Invaisive Species

Discussion

The presence of Bythotrephes in Harp Lake showed a significantly greater mean length (mm) and lower mean abundance (daphnia/m3) of the Daphnia population. However, there was no significant change in the biomass (µg/m3). The differences in length and abundance suggest that the invasive species can possibly be correlated with increased predation, competition and overall disturbance of the steady state equilibrium in the given aquatic ecosystem, Harp Lake.

The significant decrease in the abundance of Daphnia in Harp Lake may be correlated to the added predation due to the presence of Bythotrephes. It has been found that many zooplankton species significantly declined in abundance, while others completely disappeared as a direct result of the introduction of Bythotrephes into their ecosystem (Yan et al. 2002). This may allude to a hypothesis that the Bythotrephes primarily eat Daphnia. This trend can also be related to problems in reproduction. In a study by Manca et al. (2000), the increase in abundance of Bythotrephes showed the abundance of small ovigerous females virtually non existent in Harp Lake. This leads to the possibility that although Daphnia are capable of reproducing asexually, their way of sexual reproduction may be potentially eliminated, resulting in the significantly lower abundance found in Harp Lake.

Bythotrephes predation may also be the cause of the larger mean length of Daphnia found in Harp Lake. In a study by Yan et al. (2002) it was found that the abundance of large Daphnia increased with the presence of Bythotrephes. This could indicate that although the total Daphnia abundance may be lower, the majority of the remaining Daphnia were mostly all large. Larger Daphnia could be better suited for survival from predators, possibly resulting in the larger mean length found in Harp Lake.

Although the average length of the Daphnia increased, Sand Lake and Harp Lake did not show any significant difference in biomass. It is possible that although the total abundance of Daphnia decreased, the increase in mean Daphnia size perhaps made up for the lost biomass resulting from predation on smaller Daphnia. In aquatic ecosystems this suggests that the biomass of a given species remains relatively stable; there are merely simple shifts in the total ratio of species size (Dolganova. 1997). The constant biomass found in this study potentially resulted