Re: Birds as dino-killers

[Michael Bruce Habib <mbh3q@cms.mail.virginia.edu>]

On Sun, 2 Sep 2001 10:45:10 -0400 (EDT) John Bois 
<jbois@umd5.umd.edu> wrote:

> _Background extinction_ is a concept of limited value.  The idea comes (I
> think) from background radiation, the relatively constant amount of
> radiation that reaches the planet.  Biological extinction is not at all
> like this.  Each extinction has a unique set of circumstances.  There is
> no _rate_ for this.

Background extinction is actually relatively useful in some 
ways, and there are certainly rates than can be assigned.  
However, there is no single rate: the rate of background 
extinction varies between lineages and at different times, 
which is part of what makes such rates interesting.  

You find background extinction rates just like any other 
rate: it's a given unit per unit time.  In this case, you 
take the number of species (or families, etc) that suffer 
extinction over a given amount of time.  Mass extinction 
rates are found the same way.  The difference is the rate 
itself: mass extinction is the loss of many taxa in a short 
time, this produces a high rate of extinction.  At times 
not labelled as mass extinction events, species are still 
lost, and these rates are called background extinction 
rates.

It is true that each extinction has a unique set of 
circumstances, but this does not prevent one from 
determining the number of extinctions over a given time 
period.  Since there as so many "background" events, most 
causal studies are focused on mass extinction events (more 
interesting anyway, from a research standpoint, in most 
cases).  

> So...if it is true that pterosaurs suffered a gradual 
> decline toward the K/T, it is likely that this was caused 
> by birds--because birds
> were the only creatures able to reach inaccesible nesting and hiding
> places of winged pterosaurs.  If birds could do this to pterosaurs, they
> must also be able to cause trouble for small to medium-sized non-avian
> dinosaurs.  If it is true that there was a reduction in the diversity 
> of these size ranges among dinosaurs, birds must be considered prime
> suspects.

This is assuming that birds were involved in the extinction 
of pterosaurs by some sort of direct predation or attack 
scenario.  This is rather unlikely.  This is not an 
invasive species scenario (nor is it likely to be a 
human-like mass-kill scenario).  It is certainly possible 
that birds, being the other flying vertebrates at the time, 
were involved in pterosaur extinction through competition 
for nest sites, food sources, etc.  

Example: Hawks do not tend to force smaller birds into 
extinction by hunting them to death, but many smaller bird 
species are being reduced in numbers by interactions with 
introduced small birds.  By the same token, unless Mesozoic 
avians were somehow utilizing the same food or nesting 
sites as small non-avian dinosaurs, the involvement of 
birds in non-avian dinosaur extinction is unlikely.  

Keep in mind that a slow decline of pterosaurs and a 
radiation of flying birds may also simply represent birds 
radiating into open flyer niches left by pterosaur species 
in decline from factors entirely unrelated to avians.

--Mike Habib
mbh3q@virginia.edu