Biology Reference
In-Depth Information
their evolution. When rhinos were common across their range, they were highly mobile and
able to spread their genetic material among their populations and maintain a large breeding
stock. In so doing, the species built up a large genetic reservoir. This is also likely true of
other large mammals where individuals have moved long distances from their birth areas and
there has been a lot of genetic exchange among populations.
The answer to the second question also seemed straightforward. We knew that the collapse
of the rhino population was relatively recent. With more than 1,000 rhinos likely in the Chit-
wan area before 1940 or so, and a crash to 60 to 80 individuals by the early 1960s, only a
dozen or so rhino generations had elapsed before the population started rapidly expanding
again in the 1960s. McCracken explained: “Populations lose variability by rhino generation,
not year by year—a rhino doesn't breed every year, and few rhino generations had elapsed
since 1950. So little loss occurred.” Here, then, was an example of a large-mammal popula-
tion that had survived a bout of near extinction and still harbored high levels of genetic vari-
ability.
When a large species becomes especially rare or extinct, there may be many consequences
for the ecosystem of which it has been a part. Large mammals that dominate an ecosystem
and then disappear or shrink to a level at which they become “functionally extinct” no longer
perform their long-standing ecological roles, whatever those might be. In chapter 1, I de-
scribed the experience that triggered my interest in rarity—understanding the role of great-
er one-horned rhinos in the dispersal of seeds of the tree
Trewia nudiflora
and the creation
of
Trewia
woodlands in the grasslands. Lecturers in plant and animal ecology in the United
States and Europe often mention the extinct terrestrial giants only in passing. Bison, masto-
dons, giant ground sloths, North American rhinos, woolly rhinos, and their allies no longer
play their former parts as landscape engineers, so they are typically ignored except as histor-
ical curiosities or examples among the early extinctions. But through their browsing, tramp-
ling, grazing, wallowing, and manuring, giant herbivores have over time played a major role
as nature's architects, shaping the evolution of plant traits.
In most of the world today, no living laboratories remain in which to test theories about
the ecological roles of big mammals. In areas of Chitwan, however, giant herbivores lived at
such high densities that it was possible to study how plants and giant mammals might have
interacted since the Miocene epoch, over 20 million years ago. All one needed was an ele-
phant to maneuver through the grass and follow the rhinos, a bit of curiosity, and some basic
gardening skills.
The sheer size of the latrines and the dense stands of
Trewia
trees they supported were a
revelation of sorts. So was a paper I had read prior to coming to Nepal, which presented a
