Density-dependent spatial patterns and habitat associations of a dominant desert ant

Abstract

The abundance and distribution of ant colonies can reflect the relative importance of different ecological processes such as abiotic limitations and biological interactions varying with time, space and scale. We fitted a series of incrementally complex spatial models for point patterns in one and two dimensions to examine how the habitat associations and spatial arrangement of Pheidole bergi colonies change among scenarios of natural and anthropogenic variations in environmental conditions in the central Monte desert. At the habitat scale, P. bergi preferred the protected open woodland (algarrobal), with strong and persistent density variations at scales of tens of meters along most dirt roads crossing it. The 40% reduction in active colonies during the 2019-2025 drought was inversely density-dependent, increasing aggregation. Instead, colonies were 40-50% lower but more homogeneously arranged on roads crossing habitats with extensive natural or anthropic limiting factors (sand dunes and livestock-grazed algarrobal). Colony density within the protected algarrobal also showed >50% reduction between the wet 2001 and the dry 2019, with their distribution always influenced by the two-phase mosaic structure typical of arid lands. This preference for inter-patch borders and open areas rather than the interior of woody vegetated patches probably explains why the intensive anthropic impacts associated with dirt roads had no notable effect on P. bergi density. Beyond that spatial dependence, significative distance-related pairwise interactions between colonies shifted from slightly attractive in 2001, resulting in clumped distributions, towards strongly repulsive by 2019, favoring more random and overdispersed spatial patterns, contrary to the expected correlation between intraspecific competition and colony density. Pheidole bergi seems flexible enough to establish colonies in different habitats and microhabitats although with distribution and population dynamics still shaped by natural and anthropic, direct and indirect environmental constraints such as soil texture, vegetation cover or resource availability affected by multi-year droughts. The inversed density-dependent thinning and shifting relevance of inter-colony interactions with climate fluctuations observed at different spatial scales highlight the complex interplay between environmental constraints and intraspecific dynamics in the population ecology of P. bergi, an abundant, omnivorous, aggresive and probably ecologically relevant ant species in the Monte desert.