production. Diversification hedges against market
fluctuations. Reforestation acts as an investment
against price fluctuations in agricultural products.
The model has a major limitation in that natural
forest, as such, has no tangible value to local farmers
other than as potential “insurance” should they
need to clear it. So if the reclamation of abandoned
pasture fails, deforestation could result. The model
suggests that economic advantages of good pasture
management should normally buffer the possible need
for deforestation.
The conservation conundrum is that traditional
monocultures (including exotic species) pay local
benefits but result in biodiversity loss. Ecosystem
restoration enhances biodiversity but is economically
weak. And restoration is often difficult. Degradation
following agriculture represents an ongoing challenge.
Some areas that once supported forest have succeeded
to grassland, sometimes because of invasion by exotic
grass species (plate 18- 13).
One avenue toward forest restoration is with
plantings that attempt to mimic ecological succession.
Numbers of rapidly growing sun- adapted species of
plants are introduced to shade out invasive weeds and
grasses and reduce potential fire hazard. A variation
of this approach is instead to introduce plant species
more typical of mid- succession, essentially bypassing
early succession. Both of these techniques require some
proximity to established forest in order to encourage
the fauna required by particular plant species, such
as seed dispersers. One limitation of the restoration
plantings method is that it is often costly.
Another approach is plantation establishment, which
is typically a monoculture but need not be. In the
worst cases, plantations do not even consist of native
plant species. Asian Teak plantations, for example,
are thriving in parts of the Neotropics. Even when
plantations do utilize native species, the structural and
species diversity of the habitat pales in comparison
with natural forest. Plantations have strong ecological
limitations. They certainly are potentially profitable,
but they are no solution to the biodiversity problem,
quite the contrary. The closest harmonious relationship
between a plantation approach and promotion of
species diversity may be seen perhaps in the Inga
overstory of shade- coffee plantations, as discussed
in chapter 17. The use of many species rather than
monocultures in plantations offers some hope toward
encouraging biodiversity.
The Wright- Laurance Debate
Conservation biologists have recently engaged in a debate
about the future of tropical forests and biodiversity. The
debate is fundamentally between William Laurance and
Joseph Wright, both associated with the Smithsonian
Tropical Research Institute in Panama. Many tropical
ecologists have contributed to the discussion. The debate
has sometimes been characterized as a “cup half full vs.
cup half empty” argument.
Wright and colleagues assert that demographic trends
in human populations between now and mid- century
will result in a net reduction in birthrate in most tropical
regions and a large- scale movement from rural into
urban areas. This projection, based on forecasts from the
United Nations, will alleviate the trend toward increasing
deforestation and forest degradation. Secondary forests
will regenerate, and there will eventually be net increase
in rate of regrowth of forests (similar to what has occurred
in temperate nations when agriculture has been reduced
and urbanization increased). Because the demographic
trends cited above are already occurring, there will be
little change in forest cover between now and 2030 and
possibly after that time a net annual increase in forest
cover throughout most of the tropics. The tropics will
consist largely of secondary forests. While secondary
forests are less species rich than primary forests, they
will nonetheless serve well for most species. Most forest
species would be generalists, but a few specialist species
with rigid ecological requirements would be likely to
survive as well. This is the “cup half full” view.
The response by William Laurance and others is
to question the assumptions underlying the WrightPlate 18- 13. Exotic grass species have become established on
what was once dry forest in Venezuela. Photo by John Kricher.386 chapter 18 the future of the neotropics