Effects of habitat extent and forest disturbance on bird communities in lowland nepal

Abstract

Habitat loss and degradation are recognized as the major contributors to species decline and extinction, and therefore represent a key conservation challenge for biodiversity conservation. Key to the protection of biodiversity is acquisition of ecological knowledge about how anthropogenic forest disturbances affect species and how species respond to emergent landscape characteristics. Furthermore, it is also important to assess how different management approaches and land tenures influence retention of the biota of particular sites and of landscapes. However, this crucial ecological knowledge is yet to be obtained for the threatened lowland landscapes of Nepal. Protected areas cover only a small proportion of forests in lowland Nepal; the majority of forests outside the protected areas (off-reserve) have been managed by the state government. However, in recent years, community forestry programs have been increasingly popular as attempts to protect biodiversity while permitting consumptive forest use by people. It is therefore important to understand effectiveness of different forest management tenures for avifaunal conservation. I compared species richness, abundance, diversity and community composition of birds among sites in community forests, state forests and protected areas. Although sites in protected areas had the greatest richness of birds, community forests and state managed forests had complementary assemblages, supporting species not represented in protected areas. Vegetation characteristics such as large tree density, tree canopy cover and shrub density were also greater in community forests than in state-managed forests. The findings suggest that the community forestry approach appears to improve habitat quality compared to state-managed forests, and therefore can be an alternative tenure type for conservation of off-reserve forests and avifauna in the region. Subsistence forestry practices such as logging, lopping, and grazing are sources of forest disturbance in lowland Nepal. Such activities do not reduce forest area, but change habitat characteristics, potentially affecting biodiversity directly, and through interactions with landscape characteristics. I examined effects of forest use practices on species richness and abundance of forest birds, and whether landscape context such as the extent of forest cover moderates disturbance effects on birds. I found that extraction of forest products reduced forest structural complexity and altered the avifaunal community of a site. At the site level, large tree density, tree canopy cover and shrub density were important habitat characteristics, while the extent of forest cover in the landscape had the greatest influence on richness of birds. The effects of forest disturbance (livestock grazing and logging) intensity on birds depended on the extent of forest in the surrounding landscape, with strongest effects in sites with less surrounding forest. Thus, although site-level vegetation structure is important, maintenance of forest extent in the landscape is also key for avifaunal conservation in the region. Several recent studies have demonstrated that the extent of forest cover and other landscape characteristics significantly influence bird species richness. However, different foraging guilds are likely to respond to landscape characteristics in different ways. Therefore, I examined the strength and magnitude of the relationships between the extent of forest cover and estimated species richness for overall birds and for each foraging guild separately. I found that landscape-level species richness of birds positively related to the extent of forest cover in the landscape. However, the relationship varied among the foraging guilds, with strong effects for foliage-gleaning insectivores and, to a lesser extent, frugivores, but only weak effects for sallying insectivores. The relationship between estimated species richness and the extent of forest cover in the landscape was nonlinear, with species richness decreasing more steeply below about 20-30% forest cover in the landscape. Importantly, I found that the relationship between richness and forest extent varied among foraging guilds and with landscape characteristics. Therefore, generalizing relationships between species richness and the extent of forest across all species could potentially mask important relationships at the functional level. The findings of this thesis have important implications for the conservation of avifauna in multiple-use forest landscapes. Although both site-and landscape-scale forest characteristics have important influences on bird communities, the extent of forest in the landscape both directly and indirectly affects persistence of birds in these landscapes. The extent of forest in the landscape can moderate the effects of subsistence forest use practices on bird assemblages. Therefore, conservation benefits for avifauna can be maximized by maintaining both site-level habitat structures such as large trees, and the extent of forest cover at the landscape-level. This can be achieved with appropriate protection measures through reducing human pressure on forests, and restoration of degraded forest habitats, particularly those that are heavily exploited such the state-managed forests. Thus, management approaches such as community forestry for management of off-reserve forests can potentially complement protected areas and maximize conservation outcomes in the region. Such measures will improve habitat quality and increase the chance of maintaining viable populations of the full complement of avifaunal species in the lowland landscape of Nepal.