The non-indigenous ant, Solenopsis invicta, reduces loggerhead shrike and native insect abundance

The red imported fire ant (Solenopsis invicta Buren) is an aggressive, non-indigenous species that is a threat to native biota in the south-eastern United States. We determined the effect of S. invicta on loggerhead shrike (Lanius ludovicianus L.) abundance and investigated a possible mechanism of impact, which is a reduction in insect prey availability. We used a fire ant bait (hydramethylnon) to reduce fire ant populations on one randomly chosen member of each of five pairs of 202-ha study areas in the Texas coastal Bend region, and also measured shrike relative abundance and a volumetric index of insect biomass on the study areas. Loggerhead shrike relative abundance was assessed at five counting stations established along 3.2-km transects through prairie habitat on each study area during the fall of 1992 and 1993. We sampled non-S. invicta invertebrates with 13.3-L capacity UV light traps and found that insect volume, species richness, and diversity were greater on treated sites. More shrikes were observed on areas where S. invicta populations had been reduced. Both insect biomass and shrike abundance were negatively correlated to the level of S. invicta infestation. Our data suggest that shrikes may avoid areas on wintering habitats that have been invaded by S. invicta and that this avoidance may result from reduced insect availability.