Veronika Jílková

Contribution of wood ants to nutrient cycling and ecosystem function

Red wood ants ( Formica rufa group) build large and long-lasting nest mounds. Due to their construction, nest mounds and especially their centres, provide a favourable microclimate for brood development (Coenen-Staβ 1980). However, constructing and maintaining the nest in addition to feeding the colony requires a considerable amount of energy and building material. The ‘central-place foraging’ strategy (Chapter 7) employed by red wood ants – retrieving food and plant material from the area surrounding the nest – means they influence not only the properties of their nests, but also those of the wider area. Wood ants influence the functioning of forest ecosystems through altering food web structure (Chapter 8), soil properties, and nutrient transport and energy flow (Frouz and Jilkova 2008). The vast accumulation of organic materials in the nest means that once the nest is no longer in use, nutrients are released during the decomposition process which is performed by microorganisms (Lenoir et al . 2001). These nutrients are then available for plant growth. This chapter addresses the effects of wood ants on nutrient flow and plant growth in forest ecosystems. Since these effects are associated with nest building and territorial activities, they can be divided into nest- and territory-related effects. Nest-related effects Foraging for food Nest-related effects are connected with foraging, nest construction and the subsequent decomposition of organic materials. Decomposition in nests is enhanced because of the abundance of microorganisms and the favourable microclimate. Mineral nutrients are released from organic materials but predominantly remain in or near the ant nests, thus creating hot spots in nutrient-limited forest ecosystems. Wood ants feed on honeydew and solid insect prey; honeydew rarely forms less than two-thirds of the energy imported to F. rufa nests (Whittaker 1991). Ants may collect more than 80% of the excreted honeydew, which constitutes 62–94% of their diet and contains 15–20% sugars, a small percentage of amino acids and c . 70% water (Rosengren and Sundstrom 1991). Thus, approximately 13–16 kg of honeydew dry mass containing a significant amount of nutrients (Table 9.1) is transported annually into the average ant nest (Frouz et al . 1997; Jilkova et al . 2012). Once transported into the ant nest, the honeydew is redistributed and used as a source of energy (Horstmann 1974).

Methane and carbon dioxide flux in the profile of wood ant (Formica aquilonia) nests and the surrounding forest floor during a laboratory incubation

We compared methane (CH4) and carbon dioxide (CO2) fluxes in samples collected from the aboveground parts of wood ant nests and in the organic and mineral layer of the surrounding forest floor. Gas fluxes were measured during a laboratory incubation, and microbial properties (abundance of fungi, bacteria and methanotrophic bacteria) and nutrient contents (total and available carbon and nitrogen) were also determined. Both CO2 and CH4 were produced from ant nest samples, indicating that the aboveground parts of wood ant nests act as sources of both gases; in comparison, the forest floor produced about four times less CO2 and consumed rather than produced CH4 Fluxes of CH4 and CO2 were positively correlated with contents of available carbon and nitrogen. The methanotrophic community was represented by type II methanotrophic bacteria, but their abundance did not explain CH4 flux. Fungal abundance was greater in ant nest samples than in forest floor samples, but bacterial abundance was similar in both kinds of samples, suggesting that the organic materials in the nests may have been too recalcitrant for bacteria to decompose. The results indicate that the aboveground parts of wood ant nests are hot spots of CO2 and CH4 production in the forest floor.