TP has close association with diagnostic spectral variables with R
2 ranging from 0.55 to 0.72. However, GA-PLS has better performance with an average R
2 of 0.87 for aggregated dataset. GA-PLS was applied to the airborne imaging data (AISA) to map spatial distribution of TP,
Chl-a, and SDT for MR and GR. The eutrophic status was evaluated with Carlson trophic state index using TP, Chl-a, and SDT
maps derived from AISA images. Mapping results indicated that most MR belongs to mesotrophic (48.6%) and eutrophic (32.7%),
while the situation was more severe for GR with 57.8% belongs to eutrophic class, and more than 40% to hypereutrophic class
due to the high turbidity resulting from dredging practices. 相似文献
47.
Efforts to mitigate road effects are now common in new highway construction projects. For effective mitigation of road effects it is important to identify the species whose populations are reduced by roads, so that mitigation efforts can be tailored to those species. We conducted a meta-analysis using data from 75 studies that quantified the relationship between roads and/or traffic and population abundance of at least one species to determine species life history characteristics and behavioral responses to roads and/or traffic that make species or species groups prone to negative road and/or traffic effects. We found that larger mammal species with lower reproductive rates, and greater mobilities, were more susceptible to negative road effects. In addition, more mobile birds were more susceptible to negative road and/or traffic effects than less mobile birds. Amphibians and reptiles were generally vulnerable to negative road effects, and anurans (frogs and toads) with lower reproductive rates, smaller body sizes, and younger ages at sexual maturity were more negatively affected by roads and/or traffic. Species that either do not avoid roads or are disturbed by traffic were more vulnerable to negative population-level effects of roads than species that avoid roads and are not disturbed by traffic. In general, our results imply that priority for mitigation should be directed towards wide-ranging large mammals with low reproductive rates, birds with larger territories, all amphibians and reptiles, and species that do not avoid roads or are disturbed by traffic. 相似文献