A Geographic Mosaic of Plant–Pollinator Interactions in the Eastern Caribbean Islands Vinita Gowda 1,2,3 and Walter J. Kress 2 1 Department of Biological Sciences, George Washington University, Washington, DC, 20052, U.S.A. 2 Department of Botany, Smithsonian Institution, MRC-166, PO Box 37012, Washington, DC, 20013-0712, U.S.A. ABSTRACT The interplay between generalized and specialized plant–animal interactions is a core concept in understanding the evolution of mutual- isms. Within the Eastern Caribbean, Heliconia bihai is a dominant forest species in the southern island of St. Vincent where H. caribaea is virtually absent. Heliconia caribaea is most common on the northern island of St. Kitts where H. bihai is restricted to the tops of the high- est peaks. Both species are abundant on the central island of Dominica. We compared flowering patterns, nectar characteristics, and visi- tation frequency of hummingbirds in the two heliconias on the three islands to determine the extent of geographic variations in this plant–pollinator mutualism. The peak flowering season of the two heliconias was observed to be in April–May on all three islands with little within- and between-island variations. Nectar production significantly varied between species and between islands. Visitation pat- terns by the principal hummingbird pollinators also varied between the islands: (1) on Dominica, only females of a single species of hummingbird pollinated the flowers of H. bihai (sexual specialization), whereas both sexes of the same hummingbird pollinated the flow- ers of H. caribaea (species specialization); (2) on St. Vincent, both sexes of the same hummingbird pollinated the flowers of H. bihai (spe- cies specialization); and (3) on St. Kitts, only females pollinated the flowers of H. bihai (sexual specialization), whereas several species of hummingbird visited the flowers of H. caribaea (species generalization). We propose that the Heliconia–hummingbird interactions in the Eastern Caribbean represent a geographically variable coevolutionary mosaic of plant–pollinator interactions. Key words : adaptation; coevolution; Eulampis jugularis; geographic mosaic of coevolution; phenology; pollination; visitation frequency; visitation rate. EMPIRICAL STUDIES OF SPATIAL, TEMPORAL, AND COMMUNITY-LEVEL HETEROGENEITY IN PLANT .—pollinator systems are fundamental to our understanding of processes by which both plants and their pollinators evolve and maintain a mutualistic relationship (Herrera 1988, Campbell et al. 1996, Olesen & Jordano 2002, Fenster et al. 2004, Moeller 2005, Herrera et al. 2006, Anderson & Johnson 2008). To address the prevalence and importance of population- level heterogeneity across both spatial and temporal scales in coevolving interactions, Thompson (1994, 1999, 2005) proposed the Geographic Mosaic of Coevolution (GMC henceforth) to describe the patterns of mutualisms and provide testable hypothe- ses for coevolving landscapes. Three main predictions as outlined by Thompson (1999, Page S3) relevant to our current study are: (1) populations will differ in the traits shaped by an interaction; (2) traits of interacting species will be well matched in some com- munities (‘coevolutionary hot spots’) and mismatched in others (‘coevolutionary cold spots’), i.e., variable in degrees of adaptation and maladaptation (Gomulkiewicz et al. 2000); and (3) few traits associated with the mutualism will be common across the entire range of the interacting species (Thompson 1994, 2005). As a first step in testing the predictions of the GMC in the Heliconia– hummingbird system in the Eastern Caribbean Islands, we quan- tified the variation in pollinator visitation traits and interactions across the three islands within the Eastern Caribbean archipelago. Members of the Heliconiaceae are widespread in the Neo- tropics (one genus containing ~200 species), but only two native species are found in the Eastern Caribbean: H. bihai (L.) L. and H. caribaea Lam. (Berry & Kress 1991). Although both H. bihai and H. caribaea have been recorded from most of the Eastern Caribbean islands, literature and field survey of the two species from all islands show that their distribution is not homogeneous across all islands (Fig. 1) with H. caribaea dominating in terms of relative abundance on the islands north of Dominica and H. bihai dominating the islands south of Dominica. Hummingbirds (Trochilidae) are the only effective pollinators of Caribbean heliconias. Temeles and Kress (2003) showed that on two islands, St. Lucia and Dominica where four native species of hummingbirds are known, Heliconia–hummingbird interactions exhibit sexual specialization where each sex of Eulampis jugularis (the Purple-throated Carib) is specialized on one of the two native heliconias. Based on the morphological associations between flower size and bill size, they further demonstrated that trap-lining females were the primary pollinators of H. bihai, whereas territo- rial males were the primary visitors of H. caribaea (Temeles et al. 2000, Temeles & Kress 2003). A single territorial male E. jugularis perches within a well-defined clump of H. caribaea and defends all of the flowers against intruders, rarely leaving the territorial site for external foraging (Temeles et al. 2005, Gowda et al. 2012). In contrast, trap-lining females, which are the only visitors to H. bihai also feed on flowers of H. caribaea and fly between scattered plants in search of nectar (Temeles et al. 2006). Received 1 December 2011; revision accepted 26 May 2012. 3 Corresponding author; e-mail: gowda@gwmail.gwu.edu ª 2012 The Author(s) 1 Journal compilation ª 2012 by The Association for Tropical Biology and Conservation BIOTROPICA 0(0): 1–12 2012 10.1111/j.1744-7429.2012.00915.x