LETTER Interactions between spatially separated herbivores indirectly alter plant diversity Jasper van Ruijven, 1 *† Gerlinde B. De Deyn, 2 Ciska E. Raaijmakers, 2 Frank Berendse 1 and Wim H. van der Putten 2 1 Nature Conservation and Plant Ecology Group, Wageningen University and Research centre, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands 2 Department of Multitrophic interactions, Centre for Terrestrial Ecology, Netherlands Institute of Ecology (NIOO- KNAW), PO Box 40, 6666 ZG Heteren, The Netherlands †Present address: Ecology and Evolution Section, Department of Biological Sciences, Silwood Park Campus, Imperial College London, Ascot, Berkshire SL5 7PY, UK. *Correspondence: E-mail: jasper.vanruijven@wur.nl Abstract Above- and belowground herbivores promote plant diversity when selectively feeding on dominant plant species, but little is known about their combined effects. Using a model system, we show that neutral effects of an aboveground herbivore and positive effects of a belowground herbivore on plant diversity became profoundly negative when adding these herbivores in combination. The non-additive effects were explained by differences in plant preference between the aboveground- and the belowground herbivores and their consequences for indirect interactions among plant species. Simultaneous exposure to aboveground- and belowground herbivores led to plant communities being dominated by a few highly abundant species. As above- and belowground invertebrate herbivores generally differ in their mobility and local distribution patterns, our results strongly suggest that aboveground–belowground interactions contribute to local spatial hetero- geneity of diversity patterns within plant communities. Keywords Above–belowground interactions, indirect effects, insect herbivores, nematodes, plant diversity. Ecology Letters (2005) 8: 30–37 INTRODUCTION Terrestrial ecosystems consist of explicit above- and belowground subsystems. Spatially separated inhabitants of these subsystems influence each other, ecosystem properties and processes (Wardle 2002). Most changes in ecosystem processes occur through impacts of above- and below- ground organisms on plant community structure and composition (Wardle et al. 2004). Above- and belowground herbivores, pathogens, and mutualists have been identified as drivers of plant diversity, and consequently, ecosystem functioning (Crawley 1997; Olff & Ritchie 1998; van der Heijden et al. 1998; Clay & Holah 1999; Hartnett & Wilson 1999; Mulder et al. 1999; De Deyn et al. 2003). The interdependence of above- and belowground her- bivores has been acknowledged (van der Putten et al. 2001), but their joint effects on plant communities rarely have been studied (Bardgett & Wardle 2003). On a shared host plant, above- and belowground herbivores affect each other through changes in the production and allocation of primary (Seastedt et al. 1988; Masters et al. 2001; Poveda et al. 2003) and secondary (Bezemer et al. 2003) plant metabolites. Experimental studies on these kinds of interactions, however, are often limited to one plant species (van Dam et al. 2003). In reality, above- and belowground interactions take place in diverse plant communities. Under these complex conditions, above- and belowground herbivo- res may not only affect each other through changes in the host plant, but also through changes in plant community structure by affecting interactions among plant species. The established view on interactions between above- and belowground herbivores and plant community dynamics is mainly based on the application of soil and foliar insecticides in the field. These pioneering studies suggested that aboveground invertebrate herbivores mainly affect grasses, while belowground invertebrates influence dicots (Brown & Gange 1989, 1992). Most aboveground insect herbivores, however, feed on only one or a few genera or one family of Ecology Letters, (2005) 8: 30–37 doi: 10.1111/j.1461-0248.2004.00688.x Ó2004 Blackwell Publishing Ltd/CNRS