The
usual taken for granted idea of loss
of biological diversity with the construction of a dam has been investigated at
the Sharavathi river basin in this study. The Linganamakki dam was constructed
over the river Sharavathi for hydroelectric purposes in the early 1960s. The
undulating topography in the region has resulted along with submergence of vast
land area, creation of islands of varying areas within the reservoir (30 of
which harbour vegetation comprising mainly of evergreen, semievergreen and moist
deciduous forests). To determine the state of these islands, ants, one of the
well established biological indicators were sampled in ten randomly picked
islands. A one time sampling was carried out using standard collection methods
of pitfall traps, leaf litter collections, bait traps and visual collections.
Comparison of the species composition between the islands and the river basin
(earlier work) revealed that islands harboured a composition of ants that had
highly specific niche requirements as Harpegnathos
saltator (endemic species) and Leptogenys
processionalis. The overlapping canopy cover in the islands prevented the
occurrence of Pachycondyla rufipes, a
canopy gap specialist. The highly undisturbed islands had very low levels of
human interference quite unlike the river basin, thereby not providing shelter
to the invasive species Anoplolepis
longipes. Areas that do get submerged when the reservoir is full, but dry
(presently), due to lack of rains show the presence of a hot climate species Camponotus
sericeus that were abundant in scrub jungles and savanna grasslands of the
river basin. Broad leaves and shady areas being the characteristic of most of
the forests of the islands, supported O.smaragdina
that thrives in such niches only. Polyrhachis
mayri, yet another arboreal species, known to prefer undisturbed evergreen
and moist deciduous forests in the river basin, were extremely abundant in the
islands. Both the arboreal species of ants never known to nest on neighboring
trees, showed several such occurrences in some islands, hinting at a high degree
of competition in the islands. Moist deciduous forests in islands cut to pave
way for Acacia plantations (MPM) harboured a different composition dominated by
a specialist predator Diacamma rugosm,
known to be one of the most dominant ants of an acacia plantations in the river
basin. The arboreal species of ants were totally absent from Acacia plantations
except in the few remnants of moist deciduous trees (in the same island) that
provided niches for Oecophylla smaragdina.
Thus probably reducing its earlier wide distribution into the present few
trees. The arboreal ant fauna being extremely abundant along with the presence
of species preferring undisturbed dense forests coupled with the absence of
invasive species suggests the islands are biologically rich. The river basin
however affected by anthropogenic activities have posed a serious threat to
conservation evident by abundant distribution of invasive species, hot climate
species and canopy gap specialists. Damming the Sharavathi has isolated and over
time has managed to create biologically rich habitats for ants in the islands
that have lasted till now due to their inaccessibility.
Address:
Centre for Ecological Sciences,
Indian Institute of Science, Bangalore,
Karnataka, India Email: ajinhere@yahoo.co.in
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