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Although the conterminous USA has a long history of ichthyological
exploration, the description of biogeographical provinces has been ad hoc.
In this study we quantitatively determined fish faunal provinces and
interpreted them in the context of the geological history of North
America. We also evaluated influences of major river basin occupancy and
contemporary environmental factors on provincial patterns. Our data set
comprised 794 native fishes, which we used to generate a presence and
absence matrix for U.S. Geological Survey (USGS) four-digit hydrologic
units. Three nested data sets were analysed separately: primary freshwater
families, continental freshwater families (including primary and secondary
families) and all freshwater families (including primary, secondary and
peripheral families). We used clustering analysis to delimit faunal breaks
and one-way analysis of similarity (ANOSIM) to determine significance
among clusters (i.e. provinces). We used an indicator-species analysis to
identify species that contributed most to province delineations and a
similarity-percentage (SIMPER) analysis to describe the relative influence
of representatives from each category (i.e. primary, secondary,
peripheral) on provincial boundaries. Lastly, we used a parsimony
redundancy analysis to determine the roles of historical (i.e. major river
basin) and contemporary environmental factors in shaping provinces.
Analysis of the nested data sets revealed lessening provincial structure
with inclusion of more families. There were 10 primary freshwater
provinces, 9 continental freshwater provinces and 7 all freshwater
provinces. Major basin occupancy, but not contemporary environmental
factors, explained substantial variance in faunal similarities among
provinces. However, provincial boundaries did not conform strictly to
modern river basins, but reflected river-drainage connections of the
Quaternary. Provinces represent broad-scale patterns of endemism and
provide a starting point for future studies. Relative malleability of
province boundaries in the continental interior highlights this region as
biogeographically diverse and dynamic. Interior-core provinces of this
region (Central Gulf Coastal Plains, Northern Interior) have not been
recognized previously and warrant further study.
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