Oscar Johnson, Ph.D.

FGCU Junior Faculty Teaching Excellence Award

2026

Seidler Student/Faculty Scholarly Collaboration Fellowship

2025

Macias Wildlife Society grant

2024

Santa Barbara City College

A.A.

Biology/Biological Sciences, General

2008

University of California, Santa Cruz

B.S.

Ecology and Evolutionary Biology

2011

Louisiana State University

Ph.D.

Ecology and Evolutionary Biology

2021

Song complexity in suboscine birds: evolutionary drivers and ecological constraints

Yang, J., Arvind, C., Barber, R.A., Johnson, O., O’Brien, K., Stanley, R., Bravo, G.A., Buck, E.J., Claramunt, S., Brumfield, R.T., Harvey, M.G., Derryberry, E.P., and Tobias, J.A

2026-05-20

Acoustic signal complexity varies widely in animals, from single notes to highly sophisticated vocal displays. In birds, vocal complexity can evolve as an honest signal of individual quality driven by sexual selection. However, this hypothesis is rarely explored in conjunction with alternative drivers, including competition for ecological resources (social selection) and intra-group communication, both of which may favour increased signal complexity. Using Bayesian phylogenetic models, we test whether these alternative mechanisms predict the complexity of innate songs in 1288 species of suboscine passerine birds, while accounting for ecological constraints on sound production, transmission and detection. We found that overall song complexity was reduced by sexual selection (estimated from mating systems) and declined with body size and vegetation density. Conversely, note count and song length increased in territorial species, particularly those using song to defend year-round territories during the non-breeding season. These findings challenge the common assumption that sexual selection is the main driver of increased signal complexity and highlight the role of social selection via territorial competition as a factor increasing the temporal complexity of songs. Our results suggest that signal complexity depends on social, cultural and ecological contexts, reflecting a combination of multiple inter-related drivers and constraints.

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The Influence of Rivers and the Environment on Avian Taxon Composition Across Lowland Amazonia

Rego, M., Johnson, O., Fluck, E.I., Del-Rio, G., and Brumfield

2026-03-26

Aim
Assess how local variations in bird taxon composition across lowland Amazonia are associated with environmental factors and rivers.

Location
Lowland Amazonia below 500 m a.s.l., South America.

Time Period
Contemporary.

Major Taxa Studied
Birds.

Methods
We constructed maps illustrating changes in bird taxon composition across lowland Amazonia and calculated Jaccard dissimilarity between adjacent localities. We then applied geographically weighted regression (GWR) to evaluate how variation in environmental variables and riverine features (width, water discharge, meandering, floodplain extent) explains spatial turnovers in taxon composition. We used comparative phylogenetic analyses to test whether ecological traits predict cross-river taxon turnover.

Results
Geographic variation in bird taxon composition was mostly associated with the presence and physical characteristics of rivers, especially river width and discharge, which predicted composition dissimilarities along their lower courses. Away from rivers, variations in forest cover, habitat heterogeneity, and temperature seasonality were the strongest variables associated with composition turnovers. Phylogenetic analyses showed that taxa inhabiting terra firme forests were disproportionately likely to exhibit cross-river replacements, whereas dispersal-related morphological traits had limited explanatory power.

Main Conclusions
Local Amazonian bird taxon composition is primarily associated with variation in forest cover, habitat heterogeneity, and temperature seasonality, except across major rivers. The influence of river characteristics, particularly width and discharge, on bird taxon turnover highlights the role of rivers in structuring Amazonian biodiversity at local and global scales. Because these variables operate at different scales, taxon composition turnover patterns reflect a mosaic of ecological and geomorphological processes. These findings highlight the need for conservation strategies that account for bird habitat specialisation, river dynamics, and ongoing landscape changes.

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Amazonian birds in more dynamic habitats have less population genetic structure and higher gene flow

Johnson, O., Ribas, C.C., Aleixo, A., Naka, L.N., Harvey, M.G., and Brumfield, R.T.

2023-02-16

Understanding the factors that govern variation in genetic structure across species is key to the study of speciation and population genetics. Genetic structure has been linked to several aspects of life history, such as foraging strategy, habitat association, migration distance, and dispersal ability, all of which might influence dispersal and gene flow. Comparative studies of population genetic data from species with differing life histories provide opportunities to tease apart the role of dispersal in shaping gene flow and population genetic structure. Here, we examine population genetic data from sets of bird species specialized on a series of Amazonian habitat types hypothesized to filter for species with dramatically different dispersal abilities: stable upland forest, dynamic floodplain forest, and highly dynamic riverine islands. Using genome-wide markers, we show that habitat type has a significant effect on population genetic structure, with species in upland forest, floodplain forest, and riverine islands exhibiting progressively lower levels of structure. Although morphological traits used as proxies for individual-level dispersal ability did not explain this pattern, population genetic measures of gene flow are elevated in species from more dynamic riverine habitats. Our results suggest that the habitat in which a species occurs drives the degree of population genetic structuring via its impact on long-term fluctuations in levels of gene flow, with species in highly dynamic habitats having particularly elevated gene flow. These differences in genetic variation across taxa specialized in distinct habitats may lead to disparate responses to environmental change or habitat-specific diversification dynamics over evolutionary time scales.

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