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Re: Trying to UN-falsify the bolide



This is my last contribution also, just to take issue with some of John's
comments criticising what I believe to be one of the crucial mechanisms
generating the pattern of extinction at the K-T boundary, namely the
difference between green and brown food chains (veg vs detritus). 

I have found these exchanges, and particularly the cogent devil's advocacy
by John, to be highly thought-provoking (and very useful to me as an
ecologist currently writing about these sorts of things!). Thanks. MY
COMMENTS ARE IN CAPS.


At 15:45 20/08/98 -0400, John wrote:
>
>Chris and Toby provided what I believe are the best-case terrestrial
>scenarios for the bolide-as-sufficient hypothesis (albeit coupled with some
>well considered ecological contribution). Both are dependent on an
>assumption which is perhaps universally held, i.e., that suspended particles
>caused the cessation or severe retardation of photosynthesis.  An important
>and obligatory corollary to this is that dust clouds also block heat.  Under
>these scenarios mammals survive because they are small and insectivorous and
>insects are in continuous supply [MORE SO THAN GREEN VEG] [PARTLY] because
detritus remains abundant.  I will
>now attack these assumptions.

>Detritivory was not, in my view, sufficient to keep the global ecosystem
>fueled.  An important fact of detrivore life is that detritus rarely
>accumulates (ref. if necessary). [WHEN THE WORLD'S VEGETATION DIES OFF
CATASTROPHICALLY, DETRITUS WILL ACCUMULATE!] It is as valuable to organisms
who eat it
>as prey items are to us.  There is competition for it, etc.  One must assume
>then that it must be constantly replenished [ONE NEED ONLY ASSUME THAT THE
RESEVOIRS OF ORGANIC DETRITUS LASTED LONGER THAN SUFFICIENT RESEVOIRS OF
GREEN PLANTS UNDER CONDITIONS OF DARKNESS - THIS ASSUMPTION IS AS SAFE AS
ANY I CAN THINK OF. I THINK IT FORMS THE BASIS OF AN ECOLOGICALLY
DEFENSIBLE (AND LARGELY UNAVOIDABLE) EXPLANATION OF THE DIFFERENTIAL
SURVIVAL OF ANIMALS BOUND UP COMPLETELY OR PARTIALLY IN DETRITAL FOOD
CHAINS. MY ORIGINAL EXPLANATION WAS LIMITED TO THE SURVIVAL OF ECTOTHERMS
IN FRESHWATER HABITATS AND I WOULD DEFEND THIS TO THE HILT. AS AN
EXPLANATION OF DIFFERENTIAL SURVIVAL ON LAND, THE SITUATION IS MUCH LESS
CLEAR (TO ME) BUT I THINK THAT THE DIFFERENCES BETWEEN GREEN AND BROWN FOOD
CHAINS WOULD HAVE BEEN CRUCIALLY IMPORTANT HERE TOO].  The stocks of
detritus were at regular levels when the bolide hit.  And under cool
conditions, decomposers
>being completely at the mercy of temperature, no new detritus would be
>produced [?? DETRITUS CONSISTS OF INTACT ORGANIC PARTICLES. FALLEN LEAVES
ARE DETRITUS ETC. DECOMPOSERS DON'T MAKE DETRITUS, THEY DECOMPOSE IT]
>
>Perhaps the main thrust of these scenarios is the valid idea that small
>things need less absolute quantities of food and this would enable better
>survival rates [COUPLED WITH THE OBSERVATION THAT SMALL THINGS TEND TO
EXIST IN LARGER POPULATIONS WHICH ARE DIFFICULT TO WIPE OUT COMPLETELY,
THEY HAVE HIGHER BREEDING RATES AND ACCESS TO A MUCH WIDER RANGE OF
ENVIRONMENTAL SPACES].  But this also deserves scrutiny.
>Many (most) communities are structured by their vegetational environment.
>Significant plant death would result in defoliation.  Without cover, mammals
>would be at the mercy of larger predators. [IN THE DARK?] Yes, they might
take cover in
>burrows, except that, as yet, there is no evidence of fossoriality in K/T
>mammals.  This defoliation, if it occurred, would have much more serious
>effects on bird populations--many of today's species, at least, depend on it
>for predation protection. I believe the phylogeny of cross K/T birds is a
>crucial test of the bolide hypothesis [AGREE].  I would predict, simply,
that if
>modern orders of birds made it across the boundary that the bolide didn't
