Effects of Forest Fragmentation on Central Amazonian Bird Demography
Avian diversity in degraded fragmented Amazonian landscapes depends on the persistence of species in cleared and disturbed areas. Regenerating forest facilitates bird dispersal within degraded Amazonian landscapes and may tip the balance in favor of persistence in previously depauperate habitat patches. Despite the potential value of Amazonian second growth, we lack comparisons of demography among second growth, continuous forest, forest fragments in regenerating landscapes, and truly isolated fragments. Here, we used point-count and capture data to compare Amazonian bird communities among continuous forest plots, 100 ha forest fragments with adjacent second growth, 100 ha forested islands bounded by water, young and older second growth plots. We also compared differences in survival, population growth and age ratios between primary and mature secondary forest. Among foraging guilds, understory insectivores and obligate-flocking species were nearly absent in islands and young second growth. Fragments surrounded by a regenerating matrix were surprisingly species rich, suggesting that a developing matrix may mitigate extinction associated with fragmentation. Survival and population growth was lower in mature second growth relative to primary forest for all foraging guilds except frugivores, gap specialists and ant-following birds. Similarly, age ratios were skewed towards more adult insectivorous birds in older forest; these findings suggest that dominant individuals may preferentially use older forest. Our findings reinforce that true islands are extinction-driven systems with distinct, depauperate communities. Islands are not appropriate comparisons to forest fragments in some landscapes. In contrast, succession of bird communities in second growth facilitates recolonization of forest fragments, permitting fragments as small as 100 ha to support bird communities similar to continuous forest.
demography, bird, Amazon, second growth, survival, forest clearing, community dynamics, population growth
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