Research Associate, Centre for Ecolohgical Sciences,
Indian Institute of Science
Email: [email protected]
The methodology developed to assess biodiversity over the Western Ghats using remote sensing, involves a synthesis of information derived at several spatial scales.
In India we have 320 million hectares of land, and 200 million hectares of exclusive economic zone in the sea, within which are distributed some 120, 000 known and perhaps another 400, 000 as yet undescribed species of microbes, plants and animals In a country with this rich heritage of biological diversity, it is obviously not possible to census the distribution of each and every species based on field studies alone.
A two-level combination of remote sensing and field studies, can be used to derive information on the distribution of large numbers of species.This methodology appears simple – but there are several possible difficulties associated with its use over large areas. Although several species are known to exist in tight association with their habitats, some species are more wide ranging, and may occur in a wider variety of habitats. Certain groups of organisms, such as birds, may require a mixture of several ecotopes, some for foraging and others for breeding, for example. It may not be possible to relate their distribution to the presence of a single ecotope type. The spatial scale at which habitats are differentiated by the remote sensor may also crucially affect the results obtained. For example, herb species may respond to ecotopes at a very fine scale, much finer than the spatial resolution of the remote sensor.
Still, such a two scale coupling of remote sensing andfield sampling still shows great promise and, indeed, has been recommended by the Global Biodiversity Assessment.
Biogeographically, the hill chain of the Western Ghats constitutes the Malabar province of the Oriental realm, running parallel to the west coast of India from 8 ° N to 21 ° N latitudes for around 1600 km. Rising up from a relatively narrow strip of coast at its western border, the hills reach up to a height of 2800 m before they merge to the east with the Deccan plateau at an altitude of 500-600 m. The average width of this mountain range is about 100 km. This bio-region is highly species rich and under constant threat due to human pressure, and is considered one of the 18 biodiversity hot spots of the world. With its complex, heterogeneous landscapes and high levels of biodiversity, it forms an ideal ground for the testing and elaboration of this methodology.
Methods and Results
The methodology developed to assess biodiversity over the Western Ghats using remote sensing, involved a synthesis of information derived at several spatial scales. At the largest scale of mapping, satellite imagery covering the area of the Western Ghats, was collected from IRS 1 B LISS 2 sensors, during the pre-monsoon seasons of either 1992, 1993 or 1994 (depending on availability of cloud free data). The scenes were manually co-registered and pasted together to create a composite image of the study area. Using information derived from the Normalized Difference Vegetation Index (to minimize effects of inter-image variability), according to the methodology described by Nagendra and Gadgil, a relatively coarse, broad scale map of the Western Ghats was created. This map classifies the Western Ghats into 200 patches belonging to eleven ecomosaic types. Each ecomosaic type is a characteristic set of several ecotope types, both manmade such as different types of agricultural crops and plantations, and natural, such as forests in various stages of degradation. The resultant map was compared with pre-existing information on the distribution of forests, agricultural lands, climatic and topographic features, and population, for interpretation.
At the next scale of mapping, twelve landscapes belonging to five of the eleven ecomosaic types were taken up for more detailed investigations. These landscapes range in size from 9 to 54 sq. km in area. Supervised and unsupervised classifications of LISS 2, 4 band data was carried out at this scale, using single date pre-monsoon imagery collected either in 1992, 1993 or 1994. Supervised classification accuracies of these twelve landscapes ranged from 70% to 92%. Unsupervised classification accuracies were uniformly much worse.
From the supervised classification maps, information on the size, shape and inter-patch distance of ecotopes was calculated. This information is believed to affect the presence and distribution of various species within a landscape. In addition, ecotope type richness, and Shannon’s index of ecotope diversity (based on proportion of landscape area occupied by various ecotope types) were computed. Statistical analysis determined that landscapes belonging to specific ecomosaic types tended to be similar in their ecotope characteristics, thus providing us with confidence in the ecomosaic map of the Western Ghats.
Finally, at the most detailed scale of analysis, IRS 1B LISS 2 data of March 1993 was purchased for a single landscape of 27.5 sq. km from Siddapur taluk of Karnataka (400 – 600 m altitude, 14°16’- 14°19’N latitude and 74°52’- 74°54’E longitude). Supervised and unsupervised classification of this imagery mapped the landscape into seven ecotope types. 246 quadrats of 10 by 10 m were distributed in the landscape, and used to record the tree layer species distributed in these seven ecotope types. Within these, sub-quadrats of 5 by 5 m and 1 by 1 m were used to record the angiosperm species (excluding grasses, which could not be accurately identified in the field) present in the shrub and herb layers respectively.
Analysis of species composition of quadrats belonging to various ecotope types determined that ecotope types as identified by supervised classification differ significantly in their species composition. However due to increased misclassification, ecotope types identified by unsupervised classification did not differ significantly in their species composition.
This case study of the Western Ghats – West Coast moist forest region of India thus demonstrates the applicability of a multi-scale methodology employing satellite-based classification as well as field surveys, using a nested hierarchy of classification of ecological entities, for assessing species diversity. This study establishes the linkages between these seemingly disparate scales of analysis. Classification at the scale of the Western Ghats delineates ecomosaic types, which differ significantly from each other in their composition and configuration of their constituent ecotope types, as determined by classification at a finer scale. In turn, these ecotope types differ significantly in their species composition, as determined from field sampling.
Based on this, a strategy for biodiversity assessment and monitoring at regular intervals of time can be outlined for the Western Ghats and western coast of India. Ecomosaic level classification produces a map of 205 patches belonging to eleven ecomosaic types. For each ecomosaic type, the identification of 5-10 landscapes is proposed, located in different patches. This set of 50-100 landscapes, of the order of 25-50 km2, can be mapped to the ecotope level, using four band IRS LISS 2 or LISS 3 imagery. At a still lower scale, within each landscape, quadrats of 10 by 10 m may be used to sample plant distributions on ground. A schematic outline of the methodology proposed, encompassing a range of different scales of observation, and a combination of remotely sensed data and field information, is depicted in Figure 1. Fig 1: Proposed scheme of biodiversity investigations at diferent scales. Modified with permission from Current Science 75(3): 264-271
In the present classification of twelve landscapes, a total of twenty-three ecotope types were encountered. For the entire Western Ghats however, one may expect to find about 30-40 ecotope types, since in this study a few regions of the Ghats such as the high altitude shola-grasslands, were not investigated. For these 30-40 ecotope types, field sampling of various organisms, including but not restricted to plants, butterflies and birds, can be used to establish the set of species that are found associated with specific ecotope types. Within these landscapes, one must also investigate the extent to which latitude, longitude, topography and climatic factors influence the variation in species composition that is unexplained by ecotope distribution – for which purpose GIS will prove very useful.
Regular monitoring of this set of landscapes, at 2-3 year intervals, along with repeated field inventories, will enable us to monitor changes in biodiversity. Mapping logical Sciences over the past five years with the Western Ghats Biodiversity Network, a network of college teachers and students who have aided us in this investigation, we believe such a scheme to be practically feasible, in a decentralized fashion.
Previous large scale studies which use remote sensing for biodiversity assessment have been mainly carried out in the relatively species poor and homogeneous temperate ecosystems. This exercise is a first of its kind to be carried out at this scale in the tropics, explicitly investigating the linkages between information collected at such widely different spatial scales, combining remote sensing with field based species inventories. The scheme suggested here may be used as a basis to organize other programmes for assessment of biodiversity, elsewhere in the tropics too.