[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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

I wrote:

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 which John Bois replied:

Well, I don't see _anything_ simple about competition and/or predation.

I didn't say "simple", I said "direct". As in: birds out-fishing pterosaurs, and/or eating their eggs.

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 my understanding is that you need a force that knocks out large clades
before the bolide.

Not large clades, species by species extinction over millions of years. This may be the "natural " extinction rate (see below).

Competitive exclusion is defined as one species so completely outcompeting
the other that they become locally extinct. Not the case in birds and bats.
What they are doing is partitioning the resource (e.g., insects). Just as
Anolis lizards famously divvy up upper and lower branches, bats and birds
split the niche temporally. And, there _is_ some overlap. This definitely
rules out competitive exclusion. Bats and birds have not competitively
excluded each other. On the other hand, birds _may_ have competitively
excluded pterosaurs.

Competitive Exclusion: "Two species with similar requirements (cannot) coexist in the same community; one species would inevitably harvest resources and reproduce more efficiently, driving the other to local extinction. Even a slight reproductive advantage would eventually lead to the elimination of the inferior competitor and an increase in the density of the superior one." - Campbell et al., 1999

We would expect an established pterosaur to competitively exclude a bird trying to evolve into its niche. At the species level pterosaurs would have had the advantage of being established.

You are arguing that birds out-competed pterosaurs at the species level. Hence: birds out-competed pterosaurs. What I am saying is that maybe pterosaur species became extinct at the "natural" rate, and new bird species stepped into the gaps more often than new pterosaur species. This isn't competition, it is replacement.

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.

What I meant was there was a great reduction in pterosaur diversity caused
by forces other than the bolide.

The term "caused by forces" may be too strong if pterosaurs were replaced in the way I suggest. Maybe we are getting close to a matter of taste.

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

What other _global_ forces could do this?

Natural extinction rate. Not global forces.

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.

Not sure I understand: I thought you were saying that pterosaurs had
first dibs on their niche and could not be budged through competition. Now
you're saying new bird species bumped them out. How do they do this?

I don't know how many ways I can say this, but here goes: pterosaur SPECIES went extinct of their "own" accord. Birds may have been able to fill the VACANT niches faster than pterosaurs could. Their may have been very little head to head competition on the species level between pterosaurs and birds.

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.

Because birds became better competitiors.

Again, why so slow?

I agree with DM that competitive exclusion has a very
important part to play.

Yes. Birds competitively excluded pterosaurs, i.e., they became extinct and
birds didn't! Now, how did it happen?

Birds replaced pterosaurs as opportunists, by speciating into vacant niches.

Let's look at this a different way: suppose, just suppose, dinosaurs were wiped out solely by the impact. Mammals then spread into all the vacant niches. Would you then say mammals "out-competed" dinosaurs? I don't think anyone would. They were simply able to speciate faster than dinosaurs. Mammals "replaced' dinosaurs, they did not out-compete them.

So, to get back to pterosaurs and birds: if a pterosaur species became extinct (causes not to do with birds) and a bird moved into it's niche; is it fair to say the the bird out-competed the pterosaur? No, the bird replaced the pterosaur. So if it was this scenario that was played out again and again in the Cretaceous, we should say bird replaced pterosaurs, they did not cause their extinction, or out-compete them.

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/