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Re: ...bats by day, and bats, birds, by night.




On Monday, July 15, 2002, at 12:00 PM, John Bois wrote:

Outside of catastrophies (which you're not invoking) competition and
predation are the prime causes (most likely) of most extinctions.

What about changes in the environment, extinction of a food source etc.? Ecologies are VERY complicated systems, with many interdependencies. The kind of direct competition and predation you seem to be talking about may not have much to do with many extinctions.


To say these forces caused both early and late K extinctions is not a problem. You
seem to believe...but I don't know what you believe. What do you think
could have driven these species into extinction?

Changes in the environment, extinction of a food source, ect. There are many, many things that can cause extinction at the species level - few that can cause the extinction of very large clades.


...but birds have outcompeted bats
by day, and bats, birds, by night [...].
This is either as a result of an arms race between sensory
systems--acuity
of vision in birds v. echolocation in bats, or, and this is my feeling,
birds can survive in the diurnal predatory environment whereas bats
cannot.

May amount to semantics, but I'd rather say "competitive exclusion": birds
started as diurnal animals and simply didn't get nocturnal (before bats
evolved at least), but bats (as mammals) started as nocturnal animals and
were unable to get diurnal because during the day there were too many
insectivorous birds that ate their food.

Not possible. Many birds' and bats' feeding time overlaps (i.e., at dusk).
In other words, there is a rather fat margin upon which selection could
operate. I mean, it is not as if the birds eat all the insects, right? An
early-riser bat would have access to more insects, leading to more babies.
etc., etc. I think what we are seeing is a kind of niche partitioning:
birds are better competitors at some times, and bats at others. And, I
would be surprised of predation avoidance in birds wasn't part of that
equation.

We seem to be seeing competitive exclusion working rather powerfully, and you seem to agree. Birds would have faced a similar situation in trying to compete with pterosaurs (if they did).


But predator access to those islands has changed over time--with the
evolution of new/different predators.

We have no idea what *B* could or couldn't do. Ditto
for falconiforms. How can I possibly do that? You know this is pure
speculation! However, there are some facts to work with. And they favor
such speculation:

1) Most pterosaurs became extinct from causes other than the bolide.

True, but then so did most dinosaurs, and any other clade you might want to mention, because only a small percentage of large clades is extant at any one time.


2) No pterosaurs handed over their niche as a gift to rising new species.

? Maybe as pterosaur species became extinct from other causes (see above), new species of birds filled the gaps (as they reopened) than new pterosaurs.


3) Something forced them out of their niche.

Depends what you mean by "forced". Birds may, I repeat, may, have been able to speciate faster than pterosaurs. As pterosaurs became extinct, birds filled the niches faster than pterosaurs, but this didn't necessarily have anything to do with direct competition or predation.


4) The most likely things to do this is are flying creatures.

Many pterosaurs ate fish, and for this resource they competed with other fish and swimming reptiles, neither of which fly. Pterosaurs nested on land (or in trees?), in both places lived predators such as non-avian small theropods, lizards, and mammals, none of which fly.


5) The most likely flying things to do it were the only ones that
existed--birds.

I don't think the facts you present are really facts, but a series of hypothesis, which may or may not be correct. And as each one depends on the one before it, they hardly add up to an unshakeable case.


From these facts and inferences, one can posit a reasonable--if very
difficult to falsify--hypothesis. Simply, speciation and evolutionary
developments affected which creatures occupied which niches. If neornithine
developments had anything to do with this, they may well have also had
something to do with enantiornithine removal. It might even be that
enantiornithines outcompeted pterosaurs early, and neornithines, late.

Why did this process take so long? If birds were better competitors, why did they take tens of millions of years to out-compete pterosaurs? One would expect such direct competition to remove pterosaurs much faster, probably just a few million years. Maybe less.


In any case, chances are that fossils will be informative (ultimately) of
relative diversity and biogeography of these bird clades. Let's just hold
our fire till then.

I don't think this debate is going to be settled with a few more bird fossils. It seems to have more to do with how we think evolution and competition works. I agree with DM that competitive exclusion has a very important part to play.


Most pterosaurs went extinct well before the K/T, but many didn't. Birds do not seem to have been able to compete directly with Azhdarchids, and their extinction coincides with the bolide.

The bolide did it. :-)


John Conway, Palaeoartist

"All art is quite useless." - Oscar Wilde

Protosite: http://homepage.mac.com/john_conway/
Systematic ramblings: http://homepage.mac.com/john_conway/phylogenetic/