PhD ceremony Mr. J.L. Ruifrok: Herbivore-mediated structural diversity of vegetation

PhD ceremony: Mr. J.L. Ruifrok

Dissertation: Herbivore-mediated structural diversity of vegetation

Promotor(s): prof. H. Olff

Faculty: Mathematics and Natural Sciences

Large herbivores can positively affect structural diversity in vegetation, but not always do. The goal of the thesis of Jasper Ruifrok is to gain more knowledge about the conditions under which large herbivores positively affect structural diversity. Ruifrok combined modeling, descriptive and experimental approaches. He found that abiotic heterogeneity is an important predictor whether or not low-intensity grazing successfully generates and maintains lawn-rough mosaics. These mosaics consist of patches of lawn, dominated by short plants, alternating with patches of tall roughs. Ruifrok discovered that lawn and rough patches show unsynchronized succession, where rough patches are more likely to transform in woodland patches than lawn patches. Furthermore, at small spatial scales grazed lawn-rough mosaics have higher plant species richness compared to ungrazed sites. At larger spatial scales, it depends on the underlying abiotic heterogeneity whether grazing promotes plant species richness. Next Ruifrok investigated tree recruitment in the Oostvaardersplassen (OVP), a nature reserve in the Netherlands, where the herbivore community is fully bottom-up controlled. He found that tree recruitment in the OVP is strongly limited by herbivores. However, fluctuating herbivore densities in time or space may accelerate the development of structural diversity in the OVP. These fluctuations of herbivore density in time may be generated by the interaction between different-sized herbivores and their differential vulnerability to predation. The findings of Ruifrok’s thesis can contribute to successful development of structural diverse ecosystems of high ecological and social-economic value.