Institut für Ökologie, Evolution und Diversität
The associations of the plant-ant Cladomyrma with plants in Southeast Asia
- In over 100 genera of tropical angiosperms, one or more species possess specialised structures for housing ants. The longevity and intimacy of these associations has often facilitated an increasing specialisation of both the ants and the plants, leading to a number of highly specific and obligate symbioses. Early literature contained only few anecdotal reports of the ant genus Cladomyrma WHEELER inhabiting (unidentified) plants. This work presents the new findings on Cladomyrma and its host plants that accumulated over the last two decades. My studies of Cladomyrma reveal that there is a largely overlooked community of south-east Asian plant-ants and their associated plants. Currently the genus consists of at least 12 species. Cladomyrma has been thought to be restricted to the ever-wet part of the West Malesian floristic region, comprising the Malay Peninsula, Borneo, and Sumatra, but recent collections from Thailand and Vietnam indicate that species of the genus penetrate the seasonal tropical forests of Continental Asia. Cladomyrma inhabits 24 plant species belonging to a surprisingly extensive range of plant taxa: Callerya, Saraca, Spatholobus (Fabaceae), Crypteronia (Crypteroniaceae), Drypetes (Putranjivaceae), Ryparosa (Achariaceae), Strychnos (Loganiaceae), Neonauclea (Rubiaceae), Luvunga (Rutaceae) and Sphenodesme (Verbenaceae). In terms of taxonomic diversity on the genus and family level the range of hosts utilised by Cladomyrma is one of the broadest ever recorded for any live stem-nesting plant-ant lineage worldwide. This work provides a species-level overview of all Cladomyrma host plants known from Borneo, the Malay Peninsula and Sumatra, including descriptions of ant-housing structures (domatia), ant inhabitant identity, onset of colonisation during plant ontogeny, nest structure, occupancy rate, and considerations of results obtained from herbarium specimens. Both the regularity of ant association and the degree of morphological specialisation toward myrmecophytism are assessed. The behavioural traits of Cladomyrma are compatible with traits exhibited by other protective plant-ants. This work demonstrates that all species of Cladomyrma investigated (dianeae, maschwitzi, yongi, petalae) confer antiherbivore protection to young leaves of its host. The ants also attack and repell or kill herbivorous insect larvae encountered on young foliage. Cleaning behaviour appears to be a trait shared by all members of the genus, and the two species tested (maschwitzi, petalae) successfully removed termite eggs experimentally placed onto young leaves. Another trait common to all known species of the genus is that the ants preferentially patrol young shoots and leaves ('neophily'). These behavioural traits of Cladomyrma likely reduce stem damage and pathogenic infection of their host. The ants prune encroaching vegetation (tested in dianeae maschwitzi, petalae, yongi, observed in crypteroniae) and attack paper tape used to mark host plants (observed in andrei, dianeae, hobbyi, nudidorsalis, maschwitzi, yongi, petalae). If these traits combined translate into a better reproductive success of the hosts has yet to be verified. Evidence for lifetime fitness benefits is particularly difficult to quantify for the long-lived woody host plants of Cladomyrma. The predominant food source of Cladomyrma appears to be the honeydew of scale insects (Coccidae and Pseudococcidae) which the ants tend inside their nest cavities. Observations on scale insect acquisition by Cladomyrma foundress queens show that hemipteran trophobionts are not transported by the queens on their nuptial flight but they nevertheless arrive on the host plant independently of the ants. Entry into nest chambers is facilitated by small holes kept open by the foundress queen. Most Cladomyrma species have been recorded from only one or two (three) host plant species (andrei, crypteroniae, hobbyi, maschwitzi, nudidorsalis, scopulosa, yongi), but two species, Cladomyrma petalae and C. dianeae, are more catholic in their host usage; the first being a 'generalist' plant-ant colonising hosts across a broad taxonomic range, the second inhabiting several members of the genus Neonauclea. First results of host-choice experiments with C. petalae are presented and the potential mechanisms promoting host specificity are discussed. My studies of the Cladomyrma/plant associations indicate that codiversification and host shifts or host expansions, rather than cospeciation, shape the pattern of species interactions in this system. Finally, I propose a scenario in which three key traits of Cladomyrma –access to live stems, utilisation of indirect food rewards via trophobionts and 'neophily'– are hypothesised to favour niche differentiation and the acquisition of new hosts over evolutionary time.