Ecology Of Acacia-Dwelling Ant Populations 1m East Africa
Hayes-Allen, Nicola Mary
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This thesis describes a detailed study of ant populations inhabiting the swollen thorns of Acacia drepano1obium, a widespread thornbush in East Africa. The study focused primarily on populations of Crematogaster mimosae, the dominant acacia-ant associate, and to a lesser extent on the less common, related ant species, Crematogaster nigriceps. The study of each species was broken down into two parts: (1) an analysis of population structure, and (2) an analysis of population chanee. or dynamics. The first part of the study was concerned with individual ant colonies: their nature, spatial extent, structure and numerical composition. The second part involved the analysis of groups of colonies over an extended time period, to discover whether seasonal variations influenced colony productivity, and to discover how reproduction and plant colonization occurred in each ant species. Finally, through a comparison of the reproductive ecology of the two ant species, an attempt was made to define the position of each species in the thornbush ecosystem. In the following paragraphs the main fin.dings from each part of the research are summarized. Population Structure Mature colonies of both ant species occupied at least one acacia, the manner of the exploiting of plant swellings being remarkably similar; each species distributed its brood and workers through the plant thorns in a similar manner: and workers of both species were adept at entering and colonizing new swellings, as these became available at growing extremities of the host acacia. However, C. mimosae colonies were often extensive, contained multiple egg-laying queens and had poorly defined colony boundaries. C. nigriceps colonies, on the other hand, were more limited in extent, had a lower frequency of egg-laying queens, and were discrete, having very clearly defined colony boundaries. Whilst the majority of colonies of C. nigriceps were queen-producing, only a minority of C. mimosae colonies contained queen brood or virgin queens. Population Dynamics and Reproduction Marked seasonal variations in productivity were discovered in C. mimosae colonies, peak production of sexuals coinciding each year with the short rains. For the remainder of each year the emphasis was on worker production, and for a brief period each year, just before the short rains, productivity in the ant colonies reached very low levels. The 'discovery of a reproductive cycle attuned to annual seasonal change was of interest since, to the author's knowledge, such cycles have rarely been described in tropical social insects. The methods of reproduction of the to ant species were remarkably different. Mating of C. mimosae occurred on the nest . itself, with queens being subsequently reabsorbed into the mature colony. Queens of, C. nigriceps, on the contrary, left the parental colony to found (individually and unassisted) new colonies elsewhere. Evidently, the chances of an individual queen of C. nigriceps successfully rearing brood would be substantially less than for a queen of C. mimosae, which can rely on a mature ant colony to protect itself and its brood. The C. nisriceps queen might die accidentally or through predation during dispersal, might fail to find a suitable vacant nest-site, or if a nest-site is found, might be killed by coopeting ant colonies, or be socially parasitized. To compensate for this, C. nigriceps colonies produced new queens at much higher level than C. mimosae colonies. This increased queen production occurred, however, at considerable expense to worker production. Colonies of the two ant species channelled, therefore, their productive capacities in different directions: C. mimosae in the direction of increased growth and maintenance of the colony itself (building up large numbers of workers in the parent nest and extending the colony to occupy surrounding acacias), and C. nigriceps in the direction of increased production and dispersal of daughter queens to new nest-sites. On the basis of the above hypothesis, one would expect the two ant species to thrive in different types of acacia vegetation. C. mimosae, with its superior ability to expand locally from existing colonies, should eventually become the dominant species in stable, fairly dense stands of acacia. C. nisriceps should have the competitive advantage where available nest-sites are distant from existing mature colonies, i.e. where acacias are sparsely distributed and in areas where the vegetation is frequently disturbed, for example by constant uprooting and burning of the acacias. Observations 6f,the nest distributions of the two species were consistent with this expectation, showing that each ant species does indeed have a superior survival capacity in a different type of thornbush habitat. The clue to the different positions, or 'niches', of the two ant species in the acacia thornbush ecosystem was therefore to be found in their differing modes of reproduction and colonization .