Ecology 6e Chapter 16 Online Supplementary Materials
Human Disturbance: An Ancient Feature of the Biosphere
Near the border between Panama and Columbia there is a region so laced with rivers and
swamps and so thick with tropical rain forest that the Pan American Highway cannot
pass. This region, called the Darien, has become a symbol of raw tropical nature,
impenetrable and pristine. Though the Darien has resisted the passage of the Pan
American Highway, it is not as pristine as it has seemed. Native Americans occupied and
farmed the Darien continuously for about 4,000 years. They abandoned the region only
after the Spanish conquest about three centuries ago. This challenge to the generally held
view of the Darien comes from work by Mark Bush and Paul Colinvaux (1994). As a
consequence of their studies, we now know that these rain forests are not “virgin” but
have grown up since the abandonment of agriculture, covering most traces of previous
human occupation.
How did Bush and Colinvaux penetrate the secrets of the Darien? They used the same
paleoecological methods used by Margaret Davis to reconstruct the floral history of
North American forests that we discussed in chapter 1. Bush and Colinvaux looked for
lakes and swamps whose sediments might preserve a record of the Darien’s past. They
found such a record in a swamp and a small lake, Lake Wodehouse, approximately 15 km
apart, near a small mining settlement called Cana near Panama’s border with Columbia.
The researchers collected 10 m cores of sediment from Lake Wodehouse and Cana
Swamp. At the base (bottom) of the cores from Lake Wodehouse were an abundance of
spores from ferns (e.g., Triletes) and pollen from other forest plants and little pollen from
plants associated with disturbed landscapes (fig. 1). This portion of the pollen record
indicates a swamp community dominated by ferns surrounded with mature tropical forest.
Then, about 3,900 years ago, the sediment record changed abruptly with the appearance
of charcoal and corn pollen, which are present throughout the subsequent record until
nearly 310 years ago. Similar patterns occurred in Cana Swamp except that corn pollen
constituted a greater proportion of the total pollen (about 2%), indicating that corn was
being grown right on the swamp during the dry season. The sediments containing corn
pollen and charcoal also contained pollen from wild plants that live in disturbed areas
(e.g., Melastomataceae, Cecropia, Trema, and Pilea). Then, about 310 years ago, about
1.6 to 1.7 m deep in the Lake Wodehouse sediment core, charcoal and corn pollen
disappear, while the pollen of forest species increases in abundance.
Figure 1 Pollen and particulate carbon in the sediments of Lake Wodehouse, Panama,
dating from the present to 3,900 years before the present (BP) (data from Bush and
Colinvaux 1994).
Are the sedimentary records of Lake Wodehouse and Cana Swamp atypical of
tropical Central and South America? No, they are not. Other studies reveal even longer
histories of human occupation and disturbance. In another study, Mark Bush, Dolores
Piperno, and Paul Colinvaux (1989) provide evidence of 6,000 years of corn cultivation
in the Amazon River basin. They recovered this record from the sediments of Lake
Ayauch, Ecuador, a small lake within the eastern Amazon River basin at an elevation of
500 m.
The longest sediment core retrieved from Lake Ayauch was 3.26 m long and
contained sediments deposited during the last 7,100 years. The earliest portion of the
sediment record indicates that the lake was surrounded by mature forest. Then about
6,000 years ago, corn pollen appeared in lake sediments. The appearance of corn pollen
in the sediments of Lake Ayauch coincided with an increase in the pollen of Cecropia
and herbaceous vegetation, a pattern that Bush and Colinvaux also observed in Lake
Wodehouse, Panama. These changes indicate forest clearing and soil disturbance
associated with agriculture. However, the occurrence of corn pollen within the sediments
of Lake Ayauch is sporadic, a pattern suggestive of shifting agriculture. It appears that
the farmers around the lake cleared forest for their fields, farmed the land for a period,
and then moved on. After some time, the same land could be cleared once again. This
kind of shifting agriculture continued for about 5,000 years and then corn pollen
disappeared from the record about 800 years ago. The researchers suggest that people
may have stopped farming in the lake basin due to climatic change. Other studies suggest
that cultivation of corn in Panama began about 7,000 years ago.
As impressive as this 7,000-year history of agriculture may be, human disturbance in
the New World tropics is more ancient still. Substantial human disturbance of the forests
of Panama began about 11,000 years ago. Evidence comes from the sediments of Lake La
Yeguada in central Panama. Lake La Yeguada yielded a 17.5 m sediment core containing
a 14,300-year record of the surrounding landscape. This sediment core contains no record
of human disturbance for about 3,300 years. Then, about 11,000 years ago, the charcoal
content of sediment rose abruptly.
The increase in charcoal in the sediment record of La Yeguada also coincided with
the presence of Paleo-Indian artifacts across Panama. We can conclude that humans have
played a substantial role in shaping the communities and ecosystems of Panamanian
rain forests for at least 11,000 years. At the very least we can say that the exceptionally
high diversity of the New World tropics has coexisted with moderate levels of human
disturbance for at least 11,000 years. However, the present rate of clear-cutting of
Amazonian rain forest has no historical precedent in the basin and the predictions of the
intermediate disturbance model are clear. Continued severe disturbance of these tropical
rain forests will lead to reduced diversity.