Research Article |
Corresponding author: Wei Hou ( houwei@casm.ac.cn ) Academic editor: Joseph Tzanopoulos
© 2016 Wei Hou, Ulrich Walz.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Hou W, Walz U (2016) An integrated approach for landscape contrast analysis with particular consideration of small habitats and ecotones. Nature Conservation 14: 25-39. https://doi.org/10.3897/natureconservation.14.7010
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Habitat diversity is considered as an expression of biodiversity at landscape level in addition to genetic and species diversity. Thus, effective methods for measuring habitat pattern at landscape level are crucial for understanding the ecological processes. In this paper we propose to extend the commonly used model Patch Corridor Matrix Model (PCMM) for spatial pattern analysis. Originally, this model assumes discrete structures within the landscape without explicit consideration of “gradients” between patches. The gradients, often called “ecotones”, can be considered as “soft edges” which have a profound influence on adjacent ecosystems. Another part of information that has often been ignored are “small habitats” inside patches (e.g. hedgerows, tree rows, copse, and scattered trees), which leads to within-patch heterogeneity being underestimated. In this paper, the concept of landscape contrast is used to integrate the discrete and gradient landscape representations by incorporating small habitats and ecotones in methods to measure landscape heterogeneity. A height gradient is used to define the ecotones between forest and field. Then, patch contrast (i.e. Edge Contrast Index (ECON)) is calculated based on the height difference between adjacent vegetation patches. Artificial elements (e.g. traffic roads) are considered as barriers which are assigned with the highest edge contrast value. At the landscape level, a metric called Area-Weighted Edge Contrast (AWEC) is introduced to describe the landscape structure. The edge effects of ecotones, small habitats, and traffic roads are incorporated in the calculation of AWEC. Our test examples show that incorporation of ecotones and small habitats can smooth “edge effects” among patches and result in a more realistic quantification of habitat contrast. The contrast concept is especially useful in a vegetated landscape with less human impact. It could be understood as an additional interpretation to fragmentation of habitats with permeable edges among them. Consequently, this presented approach may enhance the understanding of the relationship between landscape pattern and process.
Patch corridor matrix model, gradient model, ecotones, small habitats, habitat fragmentation
Landscape metrics based on the mosaic model are often used in landscape analysis. In practice the application of this model may be over simplified by losing valuable information on the landscape structure, such as the terrain characteristics of landscape (
With the development of remote sensing technology, it is possible to direct map the small habitats or discriminate different types of habitats occurring in spatially contiguous units (
Landscape heterogeneity has been integrated into metapopulation theory by incorporating habitat fragmentation and landscape contrast (
In this paper, the ecotone is defined at a detailed spatial level as a “soft” boundary between forest and field. It has a three dimensional structure appearing as gradual blending of the two plant communities on the boundary area, where the third spatial dimension (vegetation height) is used to constrain the transition zone on forest-field boundary. It refers to mixed vegetation above the field layer but below the overstory formed by a combination of side branches of canopy trees, small trees, lianas, and shrubs. The small habitats (including single trees, tree rows, hedges, and copses) are defined by an area less than 0.5-1 hectare, a minimum width of 5 m and the occurrence in the field, isolated from forest (
Two test sites with varied landscape structure are selected from the German national park “Saxon Switzerland”, which is located in south-eastern Germany (Figure
Patch contrast is used to describe the relative difference between patches or patch classes; for example “edges” have a kind of “contrast effect”. A strong contrast value means that adjacent patches differ strongly and the transitions between them are narrow or even absent (
Examples of contrast magnitude along patch edges in “Saxon Switzerland”, Germany (Photos: Hou).
In this context, a height-based variant of the Edge Contrast Index (ECON) (
Formula 1:
pk : edge length of segment k;
dk : contrast weight of segment k;
P: total patch perimeter;
m: number of patch segments.
