How many fish can you name? Five? Fifty? How about all 829 species native to the rivers and lakes of the continental
U.S. and ? Naming all those species is impressive, but the fifteen-or-so minutes that it takes to do so would be a bit like watching water boil… only longer. Names only become interesting if we know something interesting about the species they pertain to. Canada
In the case of North American freshwater fish, one thing we do know is where each of them live (or lived, for 19 species that recently kicked the bucket); Lawrence Page and Brooks Burr recently published a brand-new field guide that has maps of the ranges of all of these fish species. But still, this isn’t all that earth-shattering to anyone but an ichthyologist or sport fisher with a checklist. What’s actually interesting is what Page and another biologist (Jason Knouft) did with these range maps.
By laying all of the maps on top of each other (using a computer, of course) they were able to show that native fish follow both a latitudinal and longitudinal diversity gradient; there tend to be more species of freshwater fish in the lakes and rivers in the southern and eastern parts of the continent than in the northern and western parts. People have been studying latitudinal diversity gradients for a long time and have come to realize that, even though it’s a cool pattern, the fact that there are more species closer to the equator really doesn’t tell us much about why there are more species closer to the equator.
If the fish are split up into the different families to which they belong (e.g. basses, sculpins, catfishes, perch, trout, etc…) we can learn a lot more about why fish diversity varies so greatly between different places. Different fish prefer different types of environments; salmon like it cold, but catfish like it hot. When the researchers correlated temperature and other environmental variables with the diversity of each of these groups, it came as no surprise that trout and salmon (Salmonidae family) diversity is higher in the frigid north and that catfish diversity is higher in the balmy south. In fact, each fish family had different aspects of the environment that they keyed in to. Not many suckers live in the mountains, but minnows can be found high or low.
In the case of North American fish vs. Latitude, the latitudinal diversity gradient appears to be losing- the pattern appears to be more of an accident of which families of fish are most prevalent and less of a general pattern found across all types of fish. Most of the differences in diversity between the north and south stem from differences in the environment and the ability of different groups of fish to survive and diversify in these environments. In the end, latitude itself doesn’t actually have much to do with it.
You can find this article at:
Knouft, J., & Page, L. (2011). Assessment of the relationships of geographic variation in species richness to climate and landscape variables within and among lineages of North American freshwater fishes Journal of Biogeography, 38 (12), 2259-2269 DOI: 10.1111/j.1365-2699.2011.02567.x