NEW PAPER OUT! The economics of optimal foraging by the red imported fire ant


First paper of 2020 is out in Environmental Entomology! Here we look at how the red imported fire ant, Solenopsis invicta, regulates its foraging behavior. This was an interesting project that we first piloted back in 2015 and built on all the way through the summer of 2018. We were interested in testing ideas from from optimal foraging theory (Figure 1 below) and developed the Diminished Returns Hypothesis that posits for social insects (1) foraging investment levels increase until diminishing gains result in a decelerating slope of return and (2) this investment level is a function of the size of the collective group. This hypothesis we argue is an analog to Charnov’s Marginal Value Theorem and in testing it we found that fire ants forage as predicated in a particular manner.

Figure 1 - Fire ant foraging
Figure 1. Hypothetical plot of the Diminishing Returns Hypothesis which predicts that as time progresses, ant colonies should invest more workers until diminishing gains result in a decelerating slope of return. Vertical dashed lines represent two time periods, discovery time (a) and the inflection point (b), that are predicted to decrease with colony size while the amount of foraging investment is predicted to vary with colony size (c). The inflection point (b) represents the hypothetical time when colonial investment towards resources slows. We visualize the inflection point here as the time along the predicted logarithmic curve where the slope of the tangent line to the origin (0,0) no longer increases (inset plot; see methods). Points along the curve represent hypothetical foraging investment (either foragers or biomass) as a function of time with a grey polygon signifying variation that may occur across colonies (c).

However, fire ant foraging behavior was also regulated, at least in part, by the size of the colony. We posit this is possibly why large fire colonies have been so successful in their invasion of the United States and compared values with native co-occuring species in southern Oklahoma (Figure 2 below). We found that fire ant foraging mass was greater than the estimated colony mass for 45% of the co-occurring native ant species (10 of 22), many of which are small myrmecines from the genera Monomorium, Myrmecina, Pheidole, Strumigenys, and Temnothorax. The estimated average colony mass of fire ants, on the other hand, was greater than 95% of the co-occurring species (21 of 22)—the lone exception being the red harvester ant, Pogonomyrmex barbatus, whose workers are often 20-fold larger than fire ants.

Figure 2 - co-occuring species
Figure 2. Comparisons between red imported fire ants (Solenopsis invicta; RIFA) and native ant species in Oklahoma. Species are arranged top to bottom by estimated colony biomass within ant subfamilies [Dolichoderinae (upward facing triangle), Dorylinae (square), Formicinae (diamond), Myrmicinae (circle), and Ponerinae (downward facing triangle)]. The x-axis is on a log10-scale. Dashed vertical lines represent either the average foraging biomass of RIFA from this study or the estimated colony biomass of RIFA. Values for worker masses and colony sizes are found in Table S1.
Taken as a whole, our results suggest that substantial biomass differences between invasive and native ants are likely one of the key reasons that species like red imported fire ants are able to dominate novel environments. A pretty fun paper overall and our first foray into the realm of testing optimality models. Check back for more updates and you can view the whole paper at the link below.

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