Thursday, October 08, 2009

What is Medean Life Part 4: Biodiversity Through Time

As some of my blog followers have been noting that I have been posting on the Medea Hypothesis as posited by Dr Peter Ward in his book of the same name. I have been working my way through what I think are the relevant parts and posting summaries (and quotes) from them. The first post about what the different hypotheses, Gaian and Medean, are. The second post covered what would make life Medean, rather than Gaian, is here. The third post is about the Medean "Events" of the past. Originally, I had planned to do this fourth post on biodiversity and biomass through deep time and into the future. I realized though that biodiversity through time and biomass through time are independent subjects and ought to be covered separately.

The complete Medea Hypothesis Review Table of Contents is here.

This time we are tackling biodiversity through deep time.

Ward paints three different possible scenarios of how biodiversity could exist through time.

Scenario One: Ever Increasing Complexity & Diversity

The first one is where over time, life has become more and more diverse. Life is growing ever more diverse and ever more complex in its interactions. The ecology through time has been becoming more and more complex in the methods that life extracts energy from its environment. As more complex ways are evolved, more opportunities for further niches arise which in turn life moves on to occupy.

This has been the relatively traditional view that the scientific field has held. In fact, it's held this since the 19th century. Work up through the end of the 20th century appears to have confirmed it. Specifically, it has been supported by the work of the late Dr Sepkoski. His work is based on surveying the scientific literature and picking up the named genera. This work has, iirc, continued by successors.

The biggest critique of this view is what is called the "Pull of the Recent." What is that? The problem cited is just the fact that the more recent the sediments, the more common they are. There are more locations for Pleistocene fossils (what is popularly called the Ice Age) than there are for Eocene (the hot house 35 million years ago). The further back in Deep Time you go, the lower number of locations where fossil bearing sediments are available. Therefore, you will get a bias to the present. Or at least will be biased to whichever eras have the most locations. Or have been the most collected from. or...There are a number of good criticisms. More later.

Scenario Two: Diversity Plateaus

The second view is that at some time in Deep Time, life hits diversity plateaus. That is to say that at different times, life had innovations that allowed it to diversify and then hit some inherent limit within the ecology: it was impossible to diversify more because all the potential niches were occupied.

Where these plateaus are is a very contentious issue. Some work indicates that the Late Paleozoic was as diverse as now. Other research suggests that the peak was actually in the Late Cretaceous as part of the Cretaceous Ecological Revolution with the introduction of flowering plants. Still others argue that life hit a step function at different times. The Paleozoic, Mesozoic, and Cenozoic were each more diverse than the last. The work, in general, is pretty new. Paleontologists are more collectors and analyzers of animals and only a handful - a very argumentative group at that - study the larger issues of diversity and ecology in Deep Time. I find the same names from 30 years ago that are arguing about it now when digging through all of this.

Third Scenario: Peak Diversity is Past

There is another, very heretical (ie not accepted) view that the diversity of life hit its peak in the past. Ward argues that, in fact, that life actually hit its peak during the Eocene as far as its diversity and we are now in the long slide to ecological simplification. Think of the old simple curves they used to do for civilizations (Toynbee's Organic Cycle of Civilizations? I think?): there was a period of youth where they would grow and expand. This was followed by a mature stance or period where expansion stopped, but maintained itself. Then, old age sets in and contraction sets in and ultimate decline, fall, and extinction follow. While not exactly excepted much in history studies these days, Ward presents that the decline stage has already set in. He asserts that life is the cause of its own decline in biodiversity. This makes it Medean.

His arguments for the passing of the peak of biodiversity fold into the next post: that biomass has been declining through time and life is the cause. We'll cover that in the next post.

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