LETTER The intermediate disturbance hypothesis applies to tropical forests, but disturbance contributes little to tree diversity Frans Bongers, 1* Lourens Poorter, 1,2 William D. Hawthorne 3 and Douglas Sheil 4 1 Forest Ecology and Forest Management Group, Centre for Ecosystem Studies, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands 2 Resource Ecology Group, Centre for Ecosystem Studies, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands 3 Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, United Kingdom 4 Institute of Tropical Forest Conservation (Mbarara University of Science and Technology), PO Box 44, Kabale, Uganda *Correspondence: E-mail: Frans.Bongers@wur.nl Abstract The intermediate disturbance hypothesis (IDH) predicts local species diversity to be maximal at an intermediate level of disturbance. Developed to explain species maintenance and diversity patterns in species-rich ecosystems such as tropical forests, tests of IDH in tropical forest remain scarce, small-scale and contentious. We use an unprecedented large-scale dataset (2504 one-hectare plots and 331 567 trees) to examine whether IDH explains tree diversity variation within wet, moist and dry tropical forests, and we analyse the underlying mechanism by determining responses within functional species groups. We find that disturbance explains more variation in diversity of dry than wet tropical forests. Pioneer species numbers increase with disturbance, shade-tolerant species decrease and intermediate species are indifferent. While diversity indeed peaks at intermediate disturbance levels little variation is explained outside dry forests, and disturbance is less important for species richness patterns in wet tropical rain forests than previously thought. Keywords Disturbance, Ghana, guild, intermediate disturbance hypothesis, shade tolerance, species density, species richness, tropical forest. Ecology Letters (2009) 12: 1–8 INTRODUCTION The intermediate disturbance hypothesis (IDH) predicts that local species diversity is maximized at an intermediate level of disturbance (Grime 1973; Connell 1978; Huston 1979; Wilkinson 1999). Although developed specifically for species-rich ecosystems such as tropical rain forests, tests of the IDH in tropical forests remain scarce (Burslem & Whitmore 1999; Sheil 1999; ter Steege 2003), small-scale (Sheil & Burslem 2003; Roxburgh et al. 2004; Shea et al. 2004) and contentious. The IDH has been expressed and interpreted in various ways combining various phenomena (Connell 1978; Huston 1979; Sheil & Burslem 2003; Roxburgh et al. 2004; Shea et al. 2004; Haddad et al. 2008). ConnellÕs (1978) original formulation proposes that in a community or forest left undisturbed, competitive exclusion will ultimately eliminate all but the late successional species from the system – a process that can be prevented by disturbance. Disturbance creates an environment in which some species such as light-demanding pioneer trees can establish and persist, adding to overall species diversity (Fig. 1). Too much disturbance also reduces overall diversity by eliminating sensitive late-succession species. Thus, there is an optimum degree of disturbance, which maximizes the number of species that can be maintained. When applied in a forest where disturbance histories are localized and patchy, the IDH includes a spatial interpretation in which the variety of local disturbance histories contributes to and maintains species richness. All these interpretations necessarily require that species differ in their need for and responses to disturbance (i.e. to events that remove or destroy biomass, or otherwise drastically change resource availability; Pickett & White 1985). In their meta-analysis covering various communities and organisms, Shea et al. (2004) found that the peaked relationship implied by the IDH was observed in a broad Ecology Letters, (2009) 12: xxx–xxx doi: 10.1111/j.1461-0248.2009.01329.x Ó 2009 Blackwell Publishing Ltd/CNRS