Agroforestry and Biodiversity Conservation in Tropical Landscapes

through human modification of part of the habitat is greater than the number

of forest species lost for the same reason.

Compositional patterns in bird communities of shifting cultivation land-

scapes are perhaps clearer than those in species richness and diversity, espe-

cially in relation to composition by feeding guilds. Even when many species

are common to all habitats in a landscape, as was the case in many of the stud-

ies cited here, variations in their relative abundances mean that compositional

similarity at the community level tends to be greatest between sites with the

most similar vegetation (e.g., primary forest and old fallows) and least between

sites at opposite extremes of the disturbance gradient (primary forest and very

young fallow; Blankenspoor 1991; Johns 1991; Andrade and Rubio-Torgler

1994; Vieira et al. 1996; Shankar Raman et al. 1998; Table 8.4). The degree

of isolation of anthropogenic habitat patches undoubtedly also plays a role in

determining compositional patterns in bird communities, although this is

largely undocumented. As an example, however, Stiles and Skutch (1989)

state that four of the five most abundant species recorded by Vieira et al.

(1996) use shaded habitats, such as cocoa plantations and older secondary for-

est, when these are adjacentto primary forest. On the same theme, Shankar

Raman et al. (1998) recognize the important role that the habitat surround-

ing patches of fallow and secondary forest may play in determining the abun-

dance, composition, and diversity of the vertebrate assemblages observed in

those patches. However, they point out that the bird assemblages of replicate

patches of fallow vegetation in given age classes separated by several kilome-

ters were more similar to each other than to those of the adjacent habitats.

These results support the conclusion that vegetation composition and struc-

ture of a given patch play major roles in determining the characteristics of bird

176 III. The Biodiversity of Agroforestry Systems

Table 8.4. Compositional similarity (Horn’s Index of Overlap) between birds
observed in contrasting vegetation types in an Amazonian rainforest, Amazonas
State, Brazil.

Unlogged Logged 35-ha Forest

Primary Forest Primary Forest Fragment Fallow Crop Fields

Unlogged
primary forest —

Logged primary
forest .75 —

35-ha forest
fragment .35 .23 —

Fallow .44 .32 .54 —

Crop fields .24 .23 .18 .55 —
Source:Johns (1991).
Note: Higher values indicate greater similarity.