Human-Altered Tree-Based Habitats and Their Value in Conserving Bird and Bat Diversity in Northeast Luzon, The Philippines - Smallholder Tree Growing for Rural Development and Environmental Services - page 356

tree-based habitats, i.e., the matrix, plays an important role in colonization and dis-
persal possibilities for forest species (Ricketts 2001). Matrix landscapes show vary-
ing degrees of hostility to forest species, which influences species' abilities to
survive outside original forest and disperse between patches of forest-like habitats
and form meta-populations (Hanski 1999). In our study, forest bird species richness
and abundance are negatively correlated to distance to contiguous forest in shrub-
land limiting the dispersal of forest birds through matrix landscapes away from
forest. In contrast, overall resident bird abundance was positively correlated to dis-
tance to forest in shrub-land; non-forest birds reach higher densities in a more open
and human-dominated landscape.
Twelve of the 15 forest bird species observed in this study, and 84 percent of
forest bird individuals, are observed within 2 km of contiguous forest. Of these,
nine forest bird species are limited to this 2 km forest-bordering zone (point count
effort was roughly similar within (63) and beyond (55) the 2 km boundary). Hughes
et al. (2002) argue that most forest birds will probably not travel more than 2 km
from nesting areas to forage areas. Only locally nesting forest birds are then really
able to sustain populations outside natural forest in a larger human-altered country-
side. Chalcophaps indica and the endemics Ixos philippinus and Dicaeum australe
are the only forest species relatively common in Gmelina forest beyond two km of
contiguous forest where we suspect they occur as breeding species. The endemic
Loriculus philippensis has been observed in small feeding groups in homegardens
beyond 2 km of contiguous forest. We suspect this species makes longer foraging
trips than other forest species to feed on flowers and nectar in gardens. We deter
from these findings that contiguous forest of the Sierra Madre functions as a source
of forest birds to the nearby non-forest landscape. However, tree-based human-
altered habitats in this landscape do not provide suitable conditions for forest birds
to establish themselves as breeding species, neither as sink nor as source popula-
tions (Pulliam 1988), with the possible exception of Gmelina forest for the three
forest bird species mentioned above. As these forest plantations are established on
deforested land, the three forest species occurring here are not a relict population
of former larger natural forest (Brooks et al. 1999). Even if patches of Gmelina
forest are suitable for a larger number of forest bird species, colonization of these
patches will have to take place through the surrounding shrub-land matrix but the
shrub-land in the Cagayan Valley seems very hostile to forest bird species. Only six
forest bird species have been observed in the shrub-land, with only 16 individuals.
The human-altered landscape of the Cagayan Valley does offer suitable habitats
for the endemic habitat generalist Centropus viridis , observed in Gmelina forest and
shrub-land, and the endemic Amaurornis olivacea , a relatively common shrub-land
species. One more open area endemic has been observed, the threatened Anas
luzonica (once). This species congregates during the wet season in several lakes in
the Cagayan Valley and disperses during the dry season to breed in shrub-land. The
largest remaining congregations of this species are found in the Cagayan Valley. The
shrub-lands are important as breeding areas to sustain this increasingly rare species,
although hunting rather than breeding habitat conversion seems a more serious threat
to the conservation of this species (Van Weerd and Van der Ploeg 2004).