Seed production, infestation, and viability in Acacia tortilis (synonym: Vachellia tortilis) and Acacia robusta (synonym: Vachellia robusta) across the Serengeti rainfall gradient

Tree recruitment in savannas proceeds in multiple stages characterized by successive filters occurring at the seed and seedling stages. The “demographic bottleneck” hypothesis suggests that such filters ultimately restrict tree density and prevent trees from dominating grasses in savannas, but many of the demographic transitions underlying this assumption have not been quantified. We investigated how short- (1–2 years) and long-term (40 + years) rainfall patterns influenced seed production, infestation, and viability for two dominant species, Acacia robusta and Acacia tortilis across the Serengeti ecosystem mean annual precipitation gradient over a two-year period. We found that neither production, nor infestation, nor viability was influenced by rainfall. Pod production differed between species and increased with tree height in A. robusta. Mean infestation proportion in 2013 was higher (mean ± SE; 0.28 ± 0.08) in A. tortilis than in A. robusta (0.11 ± 0.05) but the trend reversed in 2014, when A. tortilis (0.33 ± 0.10) had lower infestation than A. robusta (0.61 ± 0.09). Under laboratory conditions, A. tortilis and A. robusta seeds had maximum germination (= viability) proportions of 70 and 20%, respectively. Mean seed viability was more than five-fold higher (0.46 ± 0.19) in A. tortilis than in A. robusta (0.08 ± 0.10). Our study has produced important estimates for seed stage demographic dynamics that can be used for modeling tree dynamics in Serengeti system, and savannas in general

Transpiration dynamics support resource partitioning in African savanna trees and grasses

Citation: Holdo, R. M., & Nippert, J. B. (2015). Transpiration dynamics support resource partitioning in African savanna trees and grasses. Ecology, 96(6), 1466-1472. doi:10.1890/14-1986.1It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap-flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water-use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water-use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree-grass dynamics under altered precipitation regimes

Disaster Nursing: Looking to the Future in Norway

Health resilience is an integral part of disaster management and lies at the nexus between this and public health considerations. As the largest group of professionals worldwide, nurses face continual challenges in further developing their competences in disaster response and recovery. This paper investigates the trajectory, role and future expectations of the Norwegian nursing profession in health emergency and disaster planning, and offers a future research agenda for those interested in investigating the complex inter-relationship between disaster management and nursing. The paper seeks to respond to observations made in a recent Norwegian report (2013) on behalf of the Norwegian nursing association that highlights the need to investigate nurses’ knowledge in relation to emergency/disaster plans especially in the community health care system. The paper will explore the conceptual nuances between emergencies and disasters and then comments upon supplementary observations of the need for identifying disaster nursing practices and training programs to prepare nurses. By taking this approach, the paper also seeks to provide insights into how future demands of improving nurses´ capacities or implementation as leaders in disaster management – as highlighted by several International studies. In addition, by offering a future research agenda, the intention is that this may contribute to exploration of the role of nursing in handling long-term implications of disaster management

Grass competition overwhelms effects of herbivores and precipitation on early tree establishment in Serengeti

1. Savanna ecosystems span a diverse range of climates, edaphic conditions and disturbance regimes, the complexity of which has stimulated long‐standing interest in the mechanisms that maintain tree‐grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients. 2. To identify demographic bottlenecks that limit early tree establishment (0‐18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and A. tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation and tree species identity on two key life stages: germination and early seedling survival (0‐2 months) and juvenile seedling survival (2‐18 months). 3. Germination and early seedling survival increased as a function of rainfall, in the absence of herbivores and when seeds were scarified. Juvenile seedling survival, in contrast, decreased with rainfall but increased in the absence of herbivores. Grass removal had the single strongest (positive) effect on juvenile seedling survival of any treatment. Soil moisture monitoring and grass‐addition treatments revealed that grasses negatively affected seedlings in ways that were not necessarily linked to soil moisture. 4. A demographic model combining all effects across early life stages showed that the strength of grass competition on juvenile seedling survival was the key factor limiting early tree establishment. While rainfall had an unexpected opposing effect on the two life stages, the net effect of mean annual precipitation on early tree establishment was positive. 5. Synthesis: Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (1) scarification of seeds by browsers, (2) heavy rainfall to promote germination, (3) intensive grazing (but absence of browsers) and (4) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, our experimental results are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios

