species are homogenizing the global flora and fauna, which has led to extinc-
tions and population reductions of native species (Lovel 1997).
This negative impact on native species is sometimes masked by an increase
in species richness. With the influx of competing species, species numbers in
a fragmented system can increase, which creates a situation in which further
biodiversity degradation can occur through species displacements and more
local extinctions. To mitigate these problems, direct preventive measures are
needed in addition to increases in connectivity, area-to-perimeter ratios, buffer
zones, and improvements to the matrix around existing reserves (Gascon et al.
2000). The use of agroforestry outside protected areas may play a role in such
strategies by increasing connectivity and serving as buffers but may also pose
additional threats if invasive alien tree species are used (see Chapter 15, this
volume).
Exploitation
Exploitation of the natural environment has always been a part of human cul-
ture. Increases in the human population have likewise increased demands on
natural resources. These demands have reached levels that cannot be main-
tained without permanently damaging natural ecosystems and processes. For
instance, subsistence hunting in Amazonian Brazil is estimated to affect more
than 19 million individual animals per year. This hunting, coupled with
wildlife trade and demand for wildlife products such as pelts, ivory, and
organs, places serious pressure on native fauna (Harcourt and Sayer 1996).
New roads, which provide access to previously inaccessible areas for coloniza-
tion, have increased human-induced threats. In fact, even in Brazilian Amazo-
nia, every nature reserve was found to be 40 to 100 percent accessible by roads
or navigable rivers (Peres and Terborgh 1995). Landscape planners must use
knowledge of the cascading and synergistic effects of road building and settle-
ment on biodiversity and must place greater value on wildlife and natural
habitats to reduce exploitation. Agroforestry land uses including fallows and
secondary forests may help to avoid overexploitation of the timber and non-
timber resources of natural habitats and thus contribute to integrated strate-
gies of natural resource management and forest conservation (see Chapter 14,
this volume).
Global Change
Anthropogenic physical changes also threaten tropical systems. One of the
most important is the alteration of biogeochemical cycles. Carbon, nitrogen,
phosphorus, and other nutrients are cycled through natural systems. Through
industrial emissions, anthropogenic biomass burning, mining, and agriculture
runoff, among others, humans artificially increase nutrient and pollutant loads
- Biodiversity Conservation in Deforested and Fragmented Tropical Landscapes 21