Dissertations, Theses, and Capstone Projects
Date of Degree
6-2026
Document Type
Doctoral Dissertation
Degree Name
Doctor of Philosophy
Program
Biology
Advisor
Emily B. Sessa
Committee Members
Fabián A. Michelangeli
Gordon Burleigh
Ana Carolina O.Q. Carnaval
Phillip Staniczenko
Subject Categories
Biodiversity | Botany | Ecology and Evolutionary Biology | Evolution | Genomics
Keywords
Asplenium, Azolla, Climate, Dispersal, Evolution, Ferns, Niche, Species Distribution Modelling, Scaly Tree ferns, Africa
Abstract
Among vascular plants, ferns constitute one of the oldest and most ecologically versatile lineages, yet climatic regimes, dispersal history, and habitat availability strongly structure their contemporary distributions and evolutionary trajectories. Despite their deep evolutionary history and global occurrence, critical gaps remain in our understanding of how past, present, and future climate change, together with ecological filtering, shape fern distributions across space and time. To address this knowledge gap, this dissertation integrates ecological niche modeling, paleo-distribution reconstruction, comparative biogeography, phylogenetic analyses, and climatic niche analysis to examine the ecological and evolutionary processes shaping fern distributions, with a primary focus on Africa.
The first chapter examines the potential geographic distributions of three African aquatic fern species (Azolla) under present and future climatic conditions. Using species distribution modeling, the analyses reveal strong predictive performance and project substantial contractions of climatically suitable habitat, with losses reaching nearly half of current potential ranges under extreme scenarios. These projected declines are driven primarily by changes in precipitation and elevation and vary among species, reflecting differences in climatic niche structure. These results highlight the pronounced sensitivity of African freshwater systems to ongoing climate change and underscore the need for climate-informed conservation planning to safeguard aquatic biodiversity.
Building on this foundation, the second chapter extends the analysis from aquatic ferns to African tree ferns, enabling reconstruction of historical range dynamics from the Last Glacial Maximum through the African Holocene Humid Period to projected future climates, and demonstrating that Quaternary climate oscillations structured persistence, dispersal, and contemporary distribution limits. The analyses show that climatic stability, montane connectivity, and moisture availability have consistently acted as ecological filters shaping tree fern distributions. Despite increased precipitation during the African Holocene Humid Period, spatial variability in moisture limited widespread expansion, resulting in the long-term persistence of many species within climatically stable refugial areas that likely buffered them through late Pleistocene climatic instability. Species responses to climate change were heterogeneous, ranging from minimal range contractions to substantial expansions under certain scenarios, underscoring differences in ecological tolerance and dispersal capacity. These findings highlight the critical role of refugia in maintaining tree fern populations and emphasize their importance for conserving forest biodiversity and ecosystem function under ongoing climatic and anthropogenic pressures.
Whereas the second chapter provides species-level inference through ecological niche modelling of African Alsophila, the third chapter broadens the taxonomic and geographic scope to a global synthesis of scaly tree fern family Cyatheaceae, integrating distributional patterns across all four genera (Alsophila, Cyathea, Gymnosphaera and Sphaeropteris). This synthesis reveals pronounced continental asymmetries in species richness and endemism, low species-level similarity among regions, and higher generic overlap. These patterns highlight the combined roles of historical biogeography, dispersal limitation, and variation in reproductive biology and ploidy in shaping present-day distributions.
The final chapter evaluates whether tropical Africa conforms to the long-standing “Odd Man Out” pattern through a comparative analysis of elevational structure, climatic niche space, and phylogenetic signal in tropical Asplenium assemblages across Africa, Southeast Asia, and the Americas. African assemblages are concentrated at mid to high elevations and occupy a relatively narrow portion of available climatic space despite high environmental heterogeneity, whereas Southeast Asian and American assemblages span broader elevational and climatic ranges. Phylogenetic analyses reveal strong climatic structuring and bounded niche evolution in Africa, consistent with ecological filtering imposed by seasonal moisture stress and fragmented montane systems rather than intrinsic constraints on dispersal or adaptive potential.
Overall, this dissertation demonstrates that present-day fern distributions across aquatic and terrestrial systems are constrained less by intrinsic dispersal or evolutionary limitations than by the structure, continuity, and accessibility of climatic space. In Africa, recurrent signals of habitat restructuring, refugial persistence, restricted climatic occupancy, and global diversity asymmetries point to strong ecological filtering driven by seasonal moisture stress and fragmented montane landscapes.
Recommended Citation
Mwihaki, John K., "Ecological and Evolutionary Dynamics of Ferns: Potential Habitats, Range Shifts, Distribution, and Niche Estimation" (2026). CUNY Academic Works.
https://academicworks.cuny.edu/gc_etds/6789
