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Dive Dry with Dr. Bill

#413: Finicky Eaters and Food Webs

Those of us who live on islands, and receive most of our food and other goods by sea or air, are aware that sometimes the supply lines fail. Fog may sock in our Airport-in-the-Sky or rough seas prevent the barge from coming over with trailers full of Vons groceries. Such situations may present real dilemmas to residents who have very particular dietary preferences. Others may adapt to the conditions and choose something new to munch on. Imagine someone (moi perhaps) who can only cook two things... my Thai green curry or a flank steak. If I go down to Vons and there is no coconut milk for the curry and no flank steak at 50% off, I'm in trouble. Someone else may go down the same day expecting to buy Sugar Pops, but find they are absent from the shelf. If this is the only cereal their kids will eat, they are in trouble just like me. Oatmeal just isn't an option these days. However, if the kids will just as readily eat Lucky Charms, they may be... well, in luck.

The same thing occurs in natural ecosystems and their associated food chains and food webs. If a critter has a highly specialized "diet," perhaps a parasite which only attaches to a single host species, it is out of luck if that source of food disappears through pollution, over fishing, temperature change or other impact. In fact, I wonder what the sea lice that frequent the giant sea bass did when the bass numbers crashed back in the 1970s. If they had alternate hosts, they could select "door #2" and survive. If a nudibranch like the beautiful Dendronotus iris, sometimes referred to as the rainbow nudibranch, only feeds on the burrowing sea anemone (Pachycerianthus fimbriatus), what would it do if the anemones disappeared? Fortunately for it (and for us since it is quite beautiful), at least in some areas of its range it can also feed on hydroids and young jellyfish and the anemones are hardly rare... yet.

Highly specialized species and therefore very picky eaters may face serious challenges if their prey declines in number or disappears from their range. "Generalists," critters which can feed on a wide range of munchables, have a far better chance of finding food should one of its many menu items become locally extinct. People who live on islands like myself would be better served by becoming generalists rather than specialists when it comes to their food. Since my culinary skills are quite limited, unless it involves a microwave or poaching (as in boiling water), I may have to partner up with some lovely lady whose skills far surpass my own. However, I am a very accomplished dish washer, so I will carry my end of the deal.

Another aspect of "munching" (in the Macrocystis or even on land) that is important is the complexity of an ecosystem's food webs. Very finicky eaters might create a rather linear food chain in which there is only one link at each feeding level. Let's say that a snail will feed only on one species of seaweed, and is in turn eaten by only one kind of fish. That fish gets munched on only by sea lions and the sea lions meet their maker in the jaws of a great white which specializes on that blubbery fare. At each level in this simple food chain, the primary producer (seaweed), herbivore (snail), first carnivore (fish), second carnivore (sea lion) and tertiary carnivore (shark); there is but one species. If that particular seaweed (alga) disappears, the snail is out of luck... and so is the fish that feeds on it, the sea lion and the great white shark. Such linear food chains do not produce very stable ecosystems.

Let's add the element of biodiversity to the system. We now have a healthy ecosystem with many different species all involved in "the mutual eating society." Munch, munch, munch. Each species has a wide set of options. The snail in this system can feed on many different types of algae, perhaps mixing a few greens with the reds and browns. One alga disappears and it has other options. Because there are now many more species, the snail may now face competition from other algae eaters including other molluscs or fish. And with a greater range of fish present, there may be many that prey on our snail... and its other relatives that munch the algae. Harbor seals, larger predatory fish and birds such as osprey and bald eagles now compete with the sea lion for the fine finny fishes. And the great white shark will eat fish in its youth and tasty marine mammals including the harbor seal and sea lion.

This diversity creates a wide range of interlinked food webs, with many connections between the different species in the ecosystem. There are choices for most species, although a few will stick to their highly specialized diets and risk dying off if things change. Intricate food webs such as these give a much higher level of stability as ocean conditions alter through global climate change, ocean acidification, over fishing and other impacts. Reductions in biodiversity are not a good thing, but at least in these complex systems more of the species will survive. As we humans "simplify" ecosystems by harvesting selected species from them for food (or to feed our domestic animals including food species and pets), by decimating global shark populations as witnessed within my own lifetime, or through invasions of exotic species like the Asian Sargassum that arrived here from Japan and now dominates Catalina's leeward waters during the colder months creating a monoculture similar to a field of corn or wheat, we increase the risk of instability in marine systems (as well as terrestrial ones).

Ultimately we, as a species often high up on food chains and webs, are dependent on this biodiversity. This is a lesson we must learn before we kill off our preferred food supply or the sources for new medicines and other technological breakthroughs. Unfortunately such self interest is often the motivating factor... we should also learn that healthy, complex ecosystems have a value of their own beyond their utility for us alone.

© 2010 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. 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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Rainbow nudibranch climbing up tube anemone, copepod parasites on giant sea bass;
very simple food chain and complex food web.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia