extinction and community ecology

[Nick Longrich <nrlongri@midway.uchicago.edu>]

>Larry Febo wrote:
>
>> I never could understand how  "climate" could be considered a causal agent
>> in extinction. I mean,....most organisms are quite capable of migrating to a
>> more amiable enviorn , ...right???
>
>Not necessarily, no. Actually, usually, no. It is one of these multicausal
>things that result from working with ecologies. Almost everything in an
>ecosystem is connected. Suppose the climate changes so that an organism in
>question has the options of moving or dieing. It may not be able to move
>because
>it requires a specific habitat that simply isn't available near it. Take the
>koala. It feeds on eucalyptus trees. If the climate changes and it has to
>move,
>it will simply die because the trees it feeds on are not that common. Even if
>there are other places it can move to, there needs to be some way for it
>to get
>there. If the next forest of trees is a thousand miles away, the population is
>SOL (sh~ outta luck for those of you who haven't seen this expression).
>Ok, this
>is a bit simplistic and people may quibble with the exact example, but the
>meaning still stands.
>Now, even assuming that there is a way for the animal to migrate so that there
>is unbroken habitat or patches close enough together that it can traverse the
>intervening areas, that still does not mean it will be successfull. There will
>most likely be other animals in those areas that are already occupying those
>niches which will be reluctant to leave or share. That means the organism in
>question will have to battle an already entrenched species and so will
>probably
>lose. Yes, there are plenty of examples in which an invading organism wins and
>takes over, but in those cases there are other factors at work and in the vast
>majority of cases, the displaced organism simply can't compete with the native
>organisms and if it lives, lives on the fringes.
>And suppose it can overcome all of these things, it will be exposed to
>pathogens
>and predators it is unaccustomed to and will have to learn to deal with which
>kills most of them.
>So there you have it, in a very simplistic form ad only mentioning a few
>factors. The upshot is that organisms are tied to their environments and most
>can not easily just pick up and move when the weather turns bad. Some can,
>yes,
>but most don't succeed.

        All well and good in theory, does it, in fact, describe the world
we live in? There has been a lot of controversy over whether "communities",
that is, assemblages of organisms that tend to form particular arrangements
in time and space, actually exist. One of the interesting findings is that
before the end of the last ice ages, a great number of unique associations
of currently living organisms (plants, insects, e.g.) existed which today
have been replaced by completely different associations of the same
species. This may not be the death-knell of community ecology as some have
proposed but it certainly shows that it is one heck of a lot more
complicated than we like to make it out as being. So the short answer would
appear to be yes; things can move, and they can form completely novel
associations in response to climatic changes. No reason to think dinosaurs
couldn't have done exactly the same thing.


Re: the apparent disappearance of dinosaur clades before the K-T there is a
big problem with something called "edge effects" when dealing with mass
extinctions. If we are sampling the middle of a clade's range, we don't
actually have to find it to know it's there- you can always find it at some
later date and infer that it must have been there. So Bigasaurus, final
representative of the Euneobigasauriodeaformes extends from 90-70 million
years ago. We want to know if it was around 80 million years ago, and we
have it dug up twice, from 90 and 70 million years ago, so we know it had
to have existed during the intervening time. What if it had a range,
however, that had been truncated at the boundary- say, it existed only from
75-65 million years ago, half of what it might otherwise have lived? With
the exact same preservation potential seen in the first model, Bigasaurus
might preserve just once- 75 million years ago- and therefore
Euneobigasauriodeaformes appear to bite it before the boundary. Basically,
you can appear before your extinction but never after it so this limits our
ability to infer the true range of any species/clade, and groups will be
biased such as to disappear before their actual extinction. How far before
their actual extinction depends heavily on preservation potential.
        It wouldn't surprise me at all if this was what was going on.
There are ways of creating models and seeing which model (sudden Kt
extinction or pre-KT dwindling) the data actually fit, but I don't know
that anybody has ever attempted it for dinosaurs (I believe Raup originally
came up with this and applied it to KT marine. His results were ambiguous,
i think). I do think, however, that the general trend has been for this
missing pre-KT dino diversity to slowly make itself known- whether it was
Wroblewski's study about how theropod families really don't taper off
before the KT (based on the far more abundant tooth record), or the "Sandy"
site with its oviraptorosaurs and boneheads, or the study mentioned
previously which found all sorts of things almost at the boundary layer
like dromaeosaurs and ornithomimids. There are plenty of groups which were
probably around for the much or most of the North American cretaceous- such
as therizinosaurs and mononykines- which we are just now beginning to
discover. We have what, one or two therizinosaur sites, recently
discovered, a mononykine foot and a mononykine pelvis? Pretty pathetic.
It's not at all implausible that we could simply be missing large chunks of
the dinosaurian fauna, especially if the climate/geography shifted enough
to put the lambeosaurines and centrosaurines somewhere where they don't
fossilize so hot. Personally I think we are sometimes perhaps being silly
to use dinosaurs to answer these kinds of questions, as the invert people
like to taunt the vert paleontologists, we are too often forced to deal
with "datum, not data"...