MATERIAL AND METHODS

Field studies

A survey of host plants of Cladomyrma was conducted in a total of over 200 sites in Malaysia (Peninsula and Borneo) and in Sumatra. Additional information came from a few sites in Kalimantan (Indonesia, Borneo). The main study sites are presented in figure 4-1.

Figure 4-1: Main study sites on the Malay Peninsula (MP), Sumatra (SU) and Borneo (BO): 1.

Belum, Gerik, Perak, 5°31'N 101°25'E; 2. Fraser's Hill, Raub, Pahang, 3°43'N 101°44'E; 3. Ulu Gombak, Selangor, 3°19'N 101°45'E; 4. Pasoh Forest Reserve, Negeri Sembilan, 2°58'N 102°18'E; 5. Endau-Rompin NP, Pahang/Johore, 2°25'N 103°20'E; 6. Gunung Leuser NP, Aceh, 3°50'N 97°30'E; 7. Sg. Busaha, Muaranungo, Jambi, 1°41'S 101°49'E; 8. Kubah Park, Kuching, 1^st Div., Sarawak, 1°33'N 110°12'E; 9. Lambir Hills NP, Miri, 4^th Div., Sarawak, 4°12'N 114°01'E; 10. Poring Hot Springs, Ranau, Sabah, 6°03'N 116°41'E; 11. Sepilok, Sandakan, Sabah, 5°50'N 117°55'E.

In each site, the following characters were studied: plant size and ant occupancy (either by Cladomyrma or other ant species; including presence of colony foundations). Plants of all sizes were examined to determine when ant colonisation occurs and if it continues during the life of the host plant. For a subset of these plants, characteristics of domatia (if present), ant colony size, number of entrance holes and nest chamber length (as proportion of total stem length) were measured. For the tree Saraca thaipingensis, re-colonisation experiments were conducted. This species resprouts easily from cut stumps, allowing the observation of repeated ant

48 Host plants of Cladomyrma

colonisations. Ant collections were preserved in 75-80% ethanol and later identified in the laboratory, following the revision of Cladomyrma (AGOSTI, MOOG & MASCHWITZ

1999) or, for other ant taxa, the keys of HÖLLDOBLER & WILSON (1990) and BOLTON

(1994).

Herbarium studies

With few exceptions, plant specimens were collected from non-fertile individuals because ants colonised plants early in their ontogeny. Nevertheless, almost all host plants of Cladomyrma could be identified to species level by comparing dried specimens with herbarium material at several institutions: Forest Research Institute Malaysia, Kepong (KEP), Forest Research Centre, Sepilok, Sandakan (SAN), Nationaal Herbarium Nederland, Leiden (L), Royal Botanic Gardens, Kew (K) and Phillippine National Herbarium, Manila (PNH). Identification of host taxa belonging to the genera Luvunga and Neonauclea, however, was exceptionally difficult (for details see under the respective paragraphs below). In order to verify ant colonisation of Cladomyrma host plants in areas that could not be visited I systematically examined herbarium material from the herbaria K, KEP, L, PNH and SAN. Entrance holes of Cladomyrma are usually distinguishable from those made by other ant species or stemborers by their size, regular shape with clearly defined margins and regular distribution along the twigs. In addition, I checked the labels accompanying herbarium specimens for mentioning of ant occupation, however, most collectors failed to note this conspicuous trait. Few collections provided precise geographic coordinates but coordinates of most collection localities could be determined using gazeteers or topographic maps.

Greenhouse studies

It could be postulated that domatia (swollen twigs) observed in some host plants of Cladomyrma are environmentally (induced by ants or other insects) rather than genetically based and thus have to be considered as galls. As a partial test of this possibility, some ant-free host plant species were grown as saplings or rooted cuttings in greenhouses both at Frankfurt University, Germany, and at the Ulu Gombak Field Station, Malaysia, until they produced domatia.

Voucher specimens

Dried specimens of host plants are deposited at L; acccession numbers are AMO-001 to AMO-151. Additionally, alcohol material (100% ethanol) is available at L for the following Neonauclea herbarium specimens: AMO-033, AMO-065 to -067, AMO-069, AMO-071, AMO-081 to -083, and AMO-085 to -089.

