GUERRILLA VS PHALANX STRATEGIES OF RESOURCE CAPTURE - GROWTH AND STRUCTURAL PLASTICITY IN THE TRUNK TRAIL SYSTEM OF THE HARVESTER ANT MESSOR-BARBARUS

1. In this paper we investigate some structural characteristics of the trunk trails and their organization within the colonies of the harvester ant Messor barbarus. We have analysed the growth and the adaptability of these trails in relation to the availability of resources, trying to emphasize the analogies between these dendritic systems and other ramified structures found in nature. 2. A large population of colonies of this species was studied over a 3-year sequence in a Mediterranean pasture in central Spain. The different conditions of grass abundance (main feeding resource of this granivorous species) were contrasted with the structural changes found in the trunk trails at several hierarchical levels: (i) link (trunk trail segment between two consecutive branching points); (ii) tree (or trunk trail; made up by all the interconnected trunk trail segments with a common stem); (iii) nest entrance (group of trunk trails starting from the same nest entrance hole); and (iv) nest (group of trunk trails belonging to the same nest). 3. More than 90% of trunk trail trees do not branch or reach a low (second or third) branching order. Branches seem to be produced without any regular pattern, possibly due to the lack of endogenous factors directing the structure of trunk trails. 4. The structural changes in the Messor barbarus trunk trails in relation to resource availability are consistent with the 'phalanx' and 'guerilla' models proposed for stoloniferous or rhizomatous clonal plants (Lovett Doust & Lovett Doust 1982; Hutchings 1988). These models predict a decrease in the length of stolon internodes (equivalent to our 'links') and a greater rate of branching under resource-rich conditions. Such changes appear to be attained in the Messor barbarus colonies by decreasing the length of links and increasing the number of nest entrances (and, consequently, the number of trees produced). This is accomplished without a significant change in the average effort per colony in terms of trunk trail construction and maintenance. 5. Contrary to the predictions of the dispersed central place foraging model (McIver 1991), the distances between nest entrances do not change significantly under different conditions of nutrient availability. However, they decrease when the number of nest entrances per colony increases. We interpret these results as a consequence of intraspecific competition.