Wednesday, October 10, 2007

Recovery from mass extinction

Douglas H Erwin (Dept Paleobiology, Smithsonian Institute) - Lessons from the past: Biotic recoveries from mass extinctions

This paper, for a 2000 colloquium, "The Future of Evolution", is essentially a literature review in the area of recovery from mass extinction.

Although it's the extinctions that capture the imagination, environmental and species recovery can be seen as more pertinent, since this defines the pathway to the present.

Erwin characterises two types of extinction event: pulse extinction, which is too rapid to see adaptive organic response, and press extinctions, where the environment gradually turns sour (so to speak) for a large number of existing species, but for which steady adaption to environmental change is possible.

For pulse extinctions in particular, recovery is characterised as gradual expansion from more sheltered environments, best exemplified by deep water (below continental shelf) niches, which would be most resilient to changes in temperature or atmospheric oxygen.

Initial post-extinction fauna typically consists of a limited variety of widespread species. It's not obvious to me whether this is due to opportunistic expansion or good suvival of prevalent species, but the paper suggests the former. However, there's a cautionary note that that shouldn't be taken as global spread.

There is some support for the intuitive notion that more ["morphologically"] complex species suffer more in mass extinctions - ie the evolutionary clock is reset or wound back somewhat. That in itself should give pause for thought.

The paper makes several mentions of "Lazarus taxa" - species which apparently disappear at the event's boundary, only to subsequently reappear. These are often simply an illustration of the gappiness in the fossil record, but Erwin suggests this can often be due to biogeographic differentiation, ie local geography. For example, he says that extinction is less apparent in the southern hemisphere after the K/T event - the meteor in the shallow waters of Yucatan, Mexico.

A couple of minor extinction events are mentioned: the early Jurassic Toarcian event and the Late Cretaceous Cenomanian-Turonian event. In the descriptions given, I see both as bearing putative symmetry with current conditions: both are press extinctions involving marine anoxia (oxygen depletion) during relatively high sea levels and a greenhouse climate. Adaption (by anoxic-adapted species such as bivalves) is suggested rather than opportunistic expansion [by a few remaining species].
Lest this breed complacency, it's worth noting that recovery of the terrestrial (and thus atmospheric) carbon cycle took a mere 130,000 years!

From peak of extinction rates to peak of speciation (diverse and specialised fauna) is at one point set to about 3 million years - but I would find it hard to accept generalisations on this. Some interesting phenomena in the interval include:
- clades on the wane as others gradually take over, which is reversed by the extinction event, simply because of the greater resilience to the specific event. Erwin mentions cheilostome bryzoans taking over from cyclostomes, only to have the latter triumph over the conditions of the event;
- other lineages that survive the event, only to disappear as (re)speciation hits its peak.

Overall, the suggestion is that there is no clear relationship between the magnitude of an extinction and its evolutionary impact. Modelling has shown 80% of phylogenetic structure to survive a 90% overall species loss. That is, most body types survive, and subsequent speciation is only a matter of variants.

Which would bring us back to the question of the Cambrian Explosion: why a short period of evolutionary activity resulted in pretty much all the body types in existence today.

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