Spatial structure of traditional land organization allows long-term persistence of large Formica exsecta supercolony in actively managed agricultural landscape

Abstract

Semi-natural linear habitats such as field and meadow margins are often crucial in maintaining insect diversity and species distributions in agricultural landscapes. While many ant species are capable of forming large colonies (supercolonies) that can have considerable effects on the ecosystem within which they are embedded, it is not known how colony development, persistence, and spatial structure in agricultural areas may be affected by these boundary habitats. We used historical orthophotographs, extensive mapping of ant nests, and spatial statistics to study how the structure of fine-scale traditional agricultural land-use mosaic in the western Carpathians of Central Europe (Slovakia) influenced one of the largest supercolonies of Formica exsecta documented in Europe—the second largest supercolony in terms of the number of nests (~1500 nests) and significant in its spatial extent (~5 ha) and estimated population abundance (~50 million of workers). The spatial analysis indicated that a distinct linear arrangement of the majority of nests along unmanaged property boundaries contributed to the remarkable size of the colony. Further, the structure of the supercolony appeared to be variable in space and dynamic over time; while some areas appeared to be saturated by larger older nests (hot spots), other areas were characterized by clusters of smaller younger nests (cold spots) potentially suggesting colony expansion over time. Field and grassland boundaries are important components of traditional fine-scale land organization in Central Europe and elsewhere, and they appeared to provide suitable and stable nesting habitat that facilitated the growth of the second largest European supercolony of F. exsecta, while the areas further away from the boundaries were affected negatively by land management. Our results demonstrate that F. exsecta supercolonies may persist in a broader range of agricultural disturbance regimes, including intensive grassland management (e.g., mowing), when an appropriate network of habitat refugia is included into land organization (e.g., unmanaged margins).