title: Herbivore exclusion promotes a more stochastic plant community assembly in a natural grassland.
abstract: Both bottom-up (e.g., nutrients) and top-down (e.g., herbivory) forces structure plant communities, but it remains unclear how they affect the relative importance of stochastic and deterministic processes in plant community assembly. Moreover, different-sized herbivores have been shown to have contrasting effects on community structure and function, but their effects on the processes governing community assembly (i.e., how they generate the impacts on structure) remain largely unknown. We evaluated the influence of bottom-up and top-down forces on the relative importance of deterministic and stochastic processes during plant community assembly. We used the data of a 7-yr factorial experiment manipulating nutrient availability (ambient and increased) and the presence of vertebrate herbivores (>1 kg) of different body size in a floodplain grassland in The Netherlands. We used a null model that describes a community composition expected by chance (i.e., stochastic assembly) and compared the plant community composition in the different treatments with this null model (the larger the difference, the more deterministically assembled). Our results showed that herbivore exclusion promoted a more stochastic plant community assembly, whereas increased nutrients played a relatively minor role in determining the relative importance of stochasticity in community assembly. Large herbivores facilitated intermediate-sized mammal herbivores, resulting in synergistic effects of enhanced grazing pressure and a more deterministic and convergent plant community assembly. We conclude that herbivores can act as strong deterministic forces during community assembly in natural systems. Our results also reveal that although large- and intermediate-sized mammal herbivores often have contrasting effects on many community and ecosystem properties, they can also synergistically homogenize plant communities.
authors.affiliation: Laboratorio de Ecología, Instituto de Investigaciones Marinas y Costeras (IIMyC), Universidad Nacional de Mar del Plata, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), CC 1260 Correo Central, B7600WAG, Mar del Plata, Argentina.
authors.forename: Juan
authors.initials: J
authors.lastname: Alberti
authors.affiliation: Department of Aquatic Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708, PB Wageningen, The Netherlands.
authors.forename: Elisabeth S
authors.initials: ES
authors.lastname: Bakker
authors.affiliation: Institute of Botany, Czech Academy of Sciences, Dukelská 135, 37982, Tĭeboň, Czech Republic.
authors.forename: Roel
authors.initials: R
authors.lastname: van Klink
authors.affiliation: Conservation Ecology Group, Groningen Institute of Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700, CC Groningen, The Netherlands.
authors.forename: Han
authors.initials: H
authors.lastname: Olff
authors.affiliation: Conservation Ecology Group, Groningen Institute of Evolutionary Life Sciences, University of Groningen, P.O. Box 11103, 9700, CC Groningen, The Netherlands.
authors.forename: Christian
authors.initials: C
authors.lastname: Smit
mesh_terms: D000818:Animals; D044822:Biodiversity; D017753:Ecosystem; D065948:Grassland; D060434:Herbivory; D009426:Netherlands; D010944:Plants / Q000145:classification*
keywords: bottom-up effects; determinants of plant community diversity and structure; different-sized herbivores; null model; random and deterministic processes; top-down effects