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Ecological effects of paleo-extinctions

Scientists study marine mass extinctions as ecological disasters

Throughout geological time there have been a number of mass extinctions.  Until now they have been ranked by the number of species lost, not by ecological loss.

            

 

Mass extinctions are triggered by events outside the control of the involved organisms. These extinctions are distinguished in rock sections by an abrupt horizon, above which a species is no longer found. They punctuate earth history, and mark the boundaries of geological periods..

The ecology of life on earth is built around a co-dependence of organisms. The loss of one species has impacts on all the other organisms within this system. The more dominant the affected species the greater the effects to the system, as these are the groups that control energy flow.

Mary L. Droser et al. have produced a ranking for paleoecological changes (see table 1). So far work has been restricted to marine extinctions of the late Ordovician, and Devonian periods. (Geology Vol 28: no8 p 675-678). In their ranking a first level event is the most severe, and represents the destruction of an ecosystem or the creation of another.

The extinction of the late Ordovician was one of the five largest extinctions on marine life. However, research by Drosser and her colleagues has shown that it only had fourth-level ecological impacts

By contrast the late Devonian extinction triggered second, in addition to third and forth level ecological changes. Corals, an abundant and dominant set of organisms were virtually destroyed, an event likened to the loss of trees today. Yet by taxonomic loss the extinctions were almost identical.

The Paleo-extinction model has not contributed to our knowledge of the mechanisms of species loss, but rather their relative importance. It has reminded us that the severity of an extinction is not a function of the number of species lost but their ecological importance.  A factor, which is now to some extent quantifiable.

This new model of mass extinctions, has not only provided a new way to analyze paleo-extinctions, but has also provided a model of what will happen in the extinction events which are occurring at present. This information will help prioritise conservation work.

Table 1. Definition of Paleoecological levels and signals
Level Definition Signals
First Appearance/ disappearance of an ecosystem Initial colonisation of environment
Second Structural changes within the ecosystem First appearance or change in dominant taxon
Loss/ appearance of metazoan reefs
Appearance/ disappearance of types of adaptive strategy
Third Community level changes within an established ecological structure. Appearance/ disappearance of community types "filling in" or "thinning" of adaptive strategies
Forth Community level changes Increase/ disappearance of whole communities
Variations in a group of common descent.

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