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INTRODUCTION
Human activities, such as forest fragmentation and agricultural intensification, threaten biodiversity (Sala et al. 2000, Tylianakis et al. 2008). The consequences of this biodiversity decline for ecological processes are far from being understood (Balvanera et al. 2006). Accordingly, antagonistic interactions, like insect herbivory, might be subject to changes with potentially critical outcomes for forest regeneration.
Insect herbivory is the predominant form of leaf damage in (sub-)tropical forests (García-Guzmán & Dirzo 2001). Insect herbivory on woody seedlings and saplings may not only affect growth, productivity and survival of individual plants, but also alter the dynamics and structure of (sub-)tropical forests (Maron & Crone 2006).
Insect herbivory on woody seedlings and saplings has been shown to be affected by forest fragmentation (Ruiz-Guerra et al. 2010, Wirth et al. 2008). In a recent review Wirth et al. (2008) identified three main factors at forest edges affecting herbivory, which should also be applicable to forest fragments: changes in (1) environmental conditions, (2) resource availability and quality and (3) trophic interactions. For instance, altered environmental conditions at forest edges lead to increased tree mortality in forest fragments (Laurance et al. 2001, 2006). Declined tree diversity might amplify insect herbivory in forest fragments as herbivory tends to be higher in less diverse and complex systems (Jactel & Brockerhoff 2007, McCann et al. 1998). Furthermore, the proliferation of light-demanding pioneer species at forest edges (Laurence et al. 2006) may alter resource availability and quality for herbivores in forest fragments. As pioneer trees are generally preferred by herbivores due to their higher nutritive value (Coley 1980), herbivory has been observed to be higher in pioneer-dominated forest stands (Richards & Coley 2007, Ruiz-Guerra et al. 2010).
Despite the evidence indicating that reduced forest-fragment quality should intensify insect herbivory in forest fragments, many studies have found less herbivory or a neutral response to fragmentation (Benítez-Malvido & Lemus-Albor 2005, Ruiz-Guerra et al. 2010). Possibly, fragmentation limits herbivore dispersal (Faveri et al. 2008). Consequently, also the matrix landscape surrounding forest fragments may influence herbivory (Tscharntke & Brandl 2004). Depending on permeability, matrices can complement natural habitat and facilitate dispersal of, for example, insectivorous predators or herbivores among habitat...