Interference competition, not predation, explains the negative association between wood ants (Formica rufa) and abundance of ground beetles (Coleoptera: Carabidae)

1. Intraguild interactions between predators have significant implications for how multiple predators impact on their prey. Carabid assemblages are not generally thought to be structured by interspecific competitive interactions, species relative abundances instead being determined mainly by abiotic factors. However, several field studies report negative associations between the abundance of certain carabid species and the presence of wood ants.

2. Replicated laboratory experiments were used to monitor the behaviour of individual female Abax parallelepipedus (Piller & Mitterpacher) in the presence of a range of wood ant (Formica rufa L.) densities (including ant-free controls) over 23 h. Carabids did not feed when wood ants were present. Running and defence activities were high at low ant densities but decreased with increasing ant density. At the highest ant density, all carabids were killed within the first 2 h of the experiment, but the ants did not consume the carabids after they had killed them. Carabid diurnal activity increased at the higher wood ant densities.

3. Female A. parallelepipedus and Pterostichus madidus (F.) maintained individually in the presence of five wood ants had significantly lower mean body weights and carried fewer eggs after 21 days compared to control beetles that had been kept without ants.

4. The results help to explain the reduced abundance of these two carabid species under field conditions when wood ants are present. We interpret these findings to mean that the interaction between wood ants and carabids is one of asymmetric aggressive interference competition rather than predation.