Biology Reference
In-Depth Information
favor of riverbanks where wild sugarcane swards grew. Much of Chitwan is composed of sal
forest, the dominant forest type of the northern part of India and lowland Nepal. Wildlife bio-
logist Bivash Pandav calls it “green desert.” Sal is a member of the family Dipterocarpaceae,
the most valued group of timber trees in Asia, comprising about 534 species and reaching its
highest diversity in Borneo and Sumatra. Northern India and Nepal are covered by just one
species,
Shorearobusta
; it grows tall and stout with deep-furrowed bark and straight, lightly
branched trunks and remains green almost year-round. Despite the persistent greenery, few
of the plants that grow in this forest, including the tannin-rich sal, produce leaves that herbi-
vores like to eat. Our census confirmed that in much of the park, rhinos are relatively sparse
because of the dominance of this forest type.
The rhinos also concentrated their feeding in a small area. One traditional rule in mam-
malian ecology, especially for herbivores, states that a species' home range is related to its
body size. This is a critical insight in the study of rarity. Plant- and seed-eating rodents—a
category that includes many narrow-range rarities—can still be abundant numerically. Their
home range is often measured in square meters. At the other extreme, elephants and rhinos,
according to the rule, should roam widely and live at low densities. Yet radio-collared female
rhinos used only 3.5 square kilometers annually and males a slightly larger area, in contrast to
elephants, whose home range might be as large as 30 square kilometers. The core part of the
rhinos' range was actually less than one square kilometer, remarkable for a giant herbivore.
Just as important, the home ranges of females, including those with calves, overlapped, so
many rhinos were packed into a small area. That spring, the density in the riverine grasslands
of Chitwan reached thirteen adults per square kilometer—among the highest densities ever
recorded for a giant mammal. What, we wondered, accounted for this? And it wasn't just rhi-
nos whose density was unusual, but the park's big carnivores as well. The home ranges of
Chitwan's tigers and leopards are smaller than almost anywhere else, with male tigers aver-
aging about 20 square kilometers and females around 6-10 square kilometers. For comparis-
on, in the Russian Far East a male tiger's home range may be as large as 600 square kilomet-
ers.
Happily, our long hours of logging data on elephant-back were starting to answer this ques-
tion, first for rhinos and then for tigers. The twenty-four-hour movements and feeding prefer-
ence of habituated radio-collared rhinos revealed that not all grasslands were equal for them
as feeding spots. The term “elephant grass” encompasses a host of extremely tall grass spe-
cies, many of which are highly woody and unpalatable to rhinos once the plants grow past the
shoot stage. Our results showed that rhinos prefer only about five or six species of these tall
grasses. Tall grassland accounts for about 10 percent of Chitwan National Park's vegetation,
and 90 percent of that 10 percent is dominated by two species that rhinos eat only as young
shoots, one in the genus
Themeda
, the other in
Narenga
. The remaining sliver of grass habitat
directly adjacent to the riverbed, covered by a species known in Nepali as
kans
(
Saccharum
