Re: Life's complexity: self-organization, evolution or both?

John Harshman wrote:

Bill Morse wrote:

r norman wrote:


On Tue, 18 Sep 2007 02:22:08 GMT, John Harshman
<jharshman.diespamdie@xxxxxxxxxxx> wrote:


Bill Morse wrote:


r norman wrote:



On Mon, 17 Sep 2007 03:03:43 GMT, Bill Morse
<wdNOSPAmorse@xxxxxxxxxxxxxxxx> wrote:



Perplexed in Peoria wrote:



(snip)



That may well be true, but organism complexity and ecosystem
complexity are completely different animals. NS (i.e. reproduction
and genetic inheritance) is central in explaining organism
complexity. But NS isn't much involved in explaining ecosystem
complexity.

Arguable. One doesn't ordinarily think of ecosystems as having excess
reproduction. But they might be considered to have heritable fitness.
I don't think one should completely dismiss the idea of ecosystems
competing.



Ecosystems are MUCH simpler than organisms, and their evolution is
something even a physicist can understand.

This seems extraordinarily unlikely on the face of it. Ecosystems are
composed of a large number of organisms. All our experience with
other systems tells us that aggregations of large numbers of subunits
results in greater complexity than that exhibited by any of the
subunits.

It depends on how you count "number of subunits". Also, the patterns
of interaction between units within an organism are quite intense
whereas those between units within an ecosystem are rather weaker.
Yes, if somebody catches and eats you, that might be considered
intense from your point of view. But you have to look at the
population and the probability of being eaten. The nervous and
hormonal control systems, not to mention the genetic regulatory
control systems, within an organism and within a cell are much more
deterministic.


While I grant that there is a difference in intensity of interaction,
the definition is not all that clear. How does one treat endosymbionts?
Are they part of the larger organism or are they and the larger
organism part of a mini-ecosystem? And how does one treat eusocial
insects, remembering that ants comprise a huge fraction of the
organisms in many terrestrial ecosystems?

My initial thought in support of Pip's idea that ecosystems might be
simpler than organisms was that organisms in ecosystems might behave
statistically - I think this is in line with your argument that
interactions between units within an ecosystem are relatively weak. But
in reflecting on real ecosystems, I just don't see that statistical
methods capture much of what goes on. For instance,I'm unaware of any
statistical treatment that captures the importance of capstone species.




But without trying to measure complexity to see which one "wins", you
must consider that organisms must contribute to the success of the
ecosystem in which they live if they are to survive and reproduce. So
yes, indeed, ecosystems are strongly affected by evolution and change
with evolution. So why not just say that they evolve, even though the
mechanism is rather different from evolution of populations and
species.


I can agree with that. But I am interested in the question of whether
ecosystems also evolve by a mechanism that is similar to natural
selection. Do ecosystems compete? In the classic view of the 1960's
that I was taught, there was for instance a natural succession of
ecosystems following a disturbance. I doubt this is the current
thinking. It seems to me instead that succession as well as such
phenomena as change in vegetation mix with altitude on a given mountain
can best be explained by a model of competition between ecosystems.


I am attempting to disagree with both John and Richard simultaneously (no
mean feat in itself!), so I followed to this one because it contained
both of your comments.


That made no sense to me unless your last word was intended to be
"individuals" instead of "ecosystems". All succession would be is the
ability of different organisms to outcompete others in particular
environments over particular time scales, remembering that individuals
help create each others' environments. Succession is the statistical
result of all manner of individual interactions among species adapted to
different environmental features.

Yes, I agree with John. Succession is, indeed, still taught because
it occurs and can be readily observed. The reason for succession is
that the activity of organisms within an ecosystem cause significant
changes to the physical environment, that is to the ecosystem, to
their own detriment. Succession is, indeed, an "evolution" of
ecosystems using that word not in the biological sense but as the
playing out of a series of sequential events. The change is the
result of the ability of species and individuals within those species
to compete and survive in ever changing environments.


It seems my hazy memory of my ecology courses led me to confuse
succession
with the concept of ecological communities - for some reason I had
thought that succession was marked by the same sort of shift in species
assemblages that is seen in change in vegetation with altitude.

You are correct about this. It's just the same sort, i.e. individual
species come and go depending on their own environmental requirements,
whether it's with altitude or time.

I was not entirely mistaken, however -there are many recognized
ecological communities (e.g. beech-maple forest vs. oak pine forest), and
in fact some of these exhibit sharp boundaries. Kevin Kelly mentions "the
demarcation between the high tide seaweed community and the watery edge
of the spruce forest is an extreme no-man's-land of barren beach. One can
stand on this yard-wide strip of salty desert and sense the two
superorganisms on either side, fidgeting in their separate lives.

He is either being poetic about the "superorganism" or being a bit
loony. What you see here is a situation where a great many environmental
factors change in unison and over a very short distance. Thus many
species (in fact all of them) find this to be a line they can't cross.
Please don't confuse this with some kind of mystical Gaia-type community
cohesion.

As another example, the border
between deciduous forest and wildflower prairie in the midwest is
remarkably impermeable."

Ha! What then is a savannah?

There are no savannahs in the midwestern US. As far as I can tell from a
quick study, savannahs require hotter conditions. One site noted that the
Florida everglades are an example of a savannah biome. Much of the area
between Houston and San Antonio to my mind resembles African savannah. But
in any case it is a different ecological community than deciduous forest or
wildflower prairie.


An example I am more familiar with is the
transition in the Adirondacks from the mountain conifer forest to
the "krummholtz", or cripplebrush, and slightly above that the treeline
and transition to Alpine vegetation. The transitions at lower elevations
(e.g. from northern hardwood to hardwood conifer) creep up on you as you
hike up - you can sense a change but there are still many members of the
species assemblage of the lower elevation community. But the latter two
transitions are very sudden.

That's because they involve only a single species, at least in all the
krumholzes I'm familiar with. That is, the last remaining tree species
at that altitude finds itself in an ecotone in which its growth is
increasingly stunted until it has no more capability to live. Meanwhile,
alpine plants find that bare spots suitable for their growth are
increasing steadily.

In the adirondacks the krummholz includes both black spruce and balsam fir,
and of course includes numerous other smaller species. But the point is
that the other species in the mountain conifer forest require other large
trees (as opposed to the stunted growth of the cripplebrush) in order to
survive, and once the black spruce and balsam fir can only survive in a
stunted state these other species disappear. On the other side, the species
that comprise the alpine vegetation can easily survive at lower altitudes,
but can't compete in shade conditions. The result is that we get sharp
transitions in species communities, even though the transition in other
environmentalfactors is gradual. The reason is that in these systems one of
the critical environmental factors is provided by the presence of other
species. I.e., in order to understand the transition one has to look at the
system and not just the individual, even though the behavior of the system
is determined by the individual.


Now obviously the change is due to "the ability of different organisms
to outcompete others in particular environments." But in some of these
communities, the interactions between different organisms, the
individuals helping "create each others' environments", becomes a tight
enough loop that the whole community does behave in some sense as a
superorganism.

I can't think of any such communities. There are tight interactions, but
they tend to be between species pairs at the most. And the cases in
which species pairs are always found together, never separately, are
surprisingly few. Even if species A depends absolutely on species B in
some locality, it may depend on species C in some other locality.

And this observation