Spatial Variation In Interactions Of The Semi-Myrmecophyte Humboldtia Brunonis (Fabaceae) With Ants And Other Invertebrates
Despite a long history of investigations on protective ant-plant interactions, since the late 19century (Thomas Belt 1874), a comprehensive quantitative understanding of the adaptations that facilitate these associations between plants and ants and the differential importance of these adaptations in predicting the benefits and costs to each partner and in shedding light on the evolutionary trajectories of this ecologically widespread interaction is yet to be realized. In the present study we have experimentally shown that the identity of the ant species (T. albipes), the abundance of this ant species and the composition of EFN produced by floral buds and young leaves of H. brunonis plant populations contribute to facilitating protection of floral bud inflorescences and young leaves of H. brunonis populations in a particular site. Of all the 16 ant species that inhabit the domatia of H. brunonis at several sites, the plant has found its protective ant partner only in the dolichoderine ant T. albipes and that too, only in the southernmost site In our experimental set up of three populations of H. brunonis at three different latitudes, where each site is separated from another site by at least 200 km, we have shown that only the southernmost population (Solaikolli) in protected by the ant species T. albipes. Although T. albipes is found in two of the three populations used in the present study (Sampaji and Solaikolli), it is present in a relatively larger abundance both within the domatia of H. brunonis and in the terrestrial stratum only in the southernmost site Solaikolli and is hence able to protect the EFN producing structures of H. brunonis at this site. In comparison, T. albipes is found in low abundance both in the domatia and in the terrestrial stratum in the site Sampaji and does not protect EFN producing plant parts of H. brunonis at this site. We have also experimentally shown that the young leaves of H. brunonis at this southernmost site Solaikolli are protected to a greater extent than the floral bud inflorescences, since T. albipes preferentially utilized EFN produced by young leaves compared to EFN produced by floral buds at this site. Moreover, we have also shown that the floral buds of H. brunonis in the northernmost site are unprotected for two reasons: i. the absence of the protective ant species T.albipes at this site. ii. The EPN produced by some of floral buds at this site is differentially utilized to a much lower extent than floral buds from other sites due to EFN from Agumbe being > 400-fold more viscous than the EFN produced by floral buds at the other two sites (sampaji and Solaikolli) (at 30º C). Although previous studies have independently shown that the identity of the ant partner (Janzen 1966, Schemske 1980, Horvitz and Schemske 1984, Heads 1986, Oliveira et al. 1987a, b; Jaffe et al. 1989, RicoGray and Thien 1989, Davidson et al. 1991) and the abundance of the protective ant species (Koptur 1984, Rocha and Bergallo 1992, Di Giusto et al. 2001) are important in predicting the protective outcome of the interaction between a plant species and its interacting ants, few studies have examined these two factors along with an examination of the EFN volume and composition (Inouye and Inouye 1980, Rudgers and Gardener 2004) and differential utilization of these qualitatively different EFNs produced by different populations of the same ant plant. Humboldtia brunonis and its interacting ant species provides a unique system to address questions about the evolution of ecological specialization and the evolution of preadapted plant traits that facilitate interactions between plants and ants (especially domatia), due to its unique polymorphism for the presence of caulinary domatia and its widespread abundance in the low-elevation wet-evergreen forests of the Western Ghats.