Large herbivores are key drivers of nutrient cycling in ecosystems worldwide, and hence they have an important influence on the productivity and species composition in plant communities. Classical theories describe that large herbivores can accelerate or decelerate nitrogen (N) mineralization by altering the quality and quantity of resource input (e.g. dung, urine, plant litter) into the soil food web. However, in many situations the impact of herbivores on N mineralization cannot be explained by changes in resource quality and quantity. In this paper, we aim to reconcile observations of herbivores on N mineralization that were previously regarded as contradictory. We conceptually integrate alternative pathways via which herbivores can alter N mineralization. We illustrate our new integrated perspective by using herbivore-induced soil compaction and subsequent changes in soil moisture and soil aeration as an example. We show that the net effect of herbivores on mineralization depends on the balance between herbivoreinduced changes in soil physical properties and changes in the quality and quantity of resource input into the soil food web. For example, soil compaction by herbivores can limit oxygen or water availability in wet and dry soils respectively, particularly those with a fine texture. This can result in a reduction in N mineralization regardless of changes in resource quality or quantity. In such systems the plant community will shift towards species that are adapted to waterlogging (anoxia) or drought, respectively. In contrast, soils with intermediate moisture levels are less sensitive to compaction. In these soils, N mineralization rates are primarily associated with changes in resource quality and quantity. We conclude that our integrated perspective will help us to better understand when herbivores accelerate or decelerate soil nutrient cycling and improve our understanding of the functioning of grazed ecosystems.

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Question: Thorny shrubs play keystone roles in grazed ecosystems by defending non‐protected plants against herbivores, but their establishment in grazed ecosystems is poorly understood. Which factors control establishment of recruits of thorny nurse shrubs in grazed temperate woodlands? Location: Ancient grazed temperate woodlands (52°32′N, 6°36′E), The Netherlands. Methods: We surveyed biotic and abiotic factors for saplings of thorny nurse shrubs in plots with and without saplings. To disentangle these factors, we performed a transplantation experiment over two growing seasons with nurse shrub saplings (Prunus spinosa and Crataegus monogyna) planted in two dominant vegetation types – tall unpalatable swards and short grazed lawns – half of them protected from herbivory via exclosures. Results: Plots with shrub saplings had taller surrounding vegetation, higher soil pH and higher soil moisture than plots without saplings. These plots predominantly contained unpalatable sward species, while plots without saplings mainly contained palatable lawn species. After transplantation, sapling survival was higher in exclosures than in the open, and higher in sward exclosures than in lawn exclosures. Sapling growth was higher in swards than in lawns, higher inside than outside exclosures, and higher for Prunus than Crataegus, while browsing on saplings was higher in lawns. Conclusion: Unpalatable swards form essential establishment niches for thorny shrubs in grazed temperate woodlands: they protect against herbivores before thorns fully develop in saplings, and sapling growth is better due to improved micro‐environmental conditions. Once established and thorny, shrub saplings grow out of the protective range of the swards and in turn facilitate tree seedlings, which are essential for long‐term persistence of grazed temperate woodlands. This study shows that nurse plants may start as protégés before becoming facilitators for other plants in a later life stage. This may be common for nurse plants in various ecosystems. We argue that improved understanding of establishment of nurse plants and their constraining factors is crucial for effective conservation and restoration in various ecosystems.

Please cite this article as: Ruifrok, J.L., Janzen, T., Kuijper, D.P.J., Rietkerk, M., Olff, H., Smit, C., Cyclical succession in grazed ecosystems: The importance of interactions between different-sized herbivores and different-sized predators. Theoretical Population Biology (2015), http://dx.doi.org/10. 1016/j.tpb.2015.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. (1) 26 tolerance of low quality forage and (2) vulnerability to predation, which both impact the 27 composition and dynamics of natural communities. However, no study has thus far explored how 28 the combination of these two body-size related traits affect the long-term composition and 29 dynamics of the herbivore and plant communities.We made a simple model of ordinary 30 differential equations and simulated a grassland system with three herbivore species (small, 31 medium, large) and two predator species (small, large) to investigate how the combination of 32 low-quality tolerance and predation-vulnerability structure the herbivore and plant community. 33We found that facilitation and competition between different-sized herbivores and predation by 34 especially small predators stimulates coexistence among herbivore species. Furthermore, the 35 interaction between different-sized herbivores and predators generated cyclical succession in the 36 plant community, i.e. alternating periods of short vegetation dominated by high-quality plants, 37 with periods of tall vegetation dominated by low-quality plants. Our results suggest that cyclical 38 succession in plant communities is more likely to occur when a predator predominantly preys on 39 small herbivore species. Large predators also play an important role, as their addition relaxed the 40 set of conditions under which cyclical succession occurred. Consequently, our model predictions 41 suggest that a diverse predator community plays an important role in the long-term dynamics and 42 maintenance of diversity in both the herbivore and plant community. 43 44

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