|Introduction||The Ants of Africa
CHAPTER 3 - Mosaics - Evidence from Ghana -2
Leston; Collingwood; Bolton; Evans; Evans & Leston.
|Ghana - 1||Ghana - 3||Ghana - 4||Ghana - 5||Ghana - 6|
A sad fact regarding the widely quoted work of Dennis Leston is that he actually provided very little quantitative data, most of his reports being purely subjective (Majer, 1976b) and lacking information on methods, sampling sites, etc. Part of Leston's earlier work, which was at CRIG, took place between March 1966 and September 1967, when he and D.G. Gibbs made regular pkd collections in three blocks of cocoa. The detailed publication from that work (Gibbs & Leston, 1970), however, has information on only two species of ants (Oecophylla longinoda and Odontomachus troglodytes - as Odontomachus haematodus). In his generic revisions, Bolton has frequently listed findings such as, "1966, ant ecology sample (D. Leston)" - among the locations were CRIG (s on the map); Kade (see below, and l on the map); Mt. Atewa, a primary forest area close to Kibi in the Eastern region (o on the map), from which there are numerous unique findings; Legon (essentially a savannah location); Wiawso, Western Region (Evans & Leston, 1971); and, a number of locations most of which are on the main map (Map 1), or in the area covered by the map derived from that given by Room (1971) (see Map 2). Thus, it seems clear that Leston's collection methods included pkd sprays, simple hand collections, and the undefined 'ant ecology samples'.
In his 1973 paper, Leston noted that Crematogaster striatula was the most frequent dominant, with a preference for nesting in heavily shaded places, being especially dominant in dense secondary growth, and in cocoa farms adjacent to such vegetation. Of the other Crematogaster species, he added that many dominants were of the subgenus Atopogyne, including Crematogaster africana, Crematogaster buchneri, Crematogaster depressa and Crematogaster clariventris, which all construct large nests of carton, usually high up on the trunks of cocoa shade trees or forest emergents. Another dominant referred to was Tetramorium aculeatum (as Macromischoides aculeatus) which he stated required areas of dense canopy cocoa or cocoa under fairly heavy top shade. The ground nesting Camponotus acvapimensis was defined as being largely a savannah species which almost certainly could be found wherever a grassy clearing occurred in the forest zone. Thus, its presence in cocoa was indicative of a poor canopy and the absence of shade. Atopomyrmex mocquerysi was described as being of local importance, nesting in the hollow branches of tree crowns. Another species which he thought to be a savannah species which had penetrated only the more degraded areas of the forest zone was a Lepisiota species, which he gave as Acantholepis capensis. This he thought had colonies extending over several dozen trees, nesting beneath loose bark or in crevices of cocoa, coffee or various shade trees. Pheidole was described as comprising many species, all of uncertain taxonomy, with nests frequently encountered at the base of cocoa trees, including in old pods, but with territories smaller than the other dominants. In a stylized profile diagram, he summarised the above, adding the requirement by Oecophylla longinoda for a sunny part of a good and complete canopy. He completed his review of dominants with Crematogaster castanea, which he described as a species which sometimes was dominant but mutually tolerant of Oecophylla longinoda.
Overall, he concluded there were three basic mosaic types (Leston, 1973):-
He thought that cocoa farms and primary forest understoreys were probably similar, with the latter perhaps being enriched at the canopy level.
Cedric Collingwood attempted to clarify the then situation of confusion over the identities of Crematogaster depressa and other closely related species most notably suggesting that earlier reports of Crematogaster depressa may well have concerned Crematogaster clariventris.
Barry Bolton then devised a short key to separate the six most common carton-nesting species at CRIG - Crematogaster africana, Crematogaster buchneri, Crematogaster clariventris, Crematogaster depressa and Crematogaster jullieni of the subgenus Atopogyne (although, because the queens did not conform to the original Forel characteristics of the subgenus, he remained unsure if Crematogaster buchneri and Crematogaster clariventris were indeed in the subgenus); and Crematogaster stadelmanni, which he described as the only local species in the subgenus Nematocrema. Bolton surmised that the carton-nesting of Crematogaster stadelmanni might be an example of convergent behaviour with the Atopogyne species.
Collingwood tabulated findings on the occurrence of Oecophylla longinoda on cocoa at CRIG, over a period of three to four years. Four studies gave the occurrence as 21% (pkd sprays applied to 1376 trees), 16.3% (pkd sprays applied to 100 plots of 30 trees each), 17.8% (visual counts on young cocoa, 4673 trees) and 23.5% of trees (pkd sprays applied to 64 plots of 30 trees each), and 51.9% (untreated plots, 480 trees).
Although principally concerned with the cocoa black pod disease, Harry Evans made several useful observations on the ants which revealed their potential as vectors of the disease. This potential is discussed in Chapter 4 but several species of some apparent importance in the mosaic appeared in his reports. First, Crematogaster striatula was implicated by virtue of its activities in moving surface tissue from diseased pods and using this in tent-building over coccids on pod peduncles. Several other, unnamed, species were also implicated in this way but, because of their relative rarity, were regarded as unimportant. Second, and interestingly in view of the work of entomologists, he stressed the importance of soil tent-builders, notably Camponotus acvapimensis, Pheidole megacephala and Odontomachus troglodytes (as Odontomachus haematodus). He remarked that in Ghana probably 10-20% of cocoa trees are colonised by tent-building ants.
The observations on Odontomachus troglodytes were believed to be the first report of any Ponerine ant being associated with Homoptera, let alone being a tent-builder (Evans & Leston, 1971). From the records of its distribution, they concluded that probably it is a savannah ant which penetrates into the forest zone following disturbance by man. Aphids and stictococcids were observed being tended, with tents of soil particles over aphids on flower cushions and cherelles between 0.3 and 1.0 m above ground. Tents over Stictococcus sjostedti were later found on larger pods 2.0 and 3.5 m above ground. An experiment offering Stictococcids and a mealybug, Planococcoides njalensis, confirmed the association with the former but the mealybugs were not tended, all disappearing either through migration or predation.
|Mosaics - Introduction||Mosaics - Ghana - 3||
©1998, 2003 - Brian Taylor CBiol FSB FRES
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