Host plants of Cladomyrma 49

4.3. RESULTS

4.3.1. Fabaceae - Caesalpinioideae

Saraca thaipingensis Cantley ex Prain Figures 4-2, 4-5, 4-8

Tree to 25 m; locally common; in forests, mostly in stream valleys, growing along rocky banks; lowlands up to 1,100 m altitude (HOU, LARSEN & LARSEN 1996). The tributaries and headwaters in Peninsular Malaysia are often so densely bordered by S.

thaipingensis, thus forming tunnels through the forest, that CORNER (1988) coined the term 'Saraca-streams'. Distribution: Malay Peninsula (throughout), West and Central Java; few collections north of the Kra Isthmus (South Burma and North Thailand).

Ant-housing and other ant-related structures: The twigs of S. thaipingensis do not provide swollen parts or 'weak spots' (so-called prostomata) to promote ant inhabitation and access, yet they have internodes increasing in diameter towards the nodes (Fig. 4-2B) which support the big-sized pinnate leaves (petiole and rachis up to 60 cm long; Fig. 4-2A). The pith of young internodes is relatively soft but does not degenerate to create a cavity (Fig. 4-2D). The margins of young leaflets bear 1 to 3 active extrafloral nectaries (EFN) which attract a diversity of non-resident ants (e.g., Camponotus, Crematogaster, Gnamptogenys, Rhoptromyrmex, Tapinoma, Technomyrmex). In contrast, the obligate ant partner Cladomyrma exploits the EFN only under starving conditions. Flushes of newly developed leaves are characterised by delayed greening (Fig. 4-2C). Young leaves are preferentially patrolled by Cladomyrma ants, resulting in a significant reduction of young leaf herbivory (MOOG &

MASCHWITZ 1994 and 2000, this thesis).

Ant inhabitants: The association with Cladomyrma petalae ants was first reported by MASCHWITZ et al. (1991). The association has been found in all parts of the Malay Peninsula from southern Thailand to Singapore. In small, restricted local habitats, C.

petalae may be replaced mainly by a particular Crematogaster species (sp. 1) which facultatively nests in a wide variety of plant species (see below). Occasionally, species of other ant genera can be found in abandoned Cladomyrma chambers, stem borer cavities or dead twigs (Camponotus, Monomorium, Pheidole, Tapinoma, Technomyrmex, Vombisidris).

50 Host plants of Cladomyrma

Figure 4-2: Saraca thaipingensis. A: twig and pinnate leaf of an ant-inhabited sapling; B: young internode with two Cladomyrma colony foundations. Pith is pushed out through the entrance holes by the foundresses (arrows); C: young purple leaf flush which dangles for several days before stiffening and straightening; D: dissected internode showing a founding chamber (length 2.8 cm) of a Cladomyrma petalae queen. The inner walls of the chamber turn brown shortly after excavation of the cavity.

Host plants of Cladomyrma 51

Onset of ant occupation: Colonisation of saplings usually starts in plants ! 1 m in height because only then the primary stem diameter is sufficient to allow accomodation of Cladomyrma founding females. On average, the internode diameter at point of entry was 5.7 mm ± 1.06 SD (range 3.8 to 9.8 mm; n=118). Cladomyrma foundresses are the primary colonisers of Saraca trees. The founding females of other ant taxa (e.g., Crematogaster sp. 1) lack the ability to create cavities in Saraca twigs and they rely on the presence of preformed shelters such as stipules or cavities chewed by Cladomyrma or other stem-boring insects. Support for the 'priority effect' of Cladomyrma comes from the re-colonisation experiments (Tab. 4-1).

Table 4-1: Re-colonisation experiments on Saraca thaipingensis growing in open, sunny habitat (n=6). All branches and twigs of tree stumps (> 15 cm in diameter) were cut back to initiate resprouting. After one to 25 months the newly produced sucker shoots were dissected and examined for ant inhabitants. Foundations = foundress queen still without workers but brood may be present; Natal colony = the initial founding chamber is not yet enlarged by the nanitic workers and only one nest entrance hole is present; young colony = more than one internode colonised but alate sexuals not yet produced (ergonomic stage); mature colony = alates present (reproductive stage). Superscript = ¹ overgrown by vines, only few shoots produced; ² the foundations were all found on two shoots not yet patrolled by the mature colony; ³ three young colonies had taken over neighbouring foundations; ⁴ Monomorium inhabited an abandoned Cladomyrma foundation chamber; ⁵ Technomyrmex nested between leaflets; ⁶ Technomyrmex inhabited dead wood at the tree stump outside the patrolling range of Cladomyrma.

time Cladomyrma petalae other nesting colony growth in Saraca saplings. The sun-exposed tree stumps produced unusually high numbers of new sucker shoots within a short time period, thus growth of the colony was not restricted by limited nesting space or food shortage (young shoots function as substrate for scale insects).

