20 April 2018 Abstracts

Paleoecological Investigation of the Effect of Rainbow Trout Predation on Zooplankton Body Size and Community Composition

Michael Gilray '18

Stocking fish for sport may induce food web cascades that impact the tropic state of lakes. Here, the practice of stocking rainbow trout into a Twin Cities metropolitan area lake (Square Lake) was examined to see if changes have occurred in the lake’s zooplankton community that could explain it’s declining water quality. Rainbow trout are size-selective predators that consume large-bodied herbivorous zooplankton (i.e., Daphnia pulicaria) and therefore may cause decreased phytoplankton grazing pressure and more eutrophic conditions. A paleoecological study was performed that compared the sediment record of Square Lake to that of a reference lake (Big Carnelian Lake) that has never been stocked with rainbow trout. The primary focus of this study was to use exoskeletal remains (Bosmina headshields and Daphnia post-abdominal claws) and resting eggs (Daphnia ephippia) to determine if changes in the abundances of multiple zooplankton species and the body size of Daphnia species have occurred since the initiation of the trout stocking program in 1981. We found that for both proxies (post-abdominal claws and ephippia), D. pulicaria mean body size decreased significantly in Square Lake during the post-stocking years (after 1981), but the body size of D. mendotae did not decrease. No significant changes in D. pulicaria body size was seen in either lake before 1981. The shift in species composition from larger-bodied D. pulicaria to smaller-bodied D. mendotae after 1981 was not seen until late (~ 2010) in the sediment record. In addition to these findings, data on human population size and land development in the lakes’ watersheds were used to understand changes in the sediment and zooplankton community composition. For both lakes, it was found that since pre-European settlement to 1981 the sediment composition has become increasingly dominated by clastic and organic materials, and that the zooplankton populations have shifted in dominance from Bosmina to Daphnia. Differences in fish stocking practices of each lake caused opposite changes in the mean body size of their D.pulicaria populations. The rainbow trout stocking in Square Lake caused the mean body size to decrease, while the stocking of walleye into Big Carnelian Lake caused it to increase.

Conflict and detection avoidance as an alternative raiding strategy in the ant Temnothorax pilagens

Alison Bartak '18

The invasion of a host species colony by a slave making species is a crucial step in recruitment of slaves to further the continuation of the parasitic colony. Many slave making species invade by use of raids, which creates not only conflict, but escalation which ends in many casualties on the host species. Given these escalations, fewer host adult workers are then able to be taken back to the parasitic colony and enslaved. A specific slave making species, Temnothorax pilagens are able to circumvent detection by host species by use of chemical mimicry. This results in a peaceful raid where there are few to no casualties, and the parasitic species is also able to enslave adult workers of the host species.This study investigates the concept of aggressive vs. nonaggressive raiding strategies in different slave making species. They further investigate the ability of T. pilagens to exhibit chemical mimicry and avoid recognition by their host species, and possible explanations for this ability. The objective of this study was to determine the process in which they can chemically mimic other species, as well as test the aggression levels between T. pilagens and other slave making species against host species. Through testing the relative aggression towards T. pilagens, and other slave making species, this study revealed that T. pilagens had a relatively low aggression response by the host species in comparison. This study also revealed that aggression towards slave making species would prove counterproductive to those of the host species. This is due to the mass casualties the host species experiences when raids are escalated.