Dive Dry with Dr. Bill

043: Biogeography- Here, There But Not Everywhere

Homo sapiens are members of an unusual species since they can adapt to many different environments. Most biologists state they may be found anywhere on the planet, from the bitter cold of the Antarctic to the intense heat of the Kalihari desert. Of course biologists who are truly "worth their salt" know that human beings are found in less than 30% of the biological habitats on Earth. There are the rare few, like myself, who are highly amphibious but none of our species live permanently in the 70% of our planet that is water!

Biogeography is the study of the geographical distribution of living things on our planet (or, eventually, in the Universe). Where is a species found, how did it get there, and where might it go from here are questions often asked by biogeographers. I began my scientific career considering such questions as I researched how marine life might cross the San Pedro Channel and colonize islands like Santa Catalina and the other Channel Islands. I benefited from having E. O. Wilson, one of the great scientific minds in this field, as a college professor. Initially my work focused on collecting drifting kelp and analyzing the marine life that might have detached with it and traveled to the islands. Later I started looking at larval forms that drifted with the plankton before settling to the bottom to become adults.

Most species are only able to tolerate a certain range of conditions such as temperature, salinity or water motion. You would hardly expect a fish from the coral reefs of the South Pacific to survive if transplanted to the cool temperate water kelp forests of southern California. In fact the global distribution of temperate kelp forests and reef-building corals is a good example of biogeography based on temperature. Giant kelp survives best in cool waters less than 68 degrees while coral reefs are found in warm tropical and subtropical waters. Therefore coral are largely limited to the warm water belt between the Tropic of Cancer and the Tropic of Capricorn. Giant kelp can be found in the cooler waters of higher latitudes in both the northern and southern hemispheres.

Biogeographers often refer to geographic regions with similar conditions as provinces. While working in the waters off Baja California during the winter, I dive in the northern part of the Panamanian Province. Catalina and the Channel Islands are uniquely located in a transitional zone between the northern Oregonian Province and the Californian Province that begins at Point Conception. Areas where biogeographic provinces meet are often very interesting since marine life from both provinces may be found depending on local conditions. The northern and western islands in our group like San Miguel and San Nicolas often have fish, invertebrates and algae from the colder water Oregonian Province, while those to the south and east like Catalina tend to have the warmer water Californian Province species.

Two good examples of this mixing of provinces and their species here are the sea urchins and the gobies. In the Dive Park one can find both the beautiful blue-banded and the black-eyed gobies, as well as the black and red sea urchins. The black-eyed goby is found in colder waters from northern British Columbia to cenbtral Baja while the blue-banded goby is common in warmer waters from Pt. Conception into the Gulf of California (Sea of Cortez). The red sea urchin is found from Alaska to central Baja and the black urchin from the Channel Islands to the Galapagos. Therefore we have elements from the north and from the south present in our transitional waters.

Even a single island like Catalina offers a wide range of conditions... and different species. I was reminded of this while diving Bird Rock near Two Harbors. Along with the usual cast of characters from areas like the Casino Dive Park, there swam a painted greenling which I usually see in the cooler waters off the northern Channel Islands. It is important to realize that habitats like Catalina's nearshore waters are not uniform, but consist of many different microclimates and microhabitats based on local conditions.

It is obvious that the windward and leeward sides of Catalina Island differ in wave exposure. They also differ in water temperature with the more mixed windward side tending to be two or more degrees colder than the protected leeward side Avalon is located on. There is also a trend from the cooler waters of the West End to the warmer waters of the East End. In fact, the region between Long Point and the East End on Catalina has the warmest average water temperature of the island and the entire Channel Islands group.

Temperature is not the only factor determining a species' distribution. Some can only live on specific substrates, for example giant kelp on rocky bottoms or bat rays on sandy or other soft bottoms. Others require specific food items and are only found primarily where their food lives. The ochre starfish prefers areas of high wave exposure like our windward coast since it feeds on mussels which live there. All these different variables, and their interplay with the local geography, make biogeography a very complex puzzle to interpret and therefore an interesting subject for biologists and other curious Homo sapiens.

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Image caption: Warm water black, and cold watrer red sea urchins;
warm water blue-banded and cold water black-eyed gobies.

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia