Masters Theses

Date of Award

5-2002

Degree Type

Thesis

Degree Name

Master of Science

Major

Ecology and Evolutionary Biology

Major Professor

Daniel Simberloff

Committee Members

Susan E. Riechert, Karen M. Vail

Abstract

Interspecific competition between invasive hybrid imported fire ants, Solenopsis invicta x S. richteri, and native ants and the role of fire ant population density on competitive ability were quantified in two different habitat types, field and pasture, in southeastern Tennessee. To quantify the effects of competition indirectly, diversity values for ant communities were calculated in sites invaded and not invaded by fire ants within the field and pasture habitats. Sites invaded by fire ants had lower ant species diversity than sites not invaded. Further, sites invaded by fire ants had up to 6.75 times more individuals than sites not invaded, but fire ants accounted for as much as 93% of these individuals. Baiting experiments were performed in invaded sites to observe competition directly and to quantify it. Species composition and interactions at a total of 60 baits in each habitat were recorded every 15 minutes over a three-hour period. Competitive ability was described by the number of baits a species discovered first, recruited to first, and numerically controlled at the end of the three-hour observation period. Fire ants performed better than all native ant genera combined for all of these variables. Fire ants discovered and recruited to baits more quickly than all native ant genera except for Monomorium. Fire ants also recruited to a higher portion of baits discovered except for Monomorium in the pasture habitat. In addition, fire ants were present at baits in much higher densities than native ants. Very few baits were recruited to by more than one genus at the same time. When more than one genus did co-occur at a bait, the genera always spatially partitioned the bait, thus avoiding interspecific interactions. Dietary preferences were shown in this partitioning. Fire ants preferred to feed upon the hot dog (lipid and protein source) portion of the bait while other genera preferred the sugarsodium hydroxide portion of the bait (carbohydrate source). When interspecific interactions did occur, they were rarely aggressive. In fact, of 120 baits and 72 hours of observation time, only 2 aggressive interactions were observed. Despite the lack of cooccurrence of genera at baits and the lack of aggressive encounters, control of some baits did switch throughout the course of the observation period. When a switch occurred, the density of the numerically dominant genus slowly decreased while the density of the other genus at the bait slowly increased until it dominated at the bait. To examine the role of fire ant population density on competitive performance, half of all fire ant mounds in a one hectare area in each habitat were poisoned with OrthoR OrtheneR Fire Ant Killer and the same baiting experiment as described above was performed. After fire ant population density had been reduced, native ant genera discovered and recruited to more baits first than did fire ants. In fact, after fire ant nest poisoning, native ants discovered first, recruited to first, and controlled more than twice as many baits as they did before poisoning. Additionally, after fire ants were poisoned, native ants recruited to a higher portion of baits discovered and recruited to baits more quickly once they had been discovered than they did before poisoning. The reduction in fire ant population density negatively affected its competitive ability. After poisoning, fire ants discovered first and recruited first to approximately half as many baits as they did before poisoning, but they controlled the same number of baits that they did before poisoning. Fire ants recruited to fewer baits discovered in the field habitat, and their recruitment time after discovery of a bait was longer in the pasture habitat. Importantly, native ants and fire ants controlled equal numbers of baits after poisoning. Even after poisoning, fire ants were present at baits in higher densities than native ants. As occurred before poisoning, few baits were recruited to by more than one genus at the same time, very few aggressive interactions occurred, baits were spatially partitioned, dietary preferences were shown, and control of baits changed slowly over time. These results support the hypothesis that invasive fire ants are superior competitors to native ants in invaded habitats. In contrast to what other studies have reported, aggressive interactions at baits played an insignificant role in competitive outcome for resources while exploitation competition appeared to be the dominant mode of competition. Fire ant population density played a pivotal role in their competitive superiority and may be the proximate reason why introduced fire ants have had such a high level of success invading native ant communities.

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