An ecological engineer maintains consistent spatial patterning, with implications for community‐wide effects

Abstract

Abstract. In many ecosystems, foundational species create spatial patterns that structure a broader community. It is unclear, however, how robust these patterns are across large areas and strong environmental gradients, and how the landscape-level consequences of these patterns may vary. We investigated the robustness of non-random patterning in the dispersion of the western harvester ant (Pogonomyrmex occidentalis), a widely recognized ecosystem engineer of western North America. We used remote imagery to characterize the spatial structure and densities of western harvester ant mounds at sites spanning their range within the sagebrush steppe and short-grass prairie areas of Wyoming (581 3 450 km area). We found that ant mound densities varied substantially across the study region, but that mounds were strongly and consistently overdispersed (regularly patterned) across both climatic gradients and mound densities. Precipitation was the only abiotic factor that significantly affected either density or pattern, with stronger patterning among mounds at drier sites. This robustness in ecological patterning is likely to have strong effects on community function; mound dispersion increased the fraction of the landscape within typical ant foraging distances up to 30% over what density alone would predict. We estimated how patterning can modify one key ant effect at a landscape level by combining mound dispersion data with information from a seed removal experiment. Randomization tests based on these results showed that in a representative area, overdispersion could increase the mean landscape-wide seed removal rate by 16%, and decrease its spatial variance by 50%. Western harvester ants are known to affect multiple aspects of community function and structure at a relatively fine scale, and our results show that their spatial dispersion may therefore influence many features of interspecific interactions and community dynamics.

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

“…Ground visits of the plots "T Australia Jiga" and "T Australia Pilb" confirmed that the substrate is as flat and edaphically as homogeneous as for the typical FCs patterns 50 km further to the south . A lack of an effect of topographic or floristic variation on spatial patterns of insect mounds has been also reported for North American harvester ants, which do sometimes inhabit very homogeneous grasslands (Dibner et al, 2015).…”

The important role of ecological engineers in providing ecosystem services at landscape level

“…Therefore, the central target of species reintroductions should become their ecological role played in the context of system stability, rather than simply increasing the number of species. In other words, what matters is the functional diversity as well as how it is spatially patterned, even if it is still unclear whether and how spatial patterning of key species varies within most communities (Dibner, Doak & Lombardi, ). In the case studies presented by Hayward et al .…”

A multi‐scale study of Australian fairy circles using soil excavations and drone‐based image analysis

“…, Dibner et al. ). In Australia, we would expect that such ecosystem engineers would cause spatial patterns that are similar to the FCs, if they were the cause.…”

“…A regional-scale analysis of the typical signature of termite-created gaps vs. FCs may thus help to shed light on the spatial differences and thereby on the mechanisms driving the formation of the fairy-circle patterns. For example, true ecosystem engineers such as harvester ants create regularly spaced gaps in grass vegetation over a large precipitation range of 200-600 mm annual rainfall in many ecosystems of North America (Nicolai et al 2010, Dibner et al 2015. In Australia, we would expect that such ecosystem engineers would cause spatial patterns that are similar to the FCs, if they were the cause.…”