Tree Size and Canopy Cover Determine Occurrence of Two Sympatric Bird Species in the Degraded Coastal Forest Patches of Northeastern Tanzania

Abstract

Vegetation structural complexity matters in explaining species niche and in reshaping community structure of many animal taxa, especially in ecosystems facing environmental deterioration. Even though not all species respond equally to environmental degradation, habitat alteration and loss may lead to local extinction of animal taxa. Basically, activities such as clearing for agriculture and logging can reduce forest patch size and/or widen gaps between habitats, deterring some species from exhaustively exploring all potential resources within their home ranges. Under this study, we used two sympatric Greenbul birds, namely, Lowland Tiny Greenbul Phyllastrephus debilis—a forest specialist, and Yellow-bellied Greenbul Chlorocichla flaviventris—a forest generalist, to investigate their utilization of forest vertical vegetation structure in the forest patches and the associated matrices. We collected data from the coastal forests of north-eastern Tanzania, the forests that have historically been subject to modification following negative human pressure such as tree cutting and clearing for agriculture. We modelled vegetation attributes including tree heights, canopy cover, and tree girths against birds' presence/absence data. We found out that mid-sized trees with dbh between 21 and 30 cm and trees with big canopies strongly influenced our study species differently. The study revealed a directional shift in vegetation use rather than balanced coexistence, especially under high disturbance. While the forest specialist was positively influenced by these two factors, the generalist was influenced negatively. As the coastal forests of EA are under intense pressure following selective logging of trees for the purpose of obtaining building materials, we recommend safeguarding the vegetation to ensure representation of varied vertical structuring in the forest canopy layers for sustainable conservation of these forest dependent birds.