Sunday, November 4, 2007

Reef fish diets defy conventional wisdom

When you jump in the ocean in temperate areas you may notice large schools of only a few kinds of fish.

In Hawaiian and most tropical waters, the pattern is generally reversed—lots of different kinds of fish, but not that many of any single species.

If they're all competing with each other for food, shouldn't certain species eventually dominate?

Ken Longenecker of Bishop Museum, conducted studies that yield one clue to how
this all works.
His research, on reef fishes off Kane'ohe Bay, finds that reef fishes may have far
more specialized diets than was previously assumed. Certain reef fish prefer a
limited range of crustaceans, while others prefer another range.
A different way of saying that is that there's far less dietary overlap than scientists
have assumed until now.
That means that the species aren't competing for food, which may make it easier for
them to co-exist—and for more different species to coexist in the same habitat.

Longenecker wrote in the journal Copeia the results of his research: “Devil in the
Details: High-Resolution Dietary Analysis Contradicts a Basic Assumption of
Reef-Fish Diversity Models.”

In an email, Longenecker wrote that his work “shows, I think, that fishes on
coral reefs have far more specialized diets than was previously assumed. This
specialization, combined with the type and quantity of food available, appears
to influence which species and how many individuals of each are
found at a
specific location.”

Previous studies of reef fish diets have generally been unable to distinguish
in great detail what they were eating. They might have known a fish was eating
a crab, but not what kind of crab it was.

Longenecker collected bottom-feeding blennies, hawkfishes, sandburrowers,
scorpionfish and gobies. He studied their stomach contents. He went and
watched the species in the wild using SCUBA gear. He also collected samples
of what they appeared to be eating.

The findings suggest that while some species feed fairly broadly, and others
have extremely specialized diets, there is generally far less overlap in feeding
than marine scientists have believes.

One impact of the paper is that it shows that you can't guess at what might
improve a species' survival, Longenecker said.

The broad implication of the research is that sweeping generalizations may
be of limited use in reef fish ecology, conservation and management. We
need detailed information about species of interest,” he said.

Many scientists have argued that the environment is intricately
interconnected, that an ecosystem needs all its parts. Here's evidence
that a problem with a tiny crustacean could have a severe impact on the
population of a specialized reef fish, and by extension, impact on the larger
species that feed on that fish.

Paying attention to what's happening at the lower end of the food chain
may help to better manage fishes at higher trophic levels,” Longenecker said.

Trophic levels are steps in the food chain. For example, grass, a cow and a
beef eater are three distinct trophic levels—the plant, the herbivore and
the carnivore. And in the case of the reef, a bit of seaweed, the little reef
fish that eats it, and the papio that eats he reef fish are three different
trophic levels.

© 2007 Jan W. TenBruggencate

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