Monday, July 02, 2007

The critters come and go, but the niche remains the same

kw: book reviews, nonfiction, natural science, oceanography, evolution

The more things change, the more they stay the same! The common canard is, "Evolution is random", and certain elements of evolution are random, but the Selection aspect is definitely not random: a creature that is well suited to its environment will prosper, and one that isn't won't.

The largest environment on the planet, the ocean, has room for very large browsing animals. In fact, it has room for a whole range of browsing animals, just as the continental surfaces do. So we find on land that browsers range from elephants, rhinos and giraffes through horses, deer and antelopes, to woodchucks, rabbits and mice...and that's just the mammals. There are also browsing birds such as geese and emus.

In the water, which reduces the struggle against gravity and makes long-distance travel cheaper, the biggest browsers can be much bigger than elephants, so we have the whales. There are many varieties ranging from the huge Blues (30m and 100T) to the Minke and Pygmy Right whales (7m and 3T—The largest elephants reach 3T weight but are less than 3m in length). Baleen whales aren't the only browsers, of course. There are also sirenians (dugongs and manatees) and smaller browsers, both mammals and fish, and even a browsing lizard, the Galapagos marine iguana.

Pacific Gray Whales are the most frequently seen baleen whales, because their migration path follows the California coast, where they can better avoid Orca attacks. The Gray is a nice, middle-sized whale, 16m and 35T.

If you were to slip back in time millions of years, how would the California coast look? Would there be whales? What filled the "whale" niche fifty million or 100 million years ago?

David Rains Wallace has done his best to portray the changing, yet self-similar West Coast of the United States through time, at least Phanerozoic time (from just before Trilobites to today), in Neptune's Ark: from Ichthyosaurs to Orcas. Clearly, from the title, he leans more toward the carnivores. Both on land and sea, the largest carnivores are a fraction of the size of the largest herbivores, however, so I found myself speculating about them instead.

It seems baleen whales have been around only since the mid-Miocene (perhaps 15 million years). So the large-scale browsing of krill and other zooplankton didn't begin until about that time. Earlier browsers fed on eel grass and other near-shore plants and were much smaller. Thus, the "biggest animal in the sea" title goes to carnivores, by default, for most earlier times.

Thus, this Janjucetus hunderi, smaller than an Orca (5m or less, 1-2T) lived much like an Orca who hasn't any baleen whales to pursue. This image is very similar to a recent news account of a 9m Orca seen to snatch a 2m juvenile Great White Shark for lunch. Janjucetus lived in the Oligocene, about 25 million years ago. It was one of many early whales, and not the biggest.

Standing on a cliff over the California coast (actually somewhere in mid-California; the coast was still accreting in the Oligocene), you might see an early whale like this one, and smaller animals nearer the shore, some browsing on grasses or kelp, others hunting shellfish or fish. Seen in detail (you did remember your binoculars, hmmm?), there would be differences aplenty. But just look, perhaps rather lazily, and it would appear much the same as today.

Let's go back farther. 100-200 million years ago, Mosasaurs like this rather generic one ranged up to 15m and perhaps 20T. They filled the predator-whale niche that is today filled by Orcas and Sperm whales (18m and 50T). Again, while the coastal view would differ in many details from today, the general view would be similar.

If I properly understand both Wallace and other information I've read, since at least the late Triassic, the largest-animal niches in the sea were air-breathers, first reptiles, then (post-Cretaceous) mammals. I hadn't really paid as much attention to the ocean before, but now it's clear that the "Age of Reptiles" and the "Age of Mammals", at least as regards the large-bodied ecological niches, apply to both land and sea...of course, by total biomass, it has been the Age of Insects since the mid-Devonian or so.

But prior to the Permian, the largest sea creatures were Orca-sized fish. The Devonian ones were armored monsters, and later ones had shed their armor in favor of speed and maneuverability. It would be harder to observe these from a sea cliff, because they didn't need to surface to breathe.

The title of Wallace's book implies that some portion of the US West Coast has preserved creatures from the past. I am not sure this is what he meant. He surveys evolutionary trends beginning in the late Precambrian—when everything was smaller than a breadbasket—and traces the evolution of whales, sirenians, seals and sea lions, plus "sea bears" and purported "sea apes" and a number of other now-extinct families.

The ecology of coastal margins off Eastern North America was affected by large agricultural populations for at least the past several thousand years. Though humans came to the Americas from the West, they appear to have sparsely settled the West Coast, compared to the East, so perhaps Ark refers to the relatively pristine character of West Coast ecologies. Certainly today, one must go far offshore or deep into the abyss to find places with only a lightly human touch.

As he brings the story up to the present, Wallace unflinchingly portrays the naturalist-exploiters who first described the Steller's Sea Cow or the Dall Porpoise, and the ravages that followed. Only recently has our collective conscience brought about sufficient conservationist efforts that some ecologies can be preserved or allowed to rebuild. For this reason, one may go to Point Reyes, California or Cape Lookout, Oregon, and see oceanic life that still bears some semblance to the view seen by those who first crossed Beringia a dozen millennia ago, or more.

No comments: