CHAPTER 3 - Mosaics - Conclusions
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The Ants of
Africa CHAPTER 3 - Mosaics - Conclusions |
| Introduction |
Majer (1976b) wrote how his findings from Ghana supported his (1972) suggestion that the dominant ants are distributed in a three-dimensional mosaic, with both intra- and inter-specific mosaics over limited areas. Indisputably, his mapping and the pkd sampling provided abundant evidence of cocoa canopy mosaics in cocoa at Kade, plus one small area at CRIG. With the exception of Camponotus acvapimensis, however, evidence of a true third dimension, which I regard as the influence of ants which have their nests in the soil or in dead wood on the ground, plus some which can nest also in dead wood on the tree, did not come out of his studies. Room (1971) listed a good number of such species from the Mampong Cemetery Farm, but otherwise provided no quantitative data except from his cocoa canopy study (including mistletoe as an extension of the canopy). See ghana_cocoa_ants for a summary of their findings. Both authors appear to have adopted the basic assumption that only "dominants" play a significant role in the cocoa ecosystem. Hence, overtly useful studies of the ants inhabiting cocoa canopy (the pkd sampling) may have been flawed by the decision to ignore several Cataulacus and Polyrhachis species. Other species with demonstrated potential economic significance, for instance Pheidole megacephala and Odontomachus troglodytes (Evans, 1973b, Evans & Leston, 1971), did not appear in their samples. The more recent work by Belshaw & Bolton (1994b) added much to the knowledge of the ants which inhabit and forage in or on leaf litter but, unfortunately they also chose not to examine the link between the canopy and the ground, i.e. the trunks of trees, and perhaps curiously, also chose not to examine dead wood on the ground, known to be the nesting site of many species.
Majer (1975) followed Leston (1973) in suggesting that the ant mosaic in cocoa could be manipulated so as to reduce the incidence of major pests and disease vectors. For instance, the results of the ant exclusion experiment (see also Majer, 1976a) indicated high damage by the mirid, Distantiella theobroma, where Oecophylla longinoda had been excluded. The Crematogaster africana group tend mealybug vectors of swollen shot virus and thus were undesirable. His concept hinged on promoting Oecophylla longinoda and, perhaps, Tetramorium aculeatum, by achieving an adequately shaded farm with a complete cocoa canopy. This would be aided by removing all forest trees before planting, thus eliminating nest sites for the Crematogaster africana group, and by removing or burning all dead wood, reducing the chance of Crematogaster striatula and Pheidole species nesting. He argued also for use of coconut as the shade trees and to be planted in a buffer zone between the forest and the cocoa.
The danger of drawing strong conclusions from limited data or from data gathered from a limited area (a single site or farm, or a single facet of an ecosystem) was illustrated by Bigger (1975), who concluded from analysis of 14 outbreaks of cocoa swollen shoot virus that trees occupied by Crematogaster striatula were small, with poor canopy and low in associated insects. He contrasted this directly with Majer's finding (at that time available to Bigger in the form of a cyclostyled report) that the species was found in high yielding dense canopy cocoa, and had more insect species associated with it than any other dominant ant. Bigger also cast doubt on the suggestion by Leston (1973) that the spread of swollen shoot could be slowed by the encouragement of Oecophylla longinoda; this was because Oecophylla longinoda was present at eight of the 14 swollen shoot outbreaks and was the dominant at three of the sites.
A curious fact concerning the work in Ghana is the extremely limited information on the mosaic outside of the two research stations, Kade and CRIG. This is borne out by the reviews of Entwistle (1972) and Bigger (1981a), who between them had access to the many internal reports, such as those from the Capsid Team. As neither Entwistle or Bigger include any data from Leston, one has to conclude that Leston's concept was based largely on personal visual observations, some unpublished pkd surveys, and perhaps the data collected by Strickland (1951a). Why the veracity of the mosaic concept has not been tested in terms of Ghana cocoa as a whole, of other tree crops or even of indigenous forest is a mystery.
In terms of the arboreal mosaic and of the terrestrial species which ascend trees, the findings from Nigeria permit a genuine data-based appraisal of the distribution and relative abundance of ant species inhabiting cocoa. The Nigeria data also appear to remain unique in permitting some assessment of the ants on other tree crops and indigenous trees. Overall the findings from cocoa deserve to be taken very seriously by the proponents of ant mosaic manipulation who have argued for the encouragement of Oecophylla longinoda and Tetramorium aculeatum (Leston, 1973; Major, 1975). So to do would almost certainly enhance the presence of Pheidole megacephala, already one of the most abundant and widespread species, and could also favour Odontomachus troglodytes and Camponotus acvapimensis. The potential for major enhancement of black pod disease by the soil tents of all three species is clear, but this could be exacerbated by the fact that several species of small Crematogaster were found to construct tents high in the canopy using vegetable material apparently from diseased pods. This led to quite lengthy sequences of infections in lower pods and the ants were associated with Oecophylla longinoda. The status of Pheidole megacephala in Ghanaian cocoa is not very clear but Bigger (1993) excluded it from his association analysis, despite it being found on 5-10% of trees, arguing that it was of significance only "because it is an important mealy bug tender". He similarly wrote that the aphid, Toxoptera aurantii, "cannot be considered as an important pest", but in Nigeria (the E5/1 study) we found it was by far the most abundant of the insects over which tents were built by ants, especially in the early season, and thus was a major component of the seasonal black pod epidemic. Another cautionary note came out of the work in Nigeria. This was that an apparently homogeneous stand of cocoa, offering no obvious problems for the layout of, say, a fungicide trial, could have major variations in the ant populations. Thus, the stand would exhibit significant inconsistency in the availability of disease inoculum.
Although general opinion seems to attribute the "ant mosaic" concept to Dennis Leston, at least among anglophones, in part, at least, this may not be true. For instance, Schneirla (1971) writing before his death in 1968, noted -
"Actually, any tropical forest is a mosaic of zonal patterns in which adjacent sites may differ greatly as living nooks for any animal. As a result, colonies of any species of ant (or other potential doryline booty) are likely to be distributed unevenly through the forest (Wilson, 1958)".
Writing himself, Wilson (1976) examined the question of diversity and consequent prevalence of the various ant genera. Drawing on the results of his own surveys in Papua New Guinea, various South Pacific Islands, Sri Lanka and Mexico, he demonstrated how three genera Camponotus, Pheidole and Crematogaster, in descending order of prevalence, were clearly the ecologically most diversified. Interestingly, however, when separated by strata, he listed Pheidole as terricolous (T), and the others as arboricolorous (A). In West Africa, however, it is abundantly clear that most Camponotus species are terrestrial, albeit frequently foraging to a considerable height in trees. Other genera Wilson described as prevalent on a global scale are Amblyopone (T), Hypoponera (T), Odontomachus (T), Solenopsis (T), Oligomyrmex (T), Strumigenys (T), Monomorium (T), Leptothorax (T), Tapinoma (T), and Paratrechina (T). His Table 1, also shows Tetramorium (T), Polyrhachis (A) and Oecophylla (A) as high in the abundance index list, although he noted the Polyrhachis seems to be a relatively new genus spreading through the Old World but not having reached the New World. The same would apply to Oecophylla, which surely defies his definition of "success" being due to diversity as there are only two species - longinoda of Africa and smaragdina of the remaining Old World tropics (see Bolton, 1995).
Earlier, Wilson (1958), writing of the patchy distribution of ant species in New Guinea rain forests, had noted how there was a "microgeographic separation", with an extraordinarily complex mosaic of local habitats, with ants showing definite preferences for certain local environmental conditions. On top of that he visaged "geographic patchiness", with discontinuous distribution - "ant species apparently show extensive and unpredictable variation in population density over short geographic distances above and beyond that already noted with respect to microgeographic habitat segregation". However, he was writing about a few kilometers distance and although he wrote "as a result of discordant patchiness no two localities harbour exactly the same fauna", it seems to me that he may have erred in assuming "a relatively continuous, homogeneous rain forest". In terms of the likes of cocoa farms, the evidence from West Africa suggests that dominant ants may actually be responsible for much more homogeneity than that Wilson observed.
Both on cocoa and on the trees of tropical forests, other equally significant but less obvious niches may well prove to be due to the phenomenon of dominance, well established in cocoa farms, where some five or six ant species each control territories of perhaps as many as 100 trees. Within those territories a number of other, often smaller, species are able to co-habit. Each dominant, however, has a limited number of such consorts and both positive and negative associations have been established between dominants and consorts (Taylor, 1977).
A synthesis of the mosiac in Nigerian Cocoa as a combination of dominants and associated species can be seen in the summary diagram. Interestingly, there are species which appear able to reside on trees no matter which dominant is present. These seem to survive by being discreet, unthreatening, non-competitive when it comes to food resources, or by simply being evasive and quick on their feet.
Although the pattern of dominants, or the ant mosaic, is quite well studied on cocoa, little is known about it on other tree crops and even less about the situation on forest trees. What may well be true, however, is that forest clearance and replacement by tree crops does not lead to a great depletion of natural diversity. The question of niches is further explored in Chapter 5.
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©1998, 2003 - Brian Taylor CBiol FSB FRES 11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K. |
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