Human impacts on the functioning of African savannas

Human impacts increasingly threaten savanna ecosystems on the African continent. Because of strong population growth in sub-Saharan Africa, even large, protected areas, such as the Serengeti-Mara Ecosystem in East Africa, are now under great pressure due to increased migration towards the borders of these areas (so-called ‘edge areas’). This development leads to a new set of ecological challenges, which includes a better understanding of the impact of human activities in and around protected areas on the functioning of savanna ecosystems. The shift in the form and the intensity of herbivory is one of the major human-induced changes in savannas. In this thesis I investigate the consequences of this shift for 1) the resilience of the landscape to drought, 2) vegetation structure, and 3) the attractiveness of edge areas for native herbivores. I also explore the usability of camera traps in the estimation of aboveground productivity, a key ecosystem function in savannas. The results show that human impacts on savannas are predictable based on abiotic environmental variables. The recent and alarming increase in woody plants in areas grazed by livestock is, for example, progressing faster in areas with lower soil fertility and finer soil texture. A second result is that edge areas still provide habitat for native animals. This is mainly so at night and predominantly to smaller species, which can be linked to a form of safety from predators in human-modified landscapes. The insights of this thesis can contribute to new strategies for the coexistence of man and nature in savannas

Specific heat of nearly-one-dimensional tetramethyl ammonium manganese trichloride (TMMC) and tetramethyl ammonium cadmium trichloride (TMCC)

The specific heat of tetramethyl ammonium manganese trichloride and its cadmium isomorph is measured for 2&lt;T&lt;52 K. The temperature dependence of the lattice specific heat of the cadmium compound could be fitted with an expression based upon a pseudoelastic approach of the lattice vibrations in this low-dimensional system. Application of this approach to the manganese compound yields a magnetic contribution which fits the overall behavior of a one-dimensional Heisenberg model with an intrachain interaction of Jk=-6.70.5 K. Inspection of the low-temperature region yields a satisfactory agreement with the expression Cm=1.1|kTJ|+0.5|kTJ|2-0.13|kTJ|3, predicted by recent calculations.</p

Specific heat of nearly-one-dimensional tetramethyl ammonium manganese trichloride (TMMC) and tetramethyl ammonium cadmium trichloride (TMCC)

The specific heat of tetramethyl ammonium manganese trichloride and its cadmium isomorph is measured for 2&lt;T&lt;52 K. The temperature dependence of the lattice specific heat of the cadmium compound could be fitted with an expression based upon a pseudoelastic approach of the lattice vibrations in this low-dimensional system. Application of this approach to the manganese compound yields a magnetic contribution which fits the overall behavior of a one-dimensional Heisenberg model with an intrachain interaction of Jk=-6.70.5 K. Inspection of the low-temperature region yields a satisfactory agreement with the expression Cm=1.1|kTJ|+0.5|kTJ|2-0.13|kTJ|3, predicted by recent calculations.</p

Quasi-one-dimensional behavior of (CH3)2NH2MnCl3 (DMMC)

The heat capacity of dimethyl ammonium manganese trichloride has been investigated for 1.6&lt;T&lt;50 K. A transition to a three-dimensional antiferromagnetically ordered state has been observed at 3.60 K, which is supported by nuclear-magnetic-resonance and susceptibility measurements. The critical entropy did amount to 3.4%. The magnetic heat capacity in the paramagnetic region could be described very well by a S=52 Heisenberg linear chain system with Jk=-5.8±0.7 K. The data for kT|J|&lt;1.5, together with the earlier data on (CH3)4NMnCl3, corroborate the suggested low-temperature behavior of such a system.</p

Afgestudeerd, en dan…: Een internationale baan in de gemeentelijke wereld

Als student is het vaak lastig een beeld te vormen van een mogelijke baan. In een serie portretten van bestuurskundigen op de arbeidsmarkt wordt getracht meer duidelijkheid te scheppen. Ik zal hieraan een bijdrage leveren met een beschrijving van mijn werk bij het International Project Unit (IPU) van de Vereniging van Nederlandse Gemeenten (VNG}

Onder professoren

Prof.dr. Th.A.J. Toonen is één van de drie nieuwe hoogleraren die per 1 september in Leiden/Rotterdam zijn aangesteld. Een gesprek met de benjamin onder de professoren

Stageberichten

Jaco Post studeert sinds 1985 bestuurskunde aan de Universiteit Twente. In het begin van zijn derde jaar werd zijn belangstelling gewekt om bij een politieke partij stage te lopen