regrowth. The main function of gardening thus appears to be the propagation
of useful trees rather than the immediate and direct provision of crops
(Kocher-Schmid 2001), creating a blurred frontier between forest and gar-
dens, wild and cultivated. Such an activity, when carried out for centuries as
in this case, undoubtedly has profound implications for the structure, compo-
sition, and biodiversity of the forest. Jungle rubber (see Chapter 10, this vol-
ume) is a further example of a production system that straddles the limits
between shifting cultivation and other types of agroforestry.
As we have emphasized, “weeds,” which can be simply defined as all non-
crop species present in crop fields (Lambert and Arnason 1989), rank with
declining soil fertility as one of the main reasons why crop fields are left fal-
low in the tropics (Lambert and Arnason 1989; Hinvi et al. 1991; Thurston
1997). They contribute to the species richness but, by their very nature, not
to the overall conservation value of shifting cultivation landscapes. However,
they may play a role in maintaining ecosystem functions. Many common
weeds are also potential sources of medicinal and food products, and to the
forest ecologist they are the pioneer species of the succession that begins when
land is left fallow.
As long ago as 1929, Kenoyer (cited by Richards 1976) listed some of the
plant families typical of weed communities of crop fields and young fallows
on Barro Colorado Island, Panama, citing Amaranthaceae, Euphorbiaceae,
Asteraceae (Compositae), Solanaceae, and Fabaceae or Mimosoideae among
dicots and Poaceae and Cyperaceae as characteristic monocot families. The
representative nature of Kenoyer’s families is illustrated in Table 8.3, which
lists some typical herbaceous species of neotropical shifting cultivation land-
scapes on the basis of more recent literature. The list is illustrative, not exhaus-
tive, and permits the identification of some important characteristics of the
herbaceous weed flora of shifting cultivation landscapes. First, it is clear that,
as is the case for many organisms associated with human disturbance, this flora
is characterized by many species with wide geographic distributions, some of
which are known to be consequences of human introduction and some
of which are pantropical. By the criteria of geographic distribution typically
used in priority-setting exercises for conservation (Bibby et al. 1992; Myers et
al. 2001) and by their abundance in anthropogenic communities, the herba-
ceous weed flora associated with shifting cultivation is not of any particular
conservation value. On the other hand, weed floras can be diverse (Croat
1978). Although weeds are better known as one of the main agroecological
bases of the use of fallows, they may contribute to ecosystem nutrient reten-
tion in the cropping stage of some shifting cultivation systems (Lambert and
Arnason 1989), and the local diversity of weed communities may be related to
the quality and magnitude of this contribution to the maintenance of ecosys-
tem function.
The definition of weeds as noncrop species, with its lack of explicitly neg-
170 III. The Biodiversity of Agroforestry Systems