Responses of the trunk routes of a harvester ant to plant density

Abstract

Messor barbarus is a Mediterranean harvester ant that constructs physically defined trunk routes on the ground to connect nest entrances with foraging areas. Some responses of these trunk routes to plant density (and therefore resource abundance) were analyzed by testing the preferential allocation of different parts (trunk route ends, segments and branching points) in a patchy environment. Maps of grass density in four categories and Messor barbarus trunk routes were compiled for a Mediterranean pasture in Central Spain over four consecutive years. The proportions of the density categories in each year were used to calculate random expected frequencies of the trunk route points and the predominance of higher or lower grass densities. Trunk route ends discriminate and selectively reach patches with a greater abundance of resources in all study years. Branching points are also allocated preferentially in areas with higher vegetation density, but only in years with a predominance of the higher categories of grass density. In these years, the colonies of Messor barbarus have a "phalanx" strategy at a colonial level, and branching is more profuse. Finally, trunk route segments do not indicate any preference for crossing determined vegetation densities, but rather connect successive branching points or trunk route ends by the shortest route. These results concur with a model of structural strategy change ("guerilla" - "phalanx") (Hutchings 1988) at the level of trunk routes. They are probably constituted by "transitory" sections with few branches, that expand other more profusely branched sections which are more dedicated to foraging.

Spatial patterns of seed predation by harvester ants ( Messor Forel) in Mediterranean grassland and scrubland

“…The presence of trunk trails was considered to be an indication of a potential foraging area, regardless of their trophic value. Of the three Messor species, only M. barbarus constructs conspicuous trunk trails for food searching, collection and transport (Cerdan, 1989;López et al, 1993b;Reyes-López and Fernández-Haeger, 2001), while those of M. bouvieri and M. hispanica are more seasonal and less visible. We therefore focused on M. barbarus in this part of the study.…”

“…Finally, we defined the proportion of subplots crossed by trails (PST) for each plot and habitat type, and used these values as indicators of the extent of the foraging areas. The physical trunk trails of M. barbarus remain visible for several weeks even if they are not used (López et al, 1993b), eliminating the risk of bias due to the difference between the sampling period (August) and the months of maximum seed production in certain habitats (May -June in dry areas). 3) Seed predation risk.…”

“…Patterns of seed predation by Messor harvester ants are subordinate to the spatial structure of the ecosystems (Díaz, 1992;López et al, 1993a;1993b;Wilby and Shachak, 2000). In Mediterranean grassland and scrubland a large part of the environmental variability is linked to the gradients generated by the topography and the presence of dispersed tree cover (Bernáldez et al, 1969;González-Bernáldez and Pineda, 1980;Rivas Martínez et al, 1980;De Pablo et al, 1982;Gómez, 1991;González Bernáldez and Peco, 1991).…”

“…Nest-holes must be as close as possible to the foraging areas, selected mainly on the basis of seed availability (López et al, 1993a;1993b;Wilby and Shachack, 2000). Vegetation structure can also affect the accessibility of certain habitats and modify microclimate conditions, on which ant activity is heavily dependent (Cerdá et al, 1988;Kaspari, 1993).…”

Seed predation in a Mediterranean pasture: can ants modify the floristic composition of soil seed banks?

“…Many authors state that harvester ants are the main seed predators in Mediterranean grasslands (Azcárate & Peco 2003, Cerdá & Retana 1994, being the granivore Messor species the most important (López et al 1993a, Hensen 2002. Studies on floristic composition of Messor species diet show that they consume seeds from a wide variety of species, although it has been observed that their foraging strategy tends to reduce time costs by selecting preferably long propagules, which are more accessible and easily detectable (Reyes-López & Fernández-Haeger 2001, Hensen 2002.…”