Elsevier

Ecological Indicators

Volume 8, Issue 5, September 2008, Pages 657-663
Ecological Indicators

Assessing forest fragmentation in northeastern region (NER) of India using landscape matrices

https://doi.org/10.1016/j.ecolind.2007.10.002Get rights and content

Abstract

Northeastern region (NER) of India, one of the largest reserves of forests in India has so far been studied with a view to map the distribution of species or modeling the disturbance regimes and richness analysis. The present study focuses on the importance of regional level studies where the entire NER which is under the threat of forest fragmentation and degradation, is been assessed. In the present study, six historical data sets generated from remote sensing data (1972, 1982, 1987, 1989, 1993 and 1999) are used to assess forest cover loss, shape index and entropy to the degree of forest fragmentation over a multi-decadal period. The assessments have been carried out in the open (40–10% canopy density) and close (>40% canopy density) forest cover classes. The range of shape index and deviation from the actual mean in open forest and closed forest were computed separately. The patches among two categories were further analyzed based on patch area into six classes; ranging from <1 km2 to >500 km2. This also indicates variability of the forest patches. It is noteworthy that patches of area within 1–10 km2 and 10–50 km2 have been severely fragmented. This loss could be attributed to the shifting cultivation practice where the patches of moderate size are cultivated by group of families. The present study could give an insight to the patch configuration and composition in terms of shape index and the Shannon's entropy index.

Introduction

Tropical forests have a special role in biodiversity conservation. They occupy approximately 11% of the land surface and are home to 70% of the world's biodiversity (Dixton et al., 1994). In recent years, much attention has focused on tropical forests where, since 1970s around 50% of the forests have been lost to deforestation, primarily as a result of agricultural expansion (Myers and Goreau, 1991). Deforestation, mainly conversion of forests to agricultural land, continues at an alarmingly high rate—about 13 million ha/year. Global estimates on rate of tropical deforestation are 7.3 million ha/year between 2000 and 2005 (FAO, 2005). Loss of forests has negative impact on the environmental, economic and social setup of tropical countries and hence is reason for concern. Although the latest report suggests that the deforestation rates have been reduced, but in Southeast Asian countries the deforestation rates are still more than 1% per year (FAO, 2005).

Humans are dependent on the forests for all the basic needs and other services, either directly or indirectly. This creates high premium on the conservation of the remaining biodiversity. Although, at moderate level of disturbances, there can be increase in the biodiversity, habitat loss and fragmentation of natural habitat because of deforestation are the single largest threat to biodiversity (Sala et al., 2000). As humans increasingly extract the forest resources, expand and intensify the agricultural areas, there has been an increasing threat to forests. Forest insularity (degradation and fragmentation) accompanied by extraction of resources can dramatically change the overall ecological processes. Fragmentation can be defined as number of forest and non-forest patches in a unit area. Fragmentation leads to habitat loss, isolation and edge effects (Skole and Tucker, 1993, Forman, 1997) and subsequent degradation of ecosystem. In simpler terms, either landscape gets reduced in the size because of pressure from the peripheries or the habitat within a landscape get replaced by another type of land cover, thereby leading to permanent loss of habitat. A disruption in landscape patterns may therefore compromise its functional integrity by interfering with critical ecological processes such as land–atmosphere interactions. Assessing the fragmentation and continuous monitoring of the forests reserves are therefore key issues. Much emphasis has been placed on developing methods to quantify landscape patterns, which is considered a prerequisite to the study of pattern–process relationships.

