In biology, nothing is truly universal

No one should ever be granted a degree in science without being able to finishing this little gem of an aphorism: "It ain't what you don't know that gets you into trouble...

...It's what you know for sure that just ain't so."

Various sources attribute the quote to Mark Twain, or Will Rogers or Henry Wheeler Shaw. Chances are probably fair that none of them was the originator, but that's not the point. The point is, things change, especially our understanding of nature.

A perfect example appeared recently in the journal Hydrobiologia. It isn't going to rock ecology, like the recent finding that plants don't actually produce a lot of methane, but to any marine biologist who recalls the story of the sea otter, sea urchin and kelp forests, the lesson will be clear.

I must have been told the story in three different classes while enrolled at the University of British Columbia a few years back. Sea otters, it seems, are keystone species out along the west coast of North America. There used to be oodles of them, but they were almost wiped out by those who valued their furs more than their role in the ecosystem, and then, so the theory goes, everything went to pot.

Sea otters are voracious consumers of abalone and sea urchins. They love sea urchins. Anyone who has visited an aquarium exhibit featuring the otters will probably have seen them floating on their back, tearing apart their spiky dinners as only a sea otter can. Urchins, in turn, are among the only species that consume significant quantities of macroalgae such as kelp -- the leafy, tree-like algae that can grow to several metres in length. And kelp forest are vital habitat for a large number of species. Even gray whales may use kelp forests as shelter for calves at risk from killer whales.

So, take away the otters, the urchin population explodes, the kelp forests die and all you're left with are "urchin barrens" and a lot less biodiversity along your coastline, from Alaska to California. At least, that's the theory, one that's a trotted out by many a biology professor as a classic case of trophic cascade and the importance of keystone species.

Until, that is, a group of researchers asked themselves if all that was really true. As outlined in an edition of Marine Mammal News, a product of the North Pacific Universities Marine Mammal Research Consortium, based at UBC, and for which I once worked.

Do sea otters always reduce urchin populations and thus promote kelp growth? Or could other factors such as physical disturbance be equally important? What implications might this have for the way otter populations -- and the lucrative commercial urchin fishery -- are managed and conserved?

So they tested the idea. And you've probably guessed what they found.

...two experimental urchin removal treatments did not significantly increase the density of perennial or annual species of macroalgae [kelp] after two years, despite significant and persistent decreases in urchin densities," the authors write, pointing to a litany of other factors that may have influenced the local ecosystem, including: grazing by other invertebrates, the density of certain perennial kelp species such as Agarum, and the frequency at which populations of kelp and invertebrates reproduce and grow.

The authors, Sarah Carter, Glenn VanBlaricom and Brian Allen, are careful not to make the same mistake as those who embraced the original theory. All their experiment does is prove that the relationship between sea otters, urchins and kelp forests doesn't necessarily apply to all situations. In some stretches of the Pacific Northwest coast, the trophic cascade theory might be a useful description while in others it might not.

The results of this study support the suggestion that sea otters are only one of several factors that may regulate urchin populations in nearshore waters. They also refute the assertion that the sea otter-trophic cascade paradigm is universally applicable across locations or habitat types. From a conservation perspective, the authors encourage managers to ensure that any otter-urchin management policies are built on sound, reproducible science that is specific to the geographical region in question.

But that's a significant contribution to the lore of marine biology. it's good to be reminded every now and then that nature is a very messy place. Hard and fast rules that apply in all circumstances are rare.

In other words, every rule has its exception. Except this one.