4 - Where and why? Wood ant population ecology

Summary

Wood ant (Formica rufa group) populations are found across much of the northern hemisphere. However, their distribution is not universal, as there are factors which clearly limit their occurrence in the environment. Even on a local scale, within a particular population; distribution is not even, and some areas have a much higher density of nests than others. This chapter explores the population ecology of wood ants and addresses various factors that influence their distribution at different spatial scales. Specifically, this chapter will discuss: (1) the structure of wood ant mounds and how this influences where nests are found locally and within larger landscapes; (2) how wood ant dispersal strategies affect where populations are found and how they are structured; and (3) the causes and consequences of nest relocation and abandonment by wood ants.

Mound construction, properties and thermoregulation

Most red wood ant nests consist of an aboveground mound composed of organic material and a belowground part that extends into the mineral soil, potentially reaching as far down as the ground water table (e.g. Weber 1935; Gösswald 1989; Bristow et al. 1992). The aboveground part is built from needles, twigs, resin, bark, leaves and grasses collected from the surroundings (Wiśniewski 1967; Gösswald 1989; see Chapter 9). Coarser material is generally found inside the mounds where the breeding chambers are located (Wiśniewski 1967; Gösswald 1989). Finer organic material – for example needles – dominates the much denser outer layers of the mounds (Wiśniewski 1967). Some of the North American species cover only the surface of their mineral soil nests with organic material, sometimes termed ‘thatch’ (Weber 1935; Chapter 10).

Temperatures within red wood ant nests are considerably higher than air temperatures from early spring until late autumn (e.g. Lenoir et al. 2001). This allows the insects to end nest dormancy early and extend the breeding season, thus providing them with a competitive advantage over other ants. Several mechanisms have been suggested to explain how red wood ants manage to heat up their mounds in spring: (1) Forel's ‘Theory des Domes’ (see Seeley and Heinrich 1981); (2) Zahn's 1958 ‘Wärmeträgertheorie’ [heat-carrying theory]; and (3) the production of metabolic heat by either the ants themselves (Kneitz 1964, 1969, 1970; Rosengren et al. 1987; Kadochová and Frouz 2014) or by within-nest microbes (Coenen-Stass et al. 1980).