Browsing herbivores improve the state and functioning of savannas: A model assessment of alternative land-use strategies.

Changing climatic conditions and unsustainable land use are major threats to savannas worldwide. Historically, many African savannas were used intensively for livestock grazing, which contributed to widespread patterns of bush encroachment across savanna systems. To reverse bush encroachment, it has been proposed to change the cattle-dominated land use to one dominated by comparatively specialized browsers and usually native herbivores. However, the consequences for ecosystem properties and processes remain largely unclear. We used the ecohydrological, spatially explicit model EcoHyD to assess the impacts of two contrasting, herbivore land-use strategies on a Namibian savanna: grazer- versus browser-dominated herbivore communities. We varied the densities of grazers and browsers and determined the resulting composition and diversity of the plant community, total vegetation cover, soil moisture, and water use by plants. Our results showed that plant types that are less palatable to herbivores were best adapted to grazing or browsing animals in all simulated densities. Also, plant types that had a competitive advantage under limited water availability were among the dominant ones irrespective of land-use scenario. Overall, the results were in line with our expectations: under high grazer densities, we found heavy bush encroachment and the loss of the perennial grass matrix. Importantly, regardless of the density of browsers, grass cover and plant functional diversity were significantly higher in browsing scenarios. Browsing herbivores increased grass cover, and the higher total cover in turn improved water uptake by plants overall. We concluded that, in contrast to grazing-dominated land-use strategies, land-use strategies dominated by browsing herbivores, even at high herbivore densities, sustain diverse vegetation communities with high cover of perennial grasses, resulting in lower erosion risk and bolstering ecosystem services.

Potash mining effluents induce moderate effects on histopathological and physiological endpoints of adult zebrafish (Danio rerio).

Stress in fish can be caused by a variety of factors and has the potential to evoke stress responses leading to a reduction of physical condition and of health. The river Werra (Germany) presents a severe case of secondary salinisation caused by potash mining activities. The model organism Danio rerio was exposed to different ion-concentrations depicting current (HT) and future (LT) threshold values of the Werra, as well as to solutions with single-exceeding ions (Mg2+ + K+ (KMg), Mg2+ (Mg) and K+ (K)). After a six-week exposure period, cortisol levels, growth and weight were measured, gills and gonads were histologically analysed and mRNA expression of follicle stimulating hormone (FSH), luteinising hormone (LH), growth hormone (GH) and prolactin (PRL) were determined. Cortisol was still elevated in fish in the HT and K group, indicating moderate stress. However, gills revealed structural changes in zebrafish in all exposure groups, size of oocytes differed in the LT and K group, male FSH mRNA levels were elevated in the HT and LT group whereas PRL mRNA levels were lower in HT and LT for both, male and female fish. These results suggest that ion-stress induces moderate effects on a variety of biological parameters that mainly serve to adapt to elevated ion concentrations. For these reasons current and even future thresholds should be reconsidered, including thresholds for total as well as single ion concentrations. Future research looking at the effects on local fish species is needed, along with regular and long-term monitoring of environmental conditions, species abundance and diversity.

Potash mining effluents and ion imbalances cause transient osmoregulatory stress, affect gill integrity and elevate chronically plasma sulfate levels in adult common roach, Rutilus rutilus.

Secondary salinization is a growing global ecological issue. One cause is the discharge of effluents by the potash mining industry into surface waters such as the River Werra in Germany. Increases of major ions require various physiological responses of freshwater organisms to maintain the hydromineral balance of body fluids. However, only little is known about the acute and chronic effects of high concentrations and imbalances of ions on osmoregulation in freshwater teleosts. The present study aimed to elucidate the effects of potash mining effluents and different cation ratios on the osmoregulatory capacity and gill histopathology of a native fish species. Individuals of Rutilus rutilus were exposed to the currently allowed (HT) and intended future (LT) thresholds as well as to high concentrations of Mg2+ (Mg), K+ (K), and Mg2+ and K+ (Mg + K) for a period of 24 h, 7 d, 21 d and 8 wk. Plasma osmolarity, [Na+], [Mg2+], [K+], [Ca2+], [Cl-] and [SO42-] and branchial Na+/K+-ATPase activity were determined. Moreover, histological gill alterations after 21 d and muscle water content after 8 wk were examined. HT transiently (24 h) elevated plasma osmolarity, plasma [Na+] and [Ca2+], whereas [SO42-] was chronically increased even after 8 wk. Exposure to LT, Mg and Mg + K led to increased [SO42-] levels for at least 21 d. It seems that [SO42-] is mainly disturbed by multiple ions at high concentrations and long-term effects are unknown. Hydromineral homeostasis was maintained as indicated by unchanged Na+/K+-ATPase activity and muscle water content. However, mild structural alterations of the gills were observed in all exposure groups suggesting adaptational responses but with the potential to affect gas exchange capacity. Hence, the current thresholds for potash mining effluents affect osmomineral regulation in roach and further investigations should address potential impacts on reproduction in native fish species and physiological effects of SO42-.