Range: 0 < ECON ≤ 1
At a higher organizational level, it could be misleading to simply calculate the mean edge contrast for a particular patch type (class level) or for all patches (landscape level). An irregular-shaped small patch may play a disproportionately role in the overall landscape contrast. Therefore, a metric which refers to the patch proportions has been developed at the landscape level: Area-Weighted Edge Contrast (AWEC) (Formula 2). It is not only an accumulation of the edges’ contrast value; meanwhile the area proportion of each patch is also incorporated. Based on the modified ECON, AWEC can be understood as average dissimilarity in vertical structure of habitats. The lowest value of AWEC is 0 when the whole landscape is considered as one patch (landscape boundary is assigned with dissimilarity of 0), and the highest value is 1 as all patches have hard edges (maximum dissimilarity).
Formula 2:
n: number of patches in the landscape;
ai: area of patch i;
ECONi: the edge contrast value of patch k, see Formula 1.
A: area of the total landscape.
Range: 0 ≤ AWEC ≤ 1
The contrast indices are calculated firstly in a vegetated area (test site 1) from a section of Saxon Switzerland (using the nDSM with horizontal resolution of 1 m and the land cover data). The results of landscape contrast analysis are shown in Figure
The patch contrast (ECON) is defined based on the vegetation height difference. But in reality there are often artificial elements existing in a vegetated landscape, e.g. traffic roads. We assume that the edges of artificial elements have the highest (100%) contrast weight to neighboring patches. A comparative test is exemplified in test site 2 (see Figure
Patchiness and gradients are the concentrated expressions of spatial heterogeneity in the landscape (
In reality there is nearly no place without human impacts (
At the patch level, the modified edge contrast index (ECON) measures the degree of height contrast between a patch and its immediate neighborhood. ECON is a relative measure at patch level and stands for the degree of contrast in patch edge regardless of how big the patch is. At the landscape level, the edge effect have been often measured by Total Edge Contrast Index (TECI) or Contrast-Weighted Edge Density (CWED) (
The examples shown in test site 1 demonstrate that the existence of small habitats and ecotones can reduce the landscape contrast as they possess the characteristic of lower edge contrast than the patch interior. In addition, the ecotones function as buffer areas or “soft boundaries” which reduce the height contrast between forest and field. The small habitats which are normally neglected in landscape structure analysis also account for the average height of matrix. For this reason, it is necessary to incorporate these small habitats and differentiate the patch interior and its exterior, such as ecotones. Attempts that incorporation of ecotones in fragmentation metrics (e.g. effective mesh size (MESH) (
Artificial elements (e.g. traffic roads) affect not only regional or metapopulation dynamics but also have a direct effect on local population dynamics (
In this paper we present an integrated approach to analyze the landscape contrast as a means to describe landscape heterogeneity. Incorporation of gradient concept in landscape structure analysis helps to overcome the limitation of PCMM that valuable information on patch boundary is missing. Not like the gradient model, the integrated approach is still based on a classified map which contains an additional category of gradient elements, such as ecotones on forest/field boundary. Therefore, the robust metrics derived from PCMM can be adapted to quantify the landscape structure including gradients. Similar to PCMM, this approach has also limitations as the simplification of land surface may be affected by the classification schemes of land cover.
The modified contrast metrics in this study show different sensitivities to different landscape compositions. Comparison of applying contrast metrics in a vegetated landscape (Figure
Ecotones and small habitats are often ignored in landscape structure analysis. This may due to the fact that there is lack of suitable conceptual model and metrics to integrate them. Our experimental results have shown that the discussed approach (contrast metrics based on an integrated model) is efficient for implementation under different landscape composition. We suggest that greater attention should be paid to these detailed landscape elements at the local level.
This research was carried out at the Leibniz Institute of Ecological Urban and Regional Development (IOER) and partly supported by the Special Fund (No. 201211050) for Scientific Research in the Public Interest of Ministry of Land and Resources, P.R.China. We thank two reviewers for providing comments on the manuscript.