Density-dependence effects on the spatial dynamics of a Neotropical herbivore

Background/Question/Methods

Understanding the spatial dynamics of organisms is an important goal in ecology. Spatial dynamics can be influenced by both biotic and abiotic factors. More importantly, it can also be affected by the density of conspecifics. Although density-dependence has been recognized as an important mechanism affecting populations in many ways, much remains to be clarified about its influence on spatial dynamics. In this study we used a dominant Neotropical herbivore and agricultural pest, the leaf-cutter ant Atta laevigata, in the endangered Brazilian Cerrado as models to address how density-dependence affects key components of spatial dynamics. Mature colonies of these ants are known to displace their nests multiple times along their lives (also known as nest migration). However, little is known about their dynamic use of space. We then asked three questions: (1) Does density-dependence affect colony spatial dynamics? (2) If so, how? (3) Is the effect temporally consistent? To answer these questions we marked, mapped and monitored all mature A. laevigata colonies (nest mound size > 2 m²) in a 19-ha Cerrado site during four consecutive years. We then split the area into three subareas of increasing density of colonies and used maximum likelihood estimates to calculate the probability of nests arriving, persisting and leaving the area as a function of the existing density.

Results/Conclusions

Nest migration was a prevalent characteristic of leaf-cutter spatial dynamics, with 15-35% of nests changing their positions within the area yearly. We also found that colony migration was negatively affected by density. Arriving nests (hereafter known as newcomers) were more likely to leave the area than nests that were already there, especially in areas with high density of nests. In addition, nests (both newcomers and established ones) left high-density areas more often than low-density areas, and these patterns were consistent over time. This negative density-dependence can provide important insights for the drivers of local population dynamics of these ants as well as on their patterns of space utilization over broader scales. Locally, nest migration probably plays an important role in decreasing the intraspecific competition for resources such as space and food. Furthermore, it is known that leaf-cutters perform better near roads and therefore density-dependent colony migration could be a mechanism for the continuous expansion of these ants along these habitats at the landscape scale. Thus, we recommend that both nest migration and its density-dependence should be carefully considered in population dynamics and demographic studies over multiple scales.