Tropical savannas support globally important biological diversity and provide a variety of cultural and economic ecosystem-services to humans. As mosaic landscapes, they are characterized by the coexistence of woody and grassy habitats which are strongly regulated by abiotic gradients (e.g., soils, climate) and local disturbance regimes (e.g., fire, flooding and grazing). Consequentially, savannas are inherently dynamic over a variety of scales and provide an array of temporal and spatial ecological niches for wildlife. However, tropical savannas and their ecological functions have been widely degraded by intensifying human land-use and are threatened by impending climate change. Unfortunately, our scientific understanding of savanna ecology is currently limited, impeding development of appropriate conservation approaches. This is especially true of Neotropical systems. This dissertation provides new research, carried out within the Beni, a seasonally flooded Neotropical savanna in Bolivia, that aims to address critical knowledge gaps in savanna ecology. This research will contribute to developing sustainable land management and restoration initiatives to secure and protect savanna ecosystems for people and wildlife.
The overarching aim was to understand how ecological (flooding) and agricultural (managed fire and cattle grazing) disturbance influences habitats and wildlife in Beni. To achieve this, three specific objectives were identified: 1) understand how restoration, through cessation of long-term cattle grazing impacts (i) regeneration, (ii) understory structure and composition, and (iii) abundance and diversity of large mammals and nocturnal birds, within the regions’ naturally patchy gallery forests; 2) examine how compounded disturbance from flooding and fire shapes the distribution of woody plants and habitats across a savanna-grassland mosaic; and 3) determine functional responses of avian biodiversity to livestock removal across a savanna-grassland ecotone.
First, comparing grazed and restored gallery forests, I assessed the abundance and composition of tree functional types at different life stages (seedlings, saplings and trees), with focus on Attalea princeps (motacú) a keystone palm species that shapes gallery forest structure, microclimate and ecology in Beni. Additionally, I surveyed habitat structure and composition in the shrub and ground-layer and monitored occurrence and encounter rates of large mammals and nocturnal birds along dusk and evening transects. Livestock grazing resulted in lower recruitment of motacú palms in Beni’s gallery forests. Principal components analysis showed that grazed forests also had simpler, more open shrub-layers and altered ground-layer structure and composition including increased bare ground. Mammal species richness was greater on the restored site, and there were more declining, globally threatened and site-unique species. Species richness was similar for nocturnal birds in both forests, but nearly all species tended to be encountered more frequently on the restored site. Results suggest cattle negatively impact forest regeneration and alter understory structure and composition with consequences for the diversity and abundance of wildlife. However, the alleviation of grazing can promote understory regeneration over relatively short time scales (~10 years) with positive benefits for wildlife.
For objective two, I deployed ground-based flooding loggers and completed assessments of woody vegetation at forty-eight plots split evenly across four savanna-grassland habitats. Fifteen-year fire histories were quantified for each plot via fire-scar mapping using remote sensing data from Landsat 7 and 8 missions. Finally, I tested how flooding and fire histories influenced tree abundance using a hurdle model approach. Results demonstrated there is significant landscape-scale variation in environmental gradients and fire histories across habitats in Beni, resulting in variable woody plant communities. Compared to woodier formations, open grassland habitats experience longer flooding and more frequent fires. Congruent with increases in flooding and fires, the abundance of broadleaf woody plants declines between closed and open habitats. At the same time fire-adapted corky-barked broadleaved species become dominant while palms become sparse. Ultimately, the abundance of woody plants, in nearly all life stages, is shaped by compounded disturbances. Fire may be more limiting than flooding for broadleaf trees. Given the contribution of fire and flooding regimes to ecosystem structure and function, alterations to these cycles, (e.g., driven by climatic or land-use change), may have considerable consequences for the future sustainability of savanna-grassland mosaics.
For objective three, annual avian point counts were completed over three years at 48 plots distributed evenly across four savanna-grassland habitats. These were equally split over two sites with different management histories, facilitating a space-for-time experimental approach with one site having livestock removed for 0, 1 and 2 years during surveys, the other 8, 9 and 10 years. Vegetation surveys were completed in tandem to track shifts in understory structure and composition, canopy assessments were made in the final year. The breeding and resource-acquisition traits of 162 bird species were classified and used to track shifts in key metrics of functional diversity (functional richness, functional evenness and functional dispersion) and community composition over time. The functional response of bird communities to livestock removal varied across habitats. In woody grasslands, functional richness increased, while functional evenness and variability remained stable. In tandem, habitat-dependent shifts in avian functional composition occurred relating to both resource-acquisition traits, and nesting and breeding traits. Notably, the abundance of more specialized resource-acquisition traits increased, while the occurrence of social breeding strategies declined. Conversely, in open habitats functional richness did not change following livestock removal, but functional evenness and variability declined. This resulted in grassland bird communities becoming more similar and was associated with homogenization of habitat structure. Results suggest that livestock removal positively benefits ecosystem function in woody habitats, at least in the short term. However, continued monitoring is needed to determine if effects remain positive. In contrast, disturbance may be necessary to promote the spatial and temporal heterogeneity in habitats that maintains functionally diverse grassland bird communities.
Overall, my research demonstrates that livestock grazing, and frequent managed fire can have negative consequences for habitats and associated wildlife across the forest-grassland ecotone. This includes inhibiting the regeneration of forest understories, eroding savanna-grassland ecotones and degrading ecological function in avian communities associated with woody grasslands. Nonetheless, habitat regeneration and ecological functions can show signs of positive recovery over relatively short timescales. My research also shows that ecological disturbance may be critical to retaining diversity of avian function in open habitats. Thus, in both protected and working landscapes, disturbance management, for example patch burning, rotational grazing or their combination via pyric herbivory schemes could help to retain avian functional integrity in open grassland formations. In contrast, longer-term protection may be needed to support regeneration of forested habitats and woody savannas. However, fire will likely be required to maintain woody grassland structure and associated bird communities here in the long term.