|Introduction||The Ants of Africa
CHAPTER 3 - Mosaics - Evidence from Ghana - 3
|Ghana - 1||Ghana - 2||Ghana - 4||Ghana - 5||Ghana - 6|
The work of Room (1971) on cocoa as a whole can be divided into two parts. The area within which he was active is shown in the map (see Map 2).
First, he made a detailed survey of a small locality (not more than 250 m²), the Mampong Cemetery Farm, in south-east Ghana, on top of the Akwapim Hills and near the southern boundary of the "so-called forest region". Although he described this as a cocoa farm, it really seems to have been a rather neglected site. It contains two clumps of cocoa trees (with six and nine trees respectively), two belts one tree deep either side of a path, and 3-4 lone cocoa trees, see the Site Plan). Central to the cocoa area was an area of bananas and Lantana tangle. Other parts he studied were bare ground and ground with low herb cover. The whole seems to have been surrounded by maize fields, and the site had four large trees. He did, however, collect ants from most parts of the site, making manual catches in cocoa canopy and on the ground surface, sampling cocoa mistletoe, taking one-litre samples of leaf litter (using a Tullgren funnel to extract ants), and taking pieces of dead wood from trees and the ground. He surveyed this site at least once a week from December 1969 to July 1970, sampling at least two of the habitats on each visit. From the total area (not more than 250m²), he collected 128 species ant, from 48 genera. Of these, however, only 34 species actually were found on cocoa canopy and mistletoe (14 on mistletoe only). The greatest proportion of species, by far, was found on the soil or in leaf litter under shade (mostly below cocoa canopy). The missing link, however, is that he seems not to have examined tree parts below the canopy and, therefore, provided no information on the true third dimension, the vertical connection between ground level and canopy.
The second phase of his initial work was a sampling of cocoa canopy at thirty farms, scattered through southern Ghana, in an area some 80 km east-west and 65 km north-south, with the southeast edge being the coastal savannah (see Map 2). The collection, which was by hand catching specimens in the canopy and reachable from a ladder, was biased to include thirty independent samples from the territories of dominant ants, plus 37 samples from non-dominated canopy. Sampling, moreover, was solely from unshaded continuous canopy, between 0900h and 1200h, and only when the sun was shining. In total, Room collected 168 samples, with a total of 67 species of ant. Of these, 43 were additional to the canopy and mistletoe species previously found at the Mampong Cemetery Farm; although 9 had been found at other sampling parts of that farm; giving a total of 77 species from cocoa canopy (including mistletoe as a component of the canopy ecosystem, although eight species were found exclusively on the Mampong mistletoe) see ghana cocoa ants for a summary table.
Presumably following Leston's thinking (as he cited an unpublished conference paper by Leston), Room wrote
"It was known that cocoa farm canopy in Ghana contains some six common species of ant which, wherever they occur, are always dominant in terms of number of individuals. These species are mutually exclusive, and different colonies of the same species are antagonistic towards each other. This results in the territories of dominant ants being distributed like a patchwork quilt over the canopy".
In his canopy survey, the defined dominants were Oecophylla longinoda, Crematogaster africana, Crematogaster striatula, Crematogaster clariventris and Crematogaster buchneri. Only the first two had been found at the Mampong Cemetery Farm and, in the event, Crematogaster buchneri could not be found on 30 trees. The mean number of species per sample ranged from 3.5 for Crematogaster striatula to 6.8 for Crematogaster clariventris, the overall mean of about 4.0 (my approximation from Room's data) is higher than was indicated by the earlier findings of Strickland (1951b).
The results of the canopy survey led Room to add a further ten
species to the dominant category. These include four which he found
"only as dominants" -
Crematogaster wellmani (as Crematogaster boxi), Crematogaster stadelmanni,Crematogaster depressa and Crematogaster species A269;
three which he described as found either as sole dominants or in association with one of the common dominants - Crematogaster castanea? (with Oecophylla longinoda) and Atopomyrmex mocquerysi and Camponotus chrysurus (both of which occurred with Crematogaster striatula);
and three which he described as having been observed (unreferenced) as common dominants in other habitats - Pheidole species at the bases of cocoa trees, and Crematogaster stadelmanni and Camponotus acvapimensis in dryer, more savannah-like areas.
