Estimates of benthic invertebrate community variability and its environmental determinants differ between snapshot and trajectory designs

Abstract Long-term data sets are essential for biodiversity research and monitoring. Researchers use 2 major approaches in the study of temporal variability of biological communities: 1) the trajectory approach (monitoring sites across several consecutive years) and 2) the snapshot approach (comparing sites among few sampling events several years apart). We used data on benthic macroinvertebrate communities in 23 near-pristine forested streams to compare these 2 approaches for different study periods ranging from 3 to 14 y. We asked whether the level of temporal turnover and the identity of the best explanatory variables underlying it were comparable across studies based on differing approaches, study periods, or total duration. The 2 approaches yielded partly different stories about the level of community variability and its environmental correlates. With the snapshot approach, variation in community similarity and factors explaining it reflected short-term (e.g., year-specific) conditions, which could be misinterpreted as long-term trends, the difference being most evident for periods that began or ended in an extreme drought year. Our results imply that snapshot studies may lead to ambiguous conclusions, whereas the trajectory approach yielded more consistent results. Trajectory data of differing length showed minor differences, apart from studies with the shortest durations. Overall, our results suggest that time sequences of ∼6 y of trajectory data (i.e., 6 generations for most benthic invertebrates in boreal streams) may be needed for the among-year similarity of macroinvertebrate communities in near-pristine streams to stabilize. If temporal replication is limited (snapshots/very short time sequences) the outcome depends strongly on the particular years included in a comparison. Based on our results, we advise caution when basing conclusions on a comparison of a few (e.g., just 2) occasions several years apart or on very short time sequences