Mutiple foundations of Cladomyrma petalae occured shortly after production of the first new shoots. Several young colonies developed during the first year after start of the experiments and apparently initiated a phase of intense intraspecific competition.

Eventually, only a single colony managed to get established on the host and other foundations or ant species, if present, were restricted to plant parts outside the

52 Host plants of Cladomyrma

patrolling range of the 'winner' colony. This colonisation process is typical for all Cladomyrma species, irrespective of host taxon.

Ant occupancy rate: Occupancy rates of Saraca thaipingensis can vary due to tree age (tree size), habitat and interspecific competition. For example, of the 280 Saraca trees located at Ulu Gombak in an area of about 2000 m², 53% were occupied by Cladomyrma petalae, while 37.5 harboured Crematogaster sp. 1 (Fig. 4-3). Saraca trees inhabited by Crematogaster sp. 1 were predominantly found at the river bank in a small strip of ca. 5 m width on both sides along the river, and colonisation rate reached 70% in the river bank strip. In contrast, trees inhabited by Cladomyrma were mostly found away from the river bank deeper in the forest, and there occupancy rate was 86%. This difference in occupation pattern between the two ant species corresponded not only to habitat type (river bank versus forest) but also to differences in height of occupied trees because Saraca trees at the river bank are, on average, taller than those of the forest. Overall, Crematogaster sp. 1 was found in a significantly larger proportion of taller trees (Mann-Whitney U-test, P < 0.0001). If only trees of 1 to 10 m height are considered, colonisation rate by Cladomyrma reached almost 90%.

Saraca thaipingensis at Ulu Gombak Gombak in relation to habitat. River bank = 5 m wide strip along each side of the river; forest = the study area up to 100 m into the forest, excluding the river bank.

Crematogaster sp. 1 ants are abundant at river and forest fringes and once they are established at one site they mostly prevent the colonisation of a Saraca by colony-founding Cladomyrma queens. Any Cladomyrma foundress encountered on the plant will be chased away or killed by the Crematogaster workers (RIEDEL 1997, pers.

observation).

Host plants of Cladomyrma 53

Differential use of Saraca by Cladomyrma and Crematogaster: The myrmicine species Crematogaster sp. 1 is not specialised to living in a particular host plant but uses many plant species in an opportunistic way (e.g., Ardisia sp., Callerya (Millettia) atropurpurea, Knema sp., Lepisanthes tetraphylla, Pometia pinnata, Tetrastigma dubium, Trigonostemon malaccanus, and Vernonia arborea). A mature Crematoagster sp. 1 colony often extends over several neighbouring plants (polydomy), usually with only a small part of each plant occupied. The workers are able to cut small chambers into internodes, however, the entrances are somewhat irregular in shape and size (Fig. 4-5D, Fig. 4-6) and the nodes are not perforated (Fig. 4-4), resulting in a nest structure of many separated small chambers distributed over several plants.

Figure 4-4: Schematic illustration of nest chambers of Crematogaster sp. 1 in a Saraca branch.

Broken line: branch sections without nest cavities; solid bold line: nest chambers, confined to one internode; solid circles: attachment of leaves (usually one leaf per internode, at least towards the twig apices).

In contrast to the true plant-ant Cladomyrma, the Crematogaster sp. 1 foundress lacks the ability to chew a founding chamber into a young stem or twig. She relies on shelters like stipules or hollows gnawed by Cladomyrma or other stem-boring insects to found a colony (Fig. 4-5A-C). This has important consequences for the establishment of a Crematogaster colony on a Saraca tree. In saplings, Cladomyrma is usually the first occupant and attains a monopoly over the host plant, preventing the colonisation by Crematogaster foundress ants. Despite this 'priority effect', small Cladomyrma colonies may secondarily be displaced by Crematogaster species when workers of a mature Crematogaster colony enlarge their territory onto a neighbouring Saraca. In staged encounters between similar-sized colonies of a Crematogaster sp. 1 and C. petalae, the latter successfully defended its host plant against invasion by Crematogaster ants (n=5; unpublished results). However, when colony size of C.

petalae was distinctly smaller (about half size) than that of Crematogaster sp. 1, the Cladomyrma colony was evicted and killed (n=6).

54 Host plants of Cladomyrma

Figure 4-5: Crematogaster sp. 1 colonising Saraca thaipingensis. A: a foundress trying to enter a Cladomyrma founding chamber by removing the pith with which the entrance hole is plugged;

B: a stipule with exit hole (arrow) tightly clasping an internode; C: stipule removed, showing a