Red imported fire ant impacts on the endangered Florida grasshopper sparrow

Red imported fire ants (Solenopsis invicta) invaded peninsular Florida morethan 30 years ago. Highlands and Polk counties, Florida, were probably first invaded inthe late 1960s. Since then, fire ants have continued both to spread and to increase inabundance. Experimental evidence has shown that red imported fire ants may have adetrimental impact on native species, both invertebrate and vertebrate, and that theseimpacts may be both direct and indirect. This segment of research was designed todetermine if fire ants have a negative impact on Florida grasshopper sparrow(Ammodramus savannarumfloridanus) populations at Avon Park Air Force BombingRange, Florida. The sampling reported herein was conducted during June and October,1997, and all analyses are restricted to data collected during those periods. As assessedby baits, fire ants were dominant on about half the sites, and absent from others. Inparticular, OQ Range sites tended to have fewer fire ants present, while Delta Trail siteswere more heavily infested, although there were exceptions. June fire ant abundance wasstrongly negatively correlated with both native ant abundance (r = -0.743, P = 0.006) andnative ant species richness (r = -0.730, P = 0.007). October fire ant abundance wasstrongly negatively correlated with both native ant abundance (r = -0.690, P = 0.013) andnative ant species richness (r = -0.736, P = 0.006). The patterns of fire ant abundance asassessed by pitfalls were very similar to those determined from bait sampling. June fireant abundance was negatively correlated (p < 0.05) with native ant richness, but otherJune comparisons were not statistically significant (P > 0.05). October fire antabundance was negatively correlated (P < 0.05) with native ant richness and abundance,but other comparisons were not statistically significant (P > 0.05). There was nosignificant correlation between overall insect biomass as assessed by light traps and fireant abundance as assessed by baits or pitfalls. Total biomass varied considerably amongthe two sample periods because of changes in overall insect abundance during differentseasons. There was a negative spatial correspondence between fire ants and nativeinvertebrates. Over most of the intensive study areas, there was a negative spatialrelationship between fire ants and the abundance of native invertebrates. Over about50% of the intensive study areas, there was a negative spatial relationship between fireants and the abundance of Florida grasshopper sparrows, although the relationship wasnot as strong as that between fire ants and native invertebrates. Fire ant and nativeinvertebrates were negatively correlated at grasshopper sparrow count locations (r =0.347,P = 0.03). A multiple regression model was fit to the data, using fire ants andnative invertebrates as independent variables, and grasshopper sparrow 100-m populationestimates (n = 39) as the dependent variable. The influence of fire ants on grasshoppersparrows was negative while the influence of native invertebrates was positive.However, the overall model, while suggestive, was not significant (r = 0.304, P = 0.17).Fire ant abundance was a better (negative) predictor of sparrow populations (P = 0.13)than was invertebrate abundance (P = 0.59). The overall model and influence of fire antson sparrow populations was suggestive of a negative influence warranting analyses ofdata for 1998 and 1999. (Document has 93 pages)

Research Work Order no. 175

Final report.