Ecological consequences of colony structure in dynamic ant nest networks

Abstract

Access to resources depends on an individual's position within the environment. This is particularly important to animals that invest heavily in nest construction, such as social insects. Many ant species have a polydomous nesting strategy: a single colony inhabits several spatially separated nests, often exchanging resources between the nests. Different nests in a polydomous colony potentially have differential access to resources, but the ecological consequences of this are unclear. In this study, we investigate how nest survival and budding in polydomous wood ant (Formica lugubris) colonies are affected by being part of a multi‐nest system. Using field data and novel analytical approaches combining survival models with dynamic network analysis, we show that the survival and budding of nests within a polydomous colony are affected by their position in the nest network structure. Specifically, we find that the flow of resources through a nest, which is based on its position within the wider nest network, determines a nest's likelihood of surviving and of founding new nests. Our results highlight how apparently disparate entities in a biological system can be integrated into a functional ecological unit. We also demonstrate how position within a dynamic network structure can have important ecological consequences.

Does cooperation mean kinship between spatially discrete ant nests?

“…However, polydomous nest networks in wood ants and other species are structured to allow efficient transport of resources (Cook et al., ), suggesting there may be a colony‐level benefit to allowing, and even promoting, resource transfer between nests. In F. lugubris, a nest's position within the colony resource flow predicts nest survival (Ellis, Franks et al., In Review; Ellis, Procter et al., In Review), and nests within a network that do not forage are more likely to be abandoned (Ellis & Robinson, ). There is, therefore, an advantage to be connected to multiple nests, which should elicit competition between nests if connections are not cooperative.…”

“…We cannot assume that carbohydrate resource movement correlates with worker movement (Ellis, Franks et al., In Review; Ellis, Procter et al., In Review); therefore, we assessed internest resource movement independently of worker movement in a subset of 10 of the 24 mapped triplets in July 2014. We restricted the resource movement assessment to 10 of the triplets containing smaller nests, because in these smaller nests, we could be confident of detecting the marked food using a sample size of 100 workers per nest.…”

“…All of these potential benefits of polydomy follow logically from the assumption that the colony is a cooperative unit, and this is reinforced by empirical evidence of cooperation in the form of resource exchange between nests (Buczkowski, ; Ellis, Franks, & Robinson, ; Ellis & Robinson, ; Gordon & Heller, ). Different methods for delineating ant colony boundaries do not always draw the same colony boundaries (Ellis, Franks & Robinson In Review; Ellis, Procter, Buckham‐Bonnett, Robinson In Review).…”

“…Polydomous trail networks are structured to allow efficient transport of resources within the colony (Cook, Franks, & Robinson, ). In the wood ant F.lugubris, nests which, due to their network position, experience a higher flow of resources are more likely to grow, reproduce, and survive from year to year than those experiencing a lower resource flow (Ellis, Franks et al., In Review; Ellis, Procter et al., In Review). Polydomous trail networks therefore represent connections between cooperating nests (which we use as a shorthand to mean cooperating ant communities, each living within its own nest), sharing workers and resources, in line with the expectations of a social insect colony.…”

“…In populations of F. lugubris, groups of nests connected by trails are often bordered by other nests to which they have no social connection, although the distance between unconnected nests can be similar to that between connected nests (D. Procter personal observation). Wood ant trails frequently persist in the same location for many years (Rosengren, ); furthermore, during mapping of trail networks of F. lugubris in the UK over 3 years, separate trail networks were never observed to become connected by trails (Ellis, Franks et al., In Review; Ellis, Procter et al., In Review), indicating stability of distinct networks of connected nests. Formica lugubris exhibits variation in dispersal strategies across its range, but in the UK, new nests are formed by budding, whereby one or several queens split off from the parent nest with a subset of the workers and form a new nest nearby (Hughes, ).…”