fertility sites) and tree crop monocultures (which may be more susceptible to
pest and disease outbreaks than agroforestry plantings; Schroth et al. 2000).
Consequently, the adoption of agroforestry may reduce the need to deforest
new areas. However, it should be stressed that sustainability is not an intrinsic
characteristic of agroforestry practices. Sustainability has both biological and
socioeconomic dimensions, and even if it is technically possible to manage a
certain land use system sustainably, it may be more advantageous for a farmer
not to do so if land for new fields and plantations is readily available or if there
is an advantage to occupying a large land area (e.g., acquiring property or land
use rights). The agroforestry-deforestation hypothesis is analyzed from a
socioeconomic and historical viewpoint in Part II of this volume.
The Agroforestry-Habitat Hypothesis
Agroforestry systems can provide habitat and resources for partially
forest-dependent native plant and animal species that would not be
able to survive in a purely agricultural landscape.The biodiversity of agroforestry systems, and of agroecosystems in general,
consists of planned and unplanned components. By their very nature, agro-
forestry systems contain more planned diversity (i.e., more planted and
selected plant species) than the corresponding monoculture crops, although
not necessarily more than some traditional mixed cropping systems (Thurston
et al. 1999). Certain agroforestry systems such as tropical homegardens, which
may contain several dozen species and varieties of trees and crops, are seen as
important reservoirs of tropical tree and crop germplasm (Torquebiau 1992).
However, not all agroforestry systems have much planned diversity; for exam-
ple, certain shaded coffee plantations essentially consist of one crop and a sin-
gle, sometimes exotic shade tree species, and live fences typically consist of
only a handful of tree species.
Of similar or greater importance for the conservation value of agroforestry
systems than their planned diversity is their unplanned diversity, that is, the
plants and animals that colonize or use the structure and habitat formed by
the planted species. Structurally heterogeneous perennial vegetation can pro-
vide more niches for native flora and fauna than structurally simpler mono-
cultures and pastures (Thiollay 1995). A humus-rich soil that is not regularly
disturbed by tillage and the permanent litter layer that usually develops under
agroforestry may also provide appropriate habitat for a diverse soil fauna and
microflora that may not be present in simpler and regularly disturbed agricul-
tural systems, although little is known about such belowground biodiversity
benefits of complex land use systems (Lavelle et al. 2003).
The role of agroforestry systems as refugia for forest-dependent species is
most relevant in landscapes that are largely devoid of natural vegetation. In
8 Introduction: Agroforestry in Biodiversity Conservation in Tropical Landscapes