Landscape matrices embraces spatial heterogeneity and pattern: how to visualize pattern, how it develops and changes through time, and its implications on population, community and ecosystem processes. Landscape ecology quite naturally is concerned with anthropogenic aspects of landscape pattern and change. It has emphasized human-perceived biotypes as patches or matrices in heterogeneous landscape. Scientists often use indices of landscape structure to quantify habitat fragmentation. An index is a single number that represents some aspect of landscape structure. For example, the ratio of total area of forest to the total area of the landscape may represent the degree to which forest may have been fragmented in the landscape. Indices have been developed to measure manifold aspects of landscape structure such as composition (amount of different cover types), configuration (arrangement of the patches) as well as shapes of patches in the landscapes. The landscape matrices are widely used to quantify patch size distribution (Cullinan and Thomas, 1992), patch shape complexity (Laurance and Yensen, 1991), core area (Laurance and Yensen, 1991), dispersion (Johnson, 1988), contagion (O’Neill et al., 1988, Li and Reynolds, 1993), interspersion, connectivity (McGarigal and Holmes, 2002), fractal dimension (Leduc et al., 1994, Ritters et al., 1995) and indices vary as a function of landscape composition (Hargis et al., 1998). With reference to the study area, studies have focused on parameters that include fragmentation, interspersion, juxtaposition (Behera et al., 2005) and have demonstrated applicability of these parameters.

Saura and Martinez-Millan (2001) examined the effect of map extent on contiguity, dominance, fractal dimension, patch density and area weighted mean shape index. Contagion index has been used to characterize landscape patterns (McGarigal and Marks, 1994, Wickham et al., 1996), and to examine the relationship between the spatial patterns and ecological processes (Turner, 1990). Similarly, the shape index (Clark, 1981, Forman and Godron, 1986) was used to assess the landscape pattern. The shape index (SI) assumes that square is a most aggregated shape in a raster data format and a circle in a vector data format (where SI = 1). The deviations from unity suggest the deviation from a perfect square shape; the higher the index, the more complex the shape.

An adequate fragmentation measure is consequently needed for policy development with respect to nature and biodiversity conservation. Developing this analogy, the mathematical representation of degradation and the concept of entropy are very close (the entropy was often used as a measure of degradation). In this way, information theory can offer an appropriate framework to approach the landscape as a spatial information system (Phipps, 1981, Margalef, 1996). Moreover, it is demonstrated that heterogeneity (pattern) and entropy can be considered as equivalent terms (Forman and Godron, 1986, Boaert et al., 2005). The present paper also attempts to understand the forest fragmentation in terms of patch characteristics. Using entropy for assessing forest fragmentation provides a compliment to this attempt. It is been one of the common tools for assessing the urban sprawl dynamics (Yeh and Li, 2001, Sudhira et al., 2004). While working on larger areas the discrepancies because of absolute values for entropy are very small and can be neglected.

Section snippets

Study site

The study area, northeastern region (NER) of India comprising of seven states, extends from 88°E to 97°E and 22°N to 29°30′N. The NER is one of the biodiversity hotspots of the world, is known for its diverse and most extensive lush forest cover. The region has one of the largest reserves of subtropical forests in India. They encompass a broad range of ecological habitats from grasslands to closed humid forests, and disturbed secondary formations to almost virgin natural forests (Rao, 1994).

Materials and methods

For the first time a nationwide forest cover mapping for the entire India using remotely sensed data was carried out by National Remote Sensing Agency (NRSA) in 1972 followed by second in 1982 and generated the first ever map for the whole country for two time periods. These forest cover estimate for the years 1972 and 1982 were obtained in the form of hard-copy maps which were converted into digital format and were used for the analysis. Forest Survey of India (FSI) is the premiere agency of

Results

As evident from the results obtained, the total forest area in the NER has drastically decreased. The loss of forest cover has been attributed to the reasons discussed above. For assessing fragmentation, the patches of close and open forests were analyzed independently to assess the forest fragmentation. Table 2, Table 3 summarizes the change in number of patches, the total area and the range of shape index along with the standard deviation among the various patch size classes throughout the

Discussion

Northeast India is one of the largest reserves of forests in India that has so far been studied in the view of mapping the distribution of species or measuring the disturbance regimes and richness analysis (Roy and Tomar, 2000, Roy and Joshi, 2002, Anonymous, 2002a). The present study could give an insight to the patch configuration and composition in terms of shape index and the Shannon's entropy index. Instead of assessing the parameters for a small area, the study focuses on the importance

Acknowledgement

The study has been funded by ISRO-GBP. We appreciate the fruitful discussion and support from Mr. H. Sudhira.

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