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Re: Copeing with mammals
Okay, time to jump on another extinction thread...
A lot of ideas flying about on this thread, and I think it is important to make
a few comments about what is really known about competition...
1) Competition as the "warfare" scenario; ie. sequestering of food, space, etc.
is a process that occurs at the level of the individual. There may be
analagous situation at the species-level (note "may be"), and the same language
is often used. However, this is dangerous, and care should be taken to keep
scale in mind.
2) We don't really know that predation and competition cause species-level
extinctions on a global scale. As recently discussed on this list, the only
likely examples of predation kills so far are islands and human consumption.
This is not to say that it hasn't happened, we just don't know yet. HP John
Bois has suggested regularly that they do, and I still think he should make a
project out of that to bring some empirical work to bear. I would eagerly read
paper. At the moment, however, it is not a safe assumption.
3) What do we mean by competition at this point? This thread has strayed such
that very minimal diet or behavioral overlap is being cited as evidence for
intense competition. That's not technically wrong, but I think it'd be more
useful to have a slightly tighter definition of what strong competition entails.
"Species don't say: "Adieu small niche...I gotta be big now". They are
squeezed out of it by other, more effective clades."
Actually, they might. Really, body size evolution is very complicated and the
causes for gigantism are not known. Some clades follow "Cope's Rule", and
other do not (which makes it a pretty poor rule, I agree). Just for a
conceptual example, it is perfectly reasonable to think that a clade might
drift towards larger size because of INTRAspecific competition (which is being
strangely ignored in this thread, I think), namely that large individuals do
better than small individuals wi
thin a species for some length of time. This need not have anything to do with
other clades. It could, naturally, but it doesn't have to implicitly. Again,
we just don't know exactly why some lineages achieve huge size, nor why some
cease to achieve small size.
> Pterosaurs clearly had issues. As a clade, their extinction was
> long and
> slow. This argues against a long series of accidental extinction
> events.It argues _for_ some kind of deficit relative to other
> clades. I have
> proposed a reasonable problem--that juveniles (or adults, for that
> matter) were not as agile in the air as neornithines, and were
> thereforemore susceptible to predation/general harrassment.
I'm not sure why a long extinction trend in a clade means it "had issues" more
than one that hits extinction rapidly. All that aside, long extinctions may be
from numerous abiotic events or from competition or both. There is no evidence
to show one or the other, except that most mass extinc
tions seem to be near in time with abiotic pertubations.
I agree that the agility argument makes nice intuitive sense, and there is some
evidence for differences in flight pattern, certainly. However, to really back
that comment up, some kind of evidence that mobility differences relate to
extinction is required. I actually have one reference for bats (I can send it
along if anyone is curious), but it appears to have more to do with dispersal
than some kind of harrassment-extinction.