Predicted Impact of Barriers to Migration on the Serengeti Wildebeest Population

The Serengeti wildebeest migration is a rare and spectacular example of a once-common biological phenomenon. A proposed road project threatens to bisect the Serengeti ecosystem and its integrity. The precautionary principle dictates that we consider the possible consequences of a road completely disrupting the migration. We used an existing spatially-explicit simulation model of wildebeest movement and population dynamics to explore how placing a barrier to migration across the proposed route (thus creating two disjoint but mobile subpopulations) might affect the long-term size of the wildebeest population. Our simulation results suggest that a barrier to migration—even without causing habitat loss—could cause the wildebeest population to decline by about a third. The driver of this decline is the effect of habitat fragmentation (even without habitat loss) on the ability of wildebeest to effectively track temporal shifts in high-quality forage resources across the landscape. Given the important role of the wildebeest migration for a number of key ecological processes, these findings have potentially important ramifications for ecosystem biodiversity, structure, and function in the Serengeti

Disease and the Dynamics of Food Webs

What models and statistical tools can best help us assess how ecosystems respond to the impact of multiple factors, such as disease, predation, fire, and rain

Anthropogenic modifications to fire regimes in the wider Serengeti‐Mara ecosystem

Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti‐Mara ecosystem, document multidecadal temporal changes and investigate the factors underlying these patterns. We used MODIS active fire and burned area products from 2001 to 2014 to identify individual fires; summarizing four characteristics for each detected fire: size, ignition date, time since last fire and radiative power. Using satellite imagery, we estimated the rate of change in the density of livestock bomas as a proxy for livestock density. We used these metrics to model drivers of variation in the four fire characteristics, as well as total number of fires and total area burned. Fires in the Serengeti‐Mara show high spatial variability—with number of fires and ignition date mirroring mean annual precipitation. The short‐term effect of rainfall decreases fire size and intensity but cumulative rainfall over several years leads to increased standing grass biomass and fuel loads, and, therefore, in larger and hotter fires. Our study reveals dramatic changes over time, with a reduction in total number of fires and total area burned, to the point where some areas now experience virtually no fire. We suggest that increasing livestock numbers are driving this decline, presumably by inhibiting fire spread. These temporal patterns are part of a global decline in total area burned, especially in savannahs, and we caution that ecosystem functioning may have been compromised. Land managers and policy formulators need to factor in rapid fire regime modifications to achieve management objectives and maintain the ecological function of savannah ecosystems

Spatial patterning of soil microbial communities created by fungus‐farming termites

Spatially overdispersed mounds of fungus‐farming termites (Macrotermitinae) are hotspots of nutrient availability and primary productivity in tropical savannas, creating spatial heterogeneity in communities and ecosystem functions. These termites influence the local availability of nutrients in part by redistributing nutrients across the landscape, but the links between termite ecosystem engineering and the soil microbes that are the metabolic agents of nutrient cycling are little understood. We used DNA metabarcoding of soils from Odontotermes montanus mounds to examine the influence of termites on soil microbial communities in a semi‐arid Kenyan savanna. We found that bacterial and fungal communities were compositionally distinct in termite‐mound topsoils relative to the surrounding savanna, and that bacterial communities were more diverse on mounds. The higher microbial alpha and beta diversity associated with mounds created striking spatial patterning in microbial community composition, and boosted landscape‐scale microbial richness and diversity. Selected enzyme assays revealed consistent differences in potential enzymatic activity, suggesting links between termite‐induced heterogeneity in microbial community composition and the spatial distribution of ecosystem functions. We conducted a large‐scale field experiment in which we attempted to simulate termites’ effects on microbes by fertilizing mound‐sized patches; this altered both bacterial and fungal communities, but in a different way than natural mounds. Elevated levels of inorganic nitrogen, phosphorus and potassium may help to explain the distinctive fungal communities in termite‐mound soils, but cannot account for the distinctive bacterial communities associated with mounds

Comment on “The extent of forest in dryland biomes”

This is the author accepted manuscript. The final version is available from American Association for the Advancement of Science via the DOI in this record.Bastin et al. (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, leading to inappropriate management policies and practices in tropical grassy ecosystems

A Disease-Mediated Trophic Cascade in the Serengeti and its Implications for Ecosystem C

The removal of rinderpest had cascading effects on herbivore populations, fire, tree density, and even ecosystem carbon in the Serengeti ecosystem of East Africa