Biodiversity and Conservation 11: 1663–1680, 2002.  2002 Kluwer Academic Publishers. Printed in the Netherlands. Herbivory in a fragmented tropical forest: patterns ´ from islands at Lago Gatun, Panama 1, 2 * A. ELIZABETH ARNOLD and NIGEL M. ASQUITH 1 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA; 2 * Department of Zoology, Duke University, Durham, NC 27708, USA; Author for correspondence (e-mail: betsya@email.arizona.edu; fax: 11-520-621-9190) Received 28 February 2001; accepted in revised form 15 October 2001 Key words: Barro Colorado Island, Forest fragments, Heisteria concinna, Herbivory, Islands, Lepidop- tera, Ouratea lucens, Panama, Protium panamense, Protium tenuifolium Abstract. By imposing density-dependent mortality upon their hosts, specialist insect herbivores are thought to contribute to the maintenance of tree diversity in tropical forests. Forest fragmentation may alter patterns of herbivory, however, which may have important implications for tree species diversity in forest remnants. To explore effects of fragmentation on patterns of herbivory, we assessed folivory by Lepidopteran larvae on saplings of four focal tree species on eight artificial, forested islands at Lago ´ Gatun, Panama. We explored the importance of island area, distance to larger land, exposure to dry season winds, tree species, and season in determining proportions of new leaves damaged by caterpillars, and proportions of leaf area lost to caterpillars, during two dry and wet seasons. We found that both measures of herbivory increased markedly with island area, that island isolation had no apparent effect on herbivory, and that interactions between season and exposure, and between tree species and season, were important determinants of herbivory rates. In addition, we observed species-specific differences in herbivory among host plants under various conditions imposed by fragmentation. We conclude that patterns of herbivory by Lepidopteran larvae are sensitive to fragmentation in this tropical forest. Differential herbivory among the four tree species considered here may have important implications for ´ tree species dynamics on the islands of Lago Gatun. Introduction In intact tropical forests, density-dependent mortality imposed by host-specific pathogens and insect herbivores is thought to contribute to the maintenance of tree species diversity (Janzen 1970; Clark and Clark 1984; Barone 1998). Insects account for ca. 70% of herbivory in some tropical forests (Coley and Barone 1996), and Lepidopteran larvae, many of which have relatively narrow diet breadths in tropical forests (DeVries 1985; Marquis and Braker 1991; Barone 1998), are among the most damaging folivores in terms of leaf area lost (e.g., Janzen 1988; Aide and ˜ Lodono 1989; Sagers 1992). Many authors have noted that Lepidopteran diversity and abundance tend to increase with forest fragmentation (Brown and Hutchings 1997; Cappuccino and Martin 1997; DeVries et al. 1997), suggesting that herbivory rates by caterpillars may be higher in more fragmented forests. However, species- abundance records in several sites suggest that deterministic or random loss of individual herbivore species is likely following fragmentation (e.g., Spitzer et al.