The Economics of Optimal Foraging by the Red Imported Fire Ant

For social organisms, foraging is often a complicated behavior where tasks are divided among numerous individuals. Here, we ask how one species, the red imported fire ant (Solenopsis invicta Buren) (Hymenoptera: Formicidae), collectively manages this behavior. We tested the Diminishing Returns Hypothesis, which posits that for social insects 1) foraging investment levels increase until diminishing gains result in a decelerating slope of return and 2) the level of investment is a function of the size of the collective group. We compared how different metrics of foraging (e.g., number of foragers, mass of foragers, and body size of foragers) are correlated and how these metrics change over time. We then tested the prediction that as fire ant colonies increase in size, both discovery time and the inflection point (i.e., the time point where colonial investment toward resources slows) should decrease while a colony's maximum foraging mass should increase. In congruence with our predictions, we found that fire ants recruited en masse toward baits, allocating 486 workers and 148 mg of biomass, on average, after 60 min: amounts that were not different 30 min prior. There was incredible variation across colonies with discovery time, the inflection point, and the maximum biomass of foragers all being significantly correlated with colony size. We suggest that biomass is a solid indicator of how social taxa invest their workforce toward resources and hypothesize ways that invasive fire ants are able to leverage their enormous workforce to dominate novel ecosystems by comparing their foraging and colony mass with co-occurring native species.

The fate of selected heavy metals and arsenic in a constructed wetland

The fate of Cd, Pb, Cu, Zn, Ni, Cr, and As in a horizontal subsurface flow-constructed wetland was studied. The concentrations of the risk elements in treated municipal wastewater, wetland sediments, and Phragmites australis biomass were determined. Most of the studied elements were removed from the wastewater with fair efficiencies. On the other hand, As was released to treated water in the wetland bed. The removal efficiencies obtained for the individual elements were as follows: 64.2, 48.7, 70.0, 93.9, 63.6, 63.8, and -236.2%, respectively. The concentrations measured in sediments were the highest for samples taken 1 m from the inflow zone. They were 4.11, 2.01, 6.01, 4.85, 3.39, 9.30, and 3.17 times higher as compared to the samples taken in the distance of 10 m. The pollutants were preferentially deposited at the front part of the wetland bed where anaerobic conditions predominated and sulfate reduction took place. There were no significant differences in the concentrations of the studied elements in the aboveground biomass (Phragmites australis) samples taken in the distances of 1, 5, and 10 m from the inflow zone. However, the concentrations measured in the belowground biomass samples were significantly higher for samples taken at the front part of the wetland bed. The individual element concentration ratios between the below- and aboveground biomass measured for samples taken 1 m from the inflow zone were 4.97, 5.97, 2.21, 1.58, 2.26, 5.70, and 2.66 for Cd, Pb, Cu, Zn, Ni, Cr, and As, respectively.