Degree

Doctor of Philosophy (PhD)

Department

Renewable Natural Resources

Document Type

Dissertation

Abstract

Twenty percent of the Brazilian Amazon has now been deforested, and deforestation rates are increasing. Yet the process of deforestation threatens biodiversity beyond the direct loss of habitat by inducing edge effects and creating forest fragments. In the tropics, among the most vulnerable birds to these human disturbances are a group of insectivorous species that forage on or near the ground. A variety of mechanisms have been proposed to explain these declines, but evidence for these hypotheses remains scare or equivocal. In this study, we examine three proposed mechanisms—physiological constraints to bright light, reduced breeding activity and nest success, and limited dispersal ability—and compare the conservation potential of degraded fragments and regenerating secondary forest with that of continuous primary forest. Using a hierarchical Bayesian framework, we first explored whether vulnerability for 64 species of birds was correlated with two characters that presumably reflect a species’ visual capacity under low light intensity. Although we found that most species (55%) were vulnerable to disturbed habitat, we did not find support for our two light sensitivity metrics. We complemented this by simultaneously synthesizing evidence of the breeding bird communities in disturbed habitats, which we found to support fewer breeding species (22-48%) and fewer breeding individuals (35- 50%) than primary forest. Moving beyond correlation, we directly tested the effects of isolation on mixed-species flocks by experimentally re-isolating three forest fragments. Re-isolation led to the deterioration and collapse of these important species interaction networks, suggesting that birds in these systems have limited dispersal ability. Altogether, we find support for two of the three proposed mechanisms that we analyzed. Our studies of mixed-species flocks further indicated that they exhibit seasonal changes, which were especially pronounced in disturbed habitats that are less buffered from the changing seasons. Even though forest fragments and secondary forest support fewer species with reduced abundances, diminished breeding bird communities, and impoverished flocks with larger seasonal contrast, they still retain a sizeable portion of the original forest-dependent community. Therefore, although these human-modified habitats cannot replace primary forest, conserving them remains a priority following deforestation in the Amazon.

Committee Chair

Stouffer, Philip C

Available for download on Saturday, March 06, 2021

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