Fire eliminates the leaves and wood while at the same
time releasing the nutrients and minerals contained
within. The ash raises the pH, making the soil less
acidic, so the soil surface, fertilized by the ash from
the biomass, tends to be alkaline. The farmer plants
crops for a few years on relatively fertile soil. Rainfall
will still act to erode the now- exposed soil and leach
minerals. The crops themselves are removed, of course,
and with them go more of the minerals. The result is
that fertility and yield decline steadily. Typically, staple
crops include manioc (various varieties), plantains,
bananas, sweet potato, pineapple, chile peppers, and
others (plate 17- 5).
Plots are normally planted as polycultures rather
than monocultures, a practice that helps with pest
control and slows the rate of natural succession. Crop
losses from predation by such creatures as agoutis are
actually anticipated and extra sweet manioc is often
planted for rodent consumption.
As available soil minerals are depleted, crop yields
typically decline sharply within the first four to five
years of cultivation. Within a few years the plot will be
abandoned, allowing natural succession to occur. The
typical time sequence for slash- and- burn agriculture is
to farm the plot for two to five years (sometimes only
for one year, sometimes for as long as seven years)
and then abandon it for at least 20 years. Ideally (but
rarely), an area just abandoned will not be recut for
nearly a hundred years or so, permitting substantial
recovery of the system. Slash- and- burn agriculture
requires constant rotation of sites and often results in
a nomadic population that must move around in the
rain forest to find suitable plots to farm. Because of
soil nutrient limitation and therefore the need to allow
forest regeneration, the human population density
remains generally low (plate 17- 6).Nonindigenous Farmers in AmazoniaShould you travel anywhere along the Orinoco,
Amazon, or the major river tributaries, you will notice
immediately that areas along rivers are inhabited by
people, particularly várzea areas. When Europeans
colonized Amazonia they bred with Amerindians, and
the descendants of those unions became the people
who today make their living by farming and fishing the
floodplains. The riverine peasantry is called caboclo,
ribereño, mestizo, or campesino, depending upon region
(plate 17- 7). These people practice agriculture on the
floodplain in a manner similar to those populations on
terra firme, with the exception that they make much
more use of a market economy rather than relying
entirely on subsistence. Because they are on a river
they have the ability to move goods. They grow rice as a
cash crop, for instance, and sell fish at market. Indeed,
the largest and most diverse fish market in Amazonia
is at Manaus, Brazil, where between 30,000 and 50,000
tons of fish are marketed annually. Riverine people
also harvest such things as Brazil nuts, palm fruits,
and rubber for commercial sale. Note that such usage
is not necessarily environmentally damaging, unless
overharvesting occurs (and it often does with game
animals, including creatures such as tapirs, manatees,
turtles, and capybara, though not so much with plants).Plate 17- 7. Young boy skillfully paddling a dugout canoe.
Photo by John Kricher.Plate 17- 6. Subsistence farmers, such as the ones who live
here, commonly practice slash- and- burn agriculture. Photo by
John Kricher.
370 chapter 17 human ecology in the tropics