THE MAGNITUDE OF EXTINCTIONS
1) Background extinctions - continuous, low level extinctions.
        Extinction rates never hit zero.
        Low rates near 5 famillies/myrs
        These extinctions may be due to small scale environmental changes,
            disease, predation, pseudoextinction, competition, etc.

    Be sure to understand the figure shown in class and handed out in class
    on December 2nd.
    Be sure to understand the different interpretations of why there appears to
    be a trend toward lower levels of background extinctions through time.
 
 

2) Mass Extinctions - these are the extinctions in which extinction rates exceed
                                background extinction rate by a factor of two or so.

    What do all the mass extinctions have in common?
        1) Many taxa become extinct. Often 30% of the biota is lost.
        2) Extinct taxa span a broad range of ecologies - typically includes
                marine and non-marine forms, plants, animals, microscopic forms and large forms.
        3) Extinctions all occured within a relatively short amount of time. Related to a single
                cause or cluster of interlinked causes.
 
 

CAUSES OF EXTINCTION
    1) Biological Causes(mostly for background extinctions)
        a) small population size and geographic range
        b) diseases - single species lost (mass extinctions unlikely)
        c) competition - few rigorous studies actually suggest that this is important

    2) Physical Causes (both background and mass extinctions)
        a) global climate change (not always sufficient evidence) Why no mass extinction in the Pleistocene?
        b) Sea level changes/ species area effect (not always sufficient evidence)
            If you kill all of the modern shelf bivalves, only 17% of the families would be killed.

    3) Extraordinary Causes (reaseon why were reluctant to invoke them as causes)
        a) Unheard of vulcanism (difficult to evaluate)
        b) Change in Solar Radiation (difficult to evaluate)
        c) Nearby supernova explosion (unlikely)
        d) Meteorites - cyclicity of extinctions.

PHASES OF EXTINCTION
        Extinction -> Survival -> Recovery

        Be sure that you can draw a figure of diversity patterns through these phases.

Holdover Taxa - usually make it into the survival phase or just into the recovery phase and then go extinct.

Progenitor Taxa - appear during the extinction or survival phase and rapidly radiate in the recovery phase.

Disaster Taxa - long-ranges species of opportunists that are typically small and morphologically simple.
    They're present in large abundances in the survival phase. Their presence usually indicates elevated environmental stress.

Lazarus Taxa - taxa that reappear after they have disappeared from the fossil record during the extinction phase
    several millions of years earlier.

    So, why is there a period of disappearance?

    Refugia Idea (Kauffman and Erwin, 1995) refugia are ecological sanctuaries.
    Population Size Declines

Elvis Taxa - (Erwin and Droser) Another idea about some Lazarus Taxa. Found that the old taxa just resemble the new taxa - due to convergent evolution. So the new taxa that appear, just look like the older ones. "In recognition of the many Elvis Impersonators who have appeared since the death of the King"