adventive species originating from riparian vegetation and swamp forest
(igapó) in Bragantina fallows.
What are the characteristics of seed dispersal processes in shifting cultiva-
tion landscapes? Studies of forest succession on abandoned neotropical pastures
have emphasized the importance of wind dispersal in the natural regeneration
of forest cover in anthropogenic habitats (Janzen 1988; Finegan and Delgado
2000). Shifting cultivation fields generally are small (less than 2 ha) in com-
parison with pastures, however, and are set in landscape mosaics that may
favor the persistence of populations of seed-dispersing vertebrates. Younger
shifting cultivation landscapes also are generally characterized by the presence
of scattered, often large trees preserved by the farmer. We therefore predict
that the role of vertebrate dispersal in vegetational dynamic processes is likely
to be greater in shifting cultivation landscapes than in those dominated by
pastures (even though cattle may sometimes disperse seeds; see Chapter 19,
this volume). This point, which is crucial with respect to biodiversity genera-
tion and maintenance among plants and interacting frugivorous and granivo-
rous animals, has not attracted the attention of most workers concerned with
fallows. In one exception to this trend, Ferguson (2001) found wind-dispersed
Lonchocarpus guatemalensisand Trichospermumsp. among the most numeri-
cally important tree species in fallows in Guatemalan subtropical moist forest,
although vertebrate-dispersed tree species such as Spondias mombinwere also
common. In the paleotropics, some of the most common pioneer species (e.g.,
Macarangaspp. and Musanga cecropioides) rely on zoochory, and their seeds are
generally short-lived. In the coastal plains of Gabon (Nasi 1997; Fuhr 1999),
where Musanga cecropioidesis absent because of the poor, sandy soils, the three
most common species colonizing old fields or savannas are Aucoumea
klaineana(a wind-dispersed, large, long-lived pioneer), Sacoglottis gabonensis
(an emergent of the mature forest with elephant-dispersed seeds), and Xylopia
aethiopica(a small, short-lived pioneer whose seeds are bird dispersed). These
results tend to support our prediction, although the wide range of life forms
and dispersal strategies among the tree species of shifting cultivation land-
scapes renders difficult any generalization about the relative importance of dis-
persal mechanisms.
Besides emphasizing the importance of wind dispersal to forest regenera-
tion in neotropical pastures, recent studies show that trees in them facilitate
succession by providing habitat for seed-dispersing vertebrates, an interaction
whose importance has long been recognized in many successional environ-
ments. As in the case of dispersal mechanisms, however, the limited work on
seed dispersal in shifting cultivation landscapes does not provide a similarly
firm basis for generalization. Ferguson (2001) found no evidence that the
Attalea cohunepalms common on his shifting cultivation sites performed such
a function and suggested that the lack of facilitation by A. cohuneresulted
from the shifting cultivation landscape providing a generally favorable habitat
- The Biodiversity and Conservation Potential of Shifting Cultivation Landscapes 163