|The Ants of
CHAPTER 3 - Mosaics - Evidence from Cameroun - 2 - Dejean & colleagues
|Introduction - Cameroun 3|
The core study was carried out at the field station established at Akok (2° 39' 21" N; 9° 54' 41"E) in the Campo Forest Reserve, Southern Province, Cameroon, during "Operation Canopée 91" (see Map 7).
The "canopy raft" used in Cameroon was a hexagonal structure built from 12 inflatable PVC tubes (0.89 m diameter; 13.6 m in length) holding in tension six triangular aramid nets, each 93m². Thanks to a dirigible, this structure of 750 kg could be temporarily (generally for one week) placed atop the crown of a large tree. The flexibility of the structure enabled it to mould to the shape of the tree crowns, making it very stable and capable of holding up to six people. Direct access to foliage and twigs was possible both around the perimeter of the raft and through the netting via 52 small "doors" in different parts of the net and through six manholes located between the diagonal elements at the center of the structure. Access to the raft was possible using the single rope technique, the rope being secured to an aluminum tripod placed at the centre of the raft.
The "canopy sled", used for the first time by scientists during this mission, was a triangular assembly of inflatable PVC tubes 9m in length and 0.89m in diameter with an aramid net stretched between them (30m²; l00 kg), that could carry three persons. The canopy sled was hung on slings 10m below the cabin of the dirigible, which, when transporting the sled, was fitted with thrust reversers enabling precision manoeuvring. The sled thus could be directed at chosen sites in the canopy. Sampling was performed by a crew of two researchers and a member of the technical team secured by ropes linked directly to the dirigible. The apparatus permitted the gathering of samples of twigs and foliage from canopy vegetation, along with samples of ants and their associated Homoptera, during morning excursions in a loop pattern from the station.
During five different placements of the "canopy raft" and six sorties of the "canopy sled", the team sampled 167 trees and 20 vines. The belief was that such methods resulted in a haphazard sampling of canopy ants, for the following reasons. First, although the criteria determining the placement of the "canopy raft" were conditioned only by the shape of the supporting tree canopy (the crown must be sufficiently large and even to support the raft), the surrounding trees, accessible along the edges, do not necessarily meet this condition. Second, the six sorties of the "canopy sled" covered different loops in the forest. The six sorties were evenly spread over the entire 360° around the station. Each tree along the loops was not systematically sampled during the sorties; rather the method corresponded to irregular alighting determined by chance (mostly the whim of the dirigible pilot based on variations in wind speed and direction). A specific study was undertaken on 40 of the above-sampled trees, using the single rope technique (from the ground or from the canopy raft) in order to reach the nests of non-dominant species. Thus, the team collected epiphytes situated on the main branches just under the canopy and, using a saw, cut up large branches hollowed by termites or borers. The same method was used on five trees where, when using the canopy sled, dominant ants were not found. The single rope technique was also used to reach epiphytes and hemi-epiphytes situated at lower levels on the trunks of trees (between four and 25m in height) and to remove 1m² of the bark of 13 trees (four squares of 50x50cm on each tree).
The Arboreal Ant Mosaic in a Cameroonian Rainforest (Hymenoptera: Formicidae) by A. Dejean, D. McKey, M. Gibernau & M. Belin (2000) - Sociobiology, 35, 403-423
The ant mosaic of the canopy of a Cameroonian rainforest was studied by directly sampling 167 large trees and 20 vines reaching the canopy level using a dirigible and the "canopy raft", the "canopy sled", and, when necessary, the single rope technique. Although plant species diversity was high (trees: 63 species from 29 families; vines: 9 species from 7 families), the results showed an ant assemblage characterized by high abundance but low diversity (28 ant species in total). There were only four dominant ant species (i.e., species with populous colonies that build their own nests, exhibit strong territoriality, and have mutually exclusive territories distributed in a mosaic pattern).
The most frequent species, Crematogaster depressa, occupied 87.4% of the trees and 85% of the vines, and its colonies reached several million workers. Other dominants were recorded at low frequencies (Crematogaster (Atopogyne) sp l: 1.8% of the trees; Oecophylla longinoda: 6.0%; Tetramorium aculeatum: on one vine).
Nine ant species (with 110 colonies) were tolerated on the territories of Cr. depressa (i.e., non-dominant species with smaller colonies). The workers of three species (Cataulacus guineensis and a Camponotus sp. were very frequent. Both latter species and Cataulacus lobata obviously shared trails with Cr. depressa, but exploited food sources independently. Camponotus brutus, regarded as nocturnal, with colonies sometimes able to occupy the entire crown of a tree, rather had the status of sub-dominant.
Extrafloral nectaries (EFN) appeared to play an important role in ant species distribution. The large ecological success of Cr. depressa is probably due to its ability to nest on trees with or without EFN. O. longinoda, which rarely tolerated non-dominant ant species, was significantly more frequently recorded on trees without EFN. While dominant ants depended principally on attended homopterans (Coccidae and Stictococcidae; globally: 300,000 to 700,000 individuals per tree), non-dominant species depended primarily on EFN.
The overall ant findings are in the Campo Summary Table
A Rainforest Ant Mosaic: The Edge Effect (Hymenoptera: Formicidae) by A. Dejean & M. Gibernau - Sociobiology, 35, 385-401
To complement the canopy survey, the ant mosaic on the edge of the rainforest was studied along 500 m of a dirt road. The plants growing along the forest edges are characterised by their rapid, continuous growth and the production of leaves and soft wood. This enables the plants to keep their position in the exposed, highly insolated conditions, with high competition and high predation.
The overall ant findings are in the Campo Summary Table
This was not a study of the ant mosaic but it does throw light on why there are few ponerine ants found within the canopy. They attributed the paucity of arboreal ponerines to the inability of such relatively primitive ants to utilise a major component of the food supplies on trees - sugary fluids. This is because the primitive structure of the crop, the storage portion of the insect gut, does not allow its distension and filling with large quantities of liquid. Such distension is known as physogastry. The sugary fluids may be the direct product of plants, from nectaries within the flowers or in the case of ant utilisation, extra-floral. The transport and subsequent sharing with nest-mates and larvae is described as trophallaxis and is a major element in the advanced development of social behaviour. Odontomachus troglodytes was already known to be able to collect sugars from Homoptera (Evans & Leston, 1971) but the species is not truly arboreal, rather being unusual in its ability to climb. Sugars are carried as a droplet held within the long mandibles but the quantity thus carried is very small.
The ant studied by Dejean & Suzzoni was Platythyrea conradti and what it has evolved is a capacity to utilise the surface tension inherent in fluid droplets to transport sizeable amounts adhering to the underside of the head and alitrunk. These collections are taken back to the nest and mostly deposited on the walls of the nest (where nestmates can imbibe, 37.3%), on the soil (15.4%, a curious fraction for an arboreal ant) or deposited on the ventral side of the larvae (33.7%). As an added social element nest-mates also feed on droplets while they are being carried (13.6%), a form of mutual behaviour Dejean & Suzzoni termed the "social bucket". The place of Platythyrea conradti in the arboreal mosaic was described as non-dominant, the species avoiding conflict with a dominant by being active only during the early hours of the day, when Dejean & Suzzoni stated dominants are only slightly active.
© 2003, 2012 - Brian Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.