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Biodiversity in the Western Ghats
Harini Nagendra
Research Associates
Centre for Ecological Sciences,Indian Institute of Science.
harini@ces.iisc.ernet.in
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 and field sampling still shows great promise
and, indeed, has been recommended by the Global Biodiversity Assessment.
Study
area
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.
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