Project

Rangeland Ecology

Rangelands cover a broad diversity of ecosystems worldwide and provide key ecosystem services at regional and global scales. Their economic and ecological importance are enormous as providers of major tourist revenues, economic services and space for free-ranging livestock or wildlife.

Our Rangeland Ecology Programme has as its mission “To improve understanding and conservation of biodiversity, ecosystem services, and human well-being in rangelands through performing state of the art ecosystem science, often in collaboration with the social sciences and veterinary sciences, and teach about this at the highest possible academic level“. Our Rangeland Ecology programme focuses on the tri-trophic interaction between resources – vegetation – herbivores, which are managed and exploited by people (Prins and Van Langevelde 2008).

The first theme of this programme is the relationship between soil resources (moisture and nutrients), the response of the vegetation, and the quality of the vegetation as food for herbivores (Olff et al. 2002, Ahrestani et al. in press). For example, we have shown that trees and understory grasses can benefit from hydraulic lift (Ludwig et al. 2003) and grasses can use this hydraulically lifted water and its associated nutrients (Ludwig et al. 2008) to the benefit of herbivores (Treydte et al. 2009). In TEMBO (The Elephant Movements and Bio-economic Optimality programme), we manipulated nutrients at different spatial scales and densities through a large scale fertilization experiment. The scale of nutrient patchiness was found to control the co-existence of trees and grasses (Van der Waal 2009). Elephant were able to select nutrient-rich patches at large scales (Pretorius et al. 2011), suggesting that the scale of nutrient patchiness can initiate a trophic cascade, which may feedback to structural changes in the vegetation via altered herbivore impact patterns (Van der Waal et al. 2011). The density of trees and grasses had consequences for movement patterns of herbivores (De Knegt et al. 2007, Hengeveld et al. 2009), as we also showed for elephant (De Knegt et al. 2010, 2011). This work resulted in a RUBICON scholarship.

The second theme is the effect of fire and herbivory on trees and grasses. Our modelling studies show that there is a positive feedback mechanism between fuel load for fire (grass biomass) and intensity of fire (Van Langevelde et al. 2003, De Knegt et al. 2008, Mourik et al. 2007, Groen et al. 2011). This feedback can cause sudden increases in tree cover (bush encroachment), which might have severe consequences for the economy of millions of people in rangelands. Experimentally we study this in South Africa (PhD student Mkize Ntuthuko).

The third theme deals with understanding tree recruitment in rangelands and how can be affected by bottom up and top down mechanisms. We have on-going field experiments in dry and wet sites in 4 continents (Africa, Australia, India and South America) within the GEST project (Global Experiment on Savanna Tree seedlings with 2 postdocs and 6 PhD students). Here we compare the importance of the different environmental variables and test whether different responses of species could be explained by their phylogenetic relatedness. As climate variability directly affects tree recruitment, we are investigating the effects of climate driven resource pulses on tree recruitment using a combination of experimental, remote sensing and modelling approaches in tropical and temperate ecosystems including deserts, shrublands, savannas, forests and peatlands) (Holmgren et al. 2006a,b, Scheffer et al. 2008; Sitters et al. 2011).

For the period 2011-2015 we will continue the work on these three themes; we currently applied for grants to continue the GEST project and the intricate similarities in climate responses in the tropical and temperate ecosystems.