>do much of anything.  Is this fair? [I DON'T THINK SO. THIS VIEW IS TOO
EXTREME. WHAT WE NEED IS *TAXON-FREE* ANALYSIS OF THIS PROBLEM]
>
>Another problem with bolide extinction hypotheses is that it they
>underestimate the complexity of mammalian trophic relationships.  There were
>likely herbivores, almost certainly frugivores, omnivores, and possible
>carnivory on larger animals (ref. if needed). [HOW DID EACH GROUP FAIR?
THIS IS THE SORT OF INFO WE NEED] No one is suggesting, I
>think, that with all the fruit, leaves, and prey gone there will now be a
>bumper crop of insects so that all can get by.  Indeed, the opposite is
>likely the case.  Claims that insect populations would be maintained at high
>levels are unsupported [NO ONE IS CLAIMING *HIGH*. ONLY THAT THEY WOULD
HAVE BEEN DIFFICULT TO ERADICATE AND THUS THAT INSECTIVORES MAY HAVE HAD
*AN EDGE* (NO MORE)].  Firstly, as noted above, detritus is limiting and
>not replenished [SEE ABOVE].  Second, many (most) insects are herbivorous.
 Third, low
>temps retard or stop insect activity, particularly fecundity.
>This does not bode well for an entire ecosystem _more_ dependent on
>insectivory than at other times!
>
>Birds, too, have diverse diets (unless Feduccias sole K/T shore-bird
>survivor cracking crabs proves true). [WE NEED TO KNOW THE BIRD PATTERN
BEFORE WE CAN MOVE BEYOND OUTRIGHT SPECULATION]
>
>Bailing out altogether, hibernating, may work for a couple of mammal
>species.  There is, however, no evidence of hibernation and it is unlikely
>to be seen in most mammals (if extant species are any guide at all).
>
>Hibernation is not a general strategy for birds inasmuch as they usually
>prefer to "fly South".   One might argue that a dust cloud might be focused
>around the tropics thus leaving a sub-arctic refugia for birds.  Perhaps
>some bird phyla could survive in this way.  But not all (again, this is
>dependent upon establishment of pre-K/T bird phylogenies which may never be
>forthcoming).
>
>Extinction by bolide invoking animal _size_ requires very small tolerances.
>A bolide may have effects as disparate as complete sterilization to pretty
>sunsets.  I have difficulty believing that the terrestrial effects of the
>bolide, whatever they were, had such a precise cutoff point--exactly at the
>intersection of large pre-K/T mammal-size and small non-avian dinosaur
size. [SIZE-EFFECTS RELATE TO ABSOLUTE FOOD DEMAND (WHICH IS THE ONLY SIZE
*PER SE* ASPECT) AND *CORRELATIONS* WITH FEEDING PATTERNS AND FECUNDITY.
"SMALL-NON-AVIAN DINOS" WERE *HUGE!!!* COMPARED WITH *MOST* MAMMALS]
>I mean what are the chances of such precision?  Surely non-avians
>must be the unluckiest taxa ever.  I argue photosynthesis cessation would
>have effects that would reach into mammal communities as well; that we
>should see less discrimination.  I realize this is arguable.
>
>Finally, Chris, at least, resorts to the well established trends (not
>rules!) in business-as-usual extinction patterns, i.e., small, more numerous
>animals are less likely to go extinct than large animals.  Even he qualifies
>this rule with "all other things being equal".  But the bolide is not (it is
>claimed) an ordinary extinction event.  As such, the immediate application
>of physical and biological forces of such an event is likely to be
>drastically different from the forces acting at regular times and producing
>"regular" extinctions!  [ER...?]For example, a world-wide cold snap of short
>duration might well affect small creatures more than large [TRUE, BUT THIS
CANNOT BE TAKEN IN ISOLATION FROM ALL THE OTHER EFFECTS OF THE BOLIDE THAT
HAVE REASONABLY BEEN HYPOTHESISED.  And the
>processes mentioned above (primarily the twin hits taken by detritivores and
>insectivores) may well be more influential than forces producing "regular"
>extinctions. [AS YOU MIGHT EXPECT, I DISAGREE]
>
>That it.
>
THAT'S IT. BEST WISHES TO ALL PROTAGONISTS!