One has to assume that he used the criterion of numerical abundance for defining dominants, as he provided no quantitative data relating to individual species numbers. Some support for the criterion being a good one comes from the fact that the mean number of other species on the canopy of trees with the "others" was 4.8, which is higher than four of his selected dominants but lower than that (6.8) for Crematogaster clariventris. Of these other dominants, however, only Crematogaster castanea? (in 34 samples), Atopomyrmex mocquerysi (in 15 samples) and Camponotus chrysurus (in 19 samples) were at all common.
The results of his surveys enabled Room to carry out association analyses for thirty-six species which occurred five or more times in the 168 samples. The positive groupings perhaps are more useful than the apparent negative associations, as the latter clearly owes more to the exclusivity of the common dominants. Thus the positive groups (shown in Fig. 5 of his paper), dominants listed first, are :
The most frequently found species in the survey, Cataulacus pygmaeus (in 63 samples, possibly actually Cataulacus traegaordhi) had no positive association with any dominant, although such an association was found with Polyrhachis decemdentata (found in 23 samples) and that was associated with Oecophylla longinoda. Similarly, Tetraponera anthracina (in 31 samples) was associated with Camponotus niveosetosus (in 52 samples), and the latter was associated with the dominants Crematogaster africana and Crematogaster clariventris. It should be noted that the redrawing of Room's Fig. 5 in Hölldobler & Wilson (1990, page 421) perpetuated the Crematogaster depressa anomaly and has Tetraponera opthalmica and Tetraponera anthracina wrongly indicated as Tetramorium species.
Room described the relation between Crematogaster castanea? and Oecophylla longinoda as a very strong association, with only the greater abundance of the latter countering a possibility that the true dominant was Crematogaster castanea? and thus posing the case for canopy dominance being wholly by Crematogasterines. In this instance co-dominance seemed a better description. Arguing, however, that the heavy shade does not allow ground species to build up populations large enough for them to compete successfully with canopy-nesting species, he implied that ground-nesters were of no great importance in Ghana cocoa. The canopy survey showed, moreover, that the importance of Crematogaster striatula was confined to the Tafo area, as it was "difficult to find in other areas". In conclusion, he argued for the patchwork of dominants being due to competition for foraging areas. In each case, the dominants had a number of associated species, perhaps twenty to thirty in number, each of which compete for a place within the territory of their dominant. The less specialised the dominant, the greater the number of subdominants, as these had to utilise resources which were not drawn on by the dominant. In situations where conditions were less than ideal for a particular dominant, there seemed to be a number of "less specialised subdominants" which could fit into particular "niches". Finally, he emphasized that his study was of insolated canopy only.
The other two papers (Room, 1972a, b) reported his studies of the fauna of cocoa mistletoe, Tapinanthus bangwensis. This parasitic plant grows only in situations receiving a lot of direct sunlight and, therefore, his ant data represent the situation on the top of unshaded canopy. Most of his results came from three farms, two near Aburi and the Mampong Cemetery Farm, plus sampling at 19 of the other farms reported in his earlier paper (1971). The sampling was by destructive removal of cocoa branches bearing a mistletoe plant and his results were broken down as follows - occurrence of insect species on 630 samples of the cocoa/mistletoe junction, total abundance of those species in the samples, and presence/absence of insect species on 175 mistletoe plants. He also dissected the base of the mistletoe plants, especially examining holes made by borers. The twenty-six insect species with the highest scores were ranked in each of the three categories.
With respect to the ranking of Crematogaster africana, however, Room (1972a) noted that the three main sites were all on the Akwapim Hills, where it was particularly common and other dominants were relatively rare. The absence of Oecophylla longinoda was attributed to its habit of tending a Homopteran, Stictococcus sjostedti, which does not attain high densities on mistletoe. In fact the overwhelmingly common Homopteran was a Pseudococcid, Cataenococcus loranthi (Strickland), which particularly inhabits boreholes in the mistletoe (Room, 1972b), and is tended by Crematogaster africana and, sometimes, Crematogaster clariventris. The borehole factor had added significance because only the smaller Crematogasterines could enter the holes, Room described this as "taking refuge". Finally, he provided some evidence, from experimental ant exclusion and removal, that Crematogaster africana protected mistletoe from leaf-feeders, noting that ants tend phloem-feeders but kill cell-content feeders (Room, 1972b).
See ghana_cocoa_ants for a summary of his results.
|Mosaics - Introduction||Mosaics - Ghana - 4||
©1998, 2003 - Brian Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.