TESTING THE SENSITIVITY OF A PALEOECOLOGIC METHOD (RANK-ABUNDANCE CURVES) USING MODERN DATA: ZOOPLANKTON RESPONSE TO INCREASING ACIDITY, LITTLE ROCK LAKE, WI

Given the current biodiversity crisis, understanding the processes of extinction is increasingly important. Taxonomic loss is often used to measure the impact of extinction events, but ecologic responses have been difficult to measure. Rank-abundance curves (RACs) quantify changes in community structure during periods of stress (which may or may not result in extinction). To determine the sensitivity of RACs to biotic responses at different temporal resolutions (i.e., the effects of time-averaging on fossil communities), RACs were applied to a long-term zooplankton data set collected during the acidification and recovery of Little Rock Lake (North Temperate Lakes LTER program). RACs have been used to infer the ecologic response of modern and fossil communities; thus, a better understanding of the sensitivity of RACs will aid in the proper application of this method in the future. Stress can be caused by disturbances on many scales: daily (e.g., temperature shifts), seasonal (e.g., ice cover), or larger disturbances (e.g., basin acidification). The shapes of RACs change predictably with stress by becoming increasingly peaked; peakedness is then measured by kurtosis. As the shape and amount of RAC change is relative to each unique community, a baseline (pre-extinction or, in this case, a control basin) is needed to determine the scale of the ecologic response. The effects of time-averaging on kurtosis were simulated by examining the original temporal resolution (biweekly) and multiple lower temporal resolutions (monthly/yearly). At the highest temporal resolution (biweekly), samples showed high variability from one sample to the next, and as pH decreased, community structure changed reflecting increasing stress (more peaked RACs). At lower temporal resolutions, kurtosis values were less variable between samples (i.e., less noise from small disturbances), but still indicated pH stress. At the lowest temporal resolution (entire experimental record versus control), there was a significant difference between experimental and control. These findings indicate that RACs are sensitive to ecological disturbances that do not result in extinction, and that time-averaging does not negatively affect the results of this metric. Thus, RACs can be useful in conservation ecology and in paleoecology.