such deforested and often densely populated landscapes, agroforestry systems
may maintain more species of plants, animals, and microorganisms from the
original ecosystems than corresponding agricultural monocultures and pas-
tures and therefore could be a better compromise between production goals
and biodiversity conservation (Thiollay 1995). It should be stressed that one
cannot evaluate this role for an agroforestry system by simply counting the
species present because these will invariably include species that are adapted to
disturbed conditions and may not need special protection. Instead, it is nec-
essary to determine whether forest-dependent and threatened species use the
agroforestry areas, the degree to which they depend on these areas for habitat
or food, and whether their populations are viable over the long term. Parts III,
IV, and V of this volume explore this hypothesis in greater detail.
The Agroforestry-Matrix Hypothesis
In landscapes that are mosaics of agricultural areas and natural vege-
tation, the conservation value of the natural vegetation remnants
(which may or may not be protected) is greater if they are embedded
in a landscape dominated by agroforestry elements than if the sur-
rounding matrix consists of crop fields and pastures largely devoid of
tree cover.This hypothesis refers to the larger-scale properties that agroforestry elements
may confer to landscapes with respect to their suitability as habitat for native
fauna and flora, that is, effects that reach beyond the limits of an individual
agroforestry system and extend to the entire landscape. In tropical land use
mosaics, ecological processes and characteristics such as microclimate, water
and nutrient fluxes, pest and disease dynamics, and the presence and dispersal
of fauna and flora may be significantly influenced by agroforestry elements.
For example, strategically placed agroforestry systems may serve as biological
corridors between patches of natural vegetation or act as stepping stones that
facilitate animal movement. Where two forest fragments are separated by a
tree crop plantation with a diversified shade canopy of rainforest remnant
trees, it should be easier for arboreal forest fauna to disperse from one frag-
ment to the other than if they had to cross an open pasture, and this may help
to reduce problems of small populations in the individual fragments by main-
taining biotic connectivity. Insects, birds, and bats, crossing from one forest
patch to another via a riparian strip or using remnant trees in a pasture as step-
ping stones, may pollinate trees that occur at low densities in the individual
patches. Birds may carry seeds from one fragment to the next, moving along
live fences, hedges, and windbreaks or flying from one isolated tree to another,
thereby enhancing seed dispersal in fragmented landscapes. Where agro-
forestry systems adjoin forest areas, they may also buffer them against the
Introduction: Agroforestry in Biodiversity Conservation in Tropical Landscapes 9