20141011 08365400 1.0 TRUE Standard deviation ellipses allow you to see if the distribution of features is elongated and has a particular orientation. The ovals on this map indicate that there’s a directional trend, primarily toward the northeast and southwest, which was stronger in the earlier years. The ellipses for earlier time periods are smaller; for later time periods, larger ellipses were generated. This indicates that the spatial extent of zebra mussels is growing over time (i.e., their range is increasing). The central tendency is useful for tracking changes in the distribution from year to year. The shift in the mean center indicates that the zebra mussels are spreading toward the southwest after having been primarily concentrated in the Great Lakes region for over a decade. zebra mussel Dreissena Polymorpha This map shows directional trends and the central tendency in zebra mussel sightings for each year. Esri, USGS, U.S. National Atlas 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