Browsing by Subject "stability"

Now showing 1 - 4 of 4
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    EFFECTS OF FIRE AND HERBIVORY ON THE STABILITY OF SAVANNA ECOSYSTEMS
    (2003) Van Langevelde, Frank; Van De Vijver, Claudius A D M; Kumar, Lalit; Van De Koppel, Johan; De Ridder, Nico; Van Andel, Jelte; Skidmore, Andrew K; Hearne, John W; Stroosnijder, Leo; Bond, William J; Prins, Herbert H T; Rietkerk, Max
    Savanna ecosystems are characterized by the co-occurrence of trees and grass-es. In this paper, we argue that the balance between trees and grasses is, to a large extent, determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous chang-es, such as bush encroachment, on sandy soils. Such changes, however, are frequently ob-served. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.
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    Presence and Persistence of Putative Lytic and Temperate Bacteriophages in Vaginal Metagenomes from South African Adolescents
    (2021-11-23) Happel, Anna-Ursula; Balle, Christina; Maust, Brandon S; Konstantinus, Iyaloo N; Gill, Katherine; Bekker, Linda-Gail; Froissart, Rémy; Passmore, Jo-Ann; Karaoz, Ulas; Varsani, Arvind; Jaspan, Heather
    The interaction between gut bacterial and viral microbiota is thought to be important in human health. While fluctuations in female genital tract (FGT) bacterial microbiota similarly determine sexual health, little is known about the presence, persistence, and function of vaginal bacteriophages. We conducted shotgun metagenome sequencing of cervicovaginal samples from South African adolescents collected longitudinally, who received no antibiotics. We annotated viral reads and circular bacteriophages, identified CRISPR loci and putative prophages, and assessed their diversity, persistence, and associations with bacterial microbiota composition. Siphoviridae was the most prevalent bacteriophage family, followed by Myoviridae, Podoviridae, Herelleviridae, and Inoviridae. Full-length siphoviruses targeting bacterial vaginosis (BV)-associated bacteria were identified, suggesting their presence in vivo. CRISPR loci and prophage-like elements were common, and genomic analysis suggested higher diversity among Gardnerella than Lactobacillus prophages. We found that some prophages were highly persistent within participants, and identical prophages were present in cervicovaginal secretions of multiple participants, suggesting that prophages, and thus bacterial strains, are shared between adolescents. The number of CRISPR loci and prophages were associated with vaginal microbiota stability and absence of BV. Our analysis suggests that (pro)phages are common in the FGT and vaginal bacteria and (pro)phages may interact.
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    The implications of bank risk-weighted capital and ownership on portfolio rebalancing, profitability, and stability: evidence from Tanzania
    (2024) Mchembe, Renatus Anathory; Toerien, Francois; Ndlovu Godfrey
    Banks are vital for economic growth and development as they mobilise and channel the flow of funds from surplus to deficit units and help to finance government expenditure by investing in government securities, among other activities. Bank activities are especially crucial in developing countries, due to underdeveloped financial markets, as they often dominate the financial sector. The banking crises of the last three decades have resulted in increased macroprudential regulations to protect the banking sector against the risk of failure emanating from banks' loan portfolios. A key element of these regulations is usually the prescription of minimum risk-weighted capital ratios. However, this can potentially affect bank profitability and in an attempt to rebalance their capital risk profile to match regulatory requirements, banks may alter the balance between bank lending (higher risk activities) and low risk activities, such as investment in government securities. Contradictory evidence exists on the relations
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    Vector theories in cosmology
    (2010) Esposito-Farèse, Gilles; Pitrou, Cyril; Uzan, Jean-Philippe
    This article provides a general study of the Hamiltonian stability and the hyperbolicity of vector field models involving both a general function of the Faraday tensor and its dual, $f(F2,Ftilde F)$, as well as a Proca potential for the vector field, $V(A2)$. In particular it is demonstrated that theories involving only $f(F2)$ do not satisfy the hyperbolicity conditions. It is then shown that in this class of models, the cosmological dynamics always dilutes the vector field. In the case of a nonminimal coupling to gravity, it is established that theories involving $R f(A2)$ or $Rf(F2)$ are generically pathologic. To finish, we exhibit a model where the vector field is not diluted during the cosmological evolution, because of a nonminimal vector field-curvature coupling which maintains second-order field equations. The relevance of such models for cosmology is discussed. Comment: 17 pages, no figure