One Species or Two? The Grebes of the Sacramento National Wildlife Refuge

One of these is not like the others. Can you tell which one is which?

The concept of a species has apparently always been a problem in biology. The definition I was taught, the idea that a species is a group of organisms that can produce viable offspring, is a real problem in my own field of geology and paleontology. How can we know if a species was capable of successfully reproducing from fossil evidence? We depend on the morphology a great deal, but it often produces problems. Consider what future paleontologists will think about fossils of all the dog breeds in existence...would a Chihuahua be considered the same species as a German Shepherd? There are debates about whether the large number of species of ceratopsian dinosaurs might actually be growth stages of a single species.

As we learn more, the idea of boundaries between species becomes even more uncertain. There are some species that are more closely related than others, and hybridization is a common phenomenon. Such hybrids may be a powerful driver of evolutionary change. Which brings us to these unique birds in the pictures today. They include a single picture of some Clark's Grebes (Aechmophorus clarkii), and some Western Grebes (Aechmophorus occidentalis). We saw them in the ponds of the Sacramento National Wildlife Refuge near Willows, California, on the first day of our recent journey through British Columbia and northern Washington state. What criteria did you use to distinguish between the two?

Biologists considered them to be the same species until 1985, when they split them because they didn't tend to interbreed, had different calls, occupied slightly different niches in their wetlands environments, and on major differences in their DNA sequences. The Western Grebes are more numerous (~110,000 vs ~11,000), but both are threatened by loss of their wetland environments. The most visible differences are the eyes, which are surrounded by black in the Western Grebe and white in the Clark's Grebe. The beak of the Clark's Grebe is brighter yellow while the Western's is more olive in color.

The Sacramento National Wildlife Refuge is one of our favorite birding spots. It has spectacular seasons in fall and winter when the migrant ducks, geese, and cranes arrive from the Arctic. The birds gather in flocks 10,000 strong, and few sights are as memorable as seeing all of the Snow or Ross's Geese taking off at once. Summer is much quieter, but is a good time to view the year-round species and the raptors. I noted 26 different species on our late June visit.