This table presents an optimum latitude for the species, based on its importance statistically in the N-S dimension. Data are presented for current (FIA -> based on actual field plot data), predicted (PRD -> based on model) and potential future GCM scenarios (GISS, GFDL, HAD, UKMO, and CCC). A low Optimum number (e.g., 50 = 500 km from the southern tip of Florida) indicates the species is primarily located in the far southern part of the eastern US (i.e., the panhandle of Florida), while a high number indicates an optimal range in the northern latitudes. See the map below for the approximate latitude of the row number indicated in the table.
Also included is the potential movement of the optimum latitude for the species, in km, according to each of the GCM scenarios. Here, the optimum can be projected to move either N or S, and is indicated in the table. Most species potentially will have their latitudinal optimum move north, but not in all cases.
Methods used to calculate the optima and movement (see
also our 1998
Ecological Monographs paper): Several steps were required to
calculate the optimum. First, the country was subdivided into a grid of
10 x 10 km cells, which included 277 rows of data for the eastern United
States. The relative IVs were calculated for each row (total IV/total area
in row), and the overall distribution by row was statistically analyzed.
The latitudinal optimum is then calculated as the mean of the interquartile
range (all the points falling between the 25th percentile and the 75th
percentile); we therefore take the mean of the bulk of the data, without
including outliers. The change is calculated (in km) by subtracting the
current modeled (PRD) latitudinal optimum from the optimum predicted by
the GCM scenario, and then multiplied by 10 to account for the 10 km cells;
if the remainder is positive, a northward migration is projected.