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Dive Dry with Dr. Bill

#651: Poisons Make Bad Neighbors

Although some say I've never grown up, back in my younger days here on the island I used to do a lot of hiking... especially after "Jaws" scared me out of the water for a few years. As a biology teacher at the former Toyon school, I developed classes in the sciences that used the island as our "natural laboratory." Yes, we still did a lot of marine biology but on a given week you were equally likely to find me and my students on top of a ridge as out on a boat. Back then, and during my years as vice president of the Conservancy, I was pretty familiar with the terrestrial life on our island.

Early on I learned about allelopathy. No, that's not some form of mental communication between plants and humans. Allelopathy refers to the production of chemicals by a plant that inhibits the growth of other plant species in its vicinity. In short, it is a chemical defense in the "war of the plant world." If a plant can "defend" the space around it, it gains a competitive advantage for moisture, nutrients and light over other plants. A good example is the chamise or greasewood (Adenostema fasciculatum) that grows in the chaparral here.

If you look where chamise is present, you will often see bare patches of soil around the plants. Little or no "understory" vegetation develops beneath them. Scientists believe that the chamise may produce chemical phytotoxins (plant poisons) that prevent the seeds of herbs and other plant species from germinating. My garden must be full of such toxins since I have a brown thumb. These chemicals are thought to be phenols which includes a wide range of naturally synthesized and synthetic chemicals. Your Bud Light and other beers may contain phenols and they are valued as antioxidants in foods such as berries, apples, onions, artichokes, beans and grains.

Now let's switch to the submarine habitats around our island. I have known for some time that the invasive Sargassum horneri that appeared in our waters 10 years ago from Japan via Long Beach produced polyphenols, chemicals with multiple phenol groups. These chemicals are part of its defense against being eaten by critters such as fish, sea urchins and abalone. This seaweed just doesn't taste good... kind of like asparagus, rhubarb or canned peas to me.

This understanding recently led me to speculate on how these polyphenols in the Asian seaweed might be affecting other algae. In warmer years when the Sargassum dominates our landscape, there is little growth of giant kelp or other native seaweeds. Of course to a large extent this is due to the competitive advantage it has in terms of light, nutrients and substrate to grow on. Then, after it dies out because it is an annual rather than a perennial like our kelp, there is little growth of native algal species. An area cleared of Sargassum last November by San Diego tech diver Peter Vanags and I is still largely barren after six months.

Currently the landscape in our dive park is mostly one of barren rocks in areas where the Sargassum thoroughly dominated a few months ago. As a kelp forest ecologist familiar with what a healthy environment should look like, this is very sad to see. Much of the area including the reefs is devoid of any significant large algal growth. My scientific mind is conditioned to form hypotheses to explain what I see. However, since I'm retired and without funding, I'm usually too lazy to actually test these hypotheses scientifically so take them for what they are worth.

If my hypothesis is correct, the polyphenols produced by the "devil weed" may not only limit critters from feeding on it, but may also inhibit native seaweeds from growing in its vicinity. This could easily explain why it gains such complete dominance over the rocks and reefs (and even the pebbles!) in warm years. In my research for this column, I did find scientific papers that indicate the polyphenols do inhibit certain diatoms and other algae from growing on the Sargassum so there is some evidence this hypothesis may be correct. Wouldn't it be nice if DuPont applied "better living through chemistry" to find something to counteract these polyphenols? Of course I'm thankful they invented neoprene so I'm warm enough to dive and come up with these columns.

© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Chamise or greasewood on Catalina and dense monoculture of non-native Sargassum horneri;
area cleared of Sargassum showing little native growth and diverse native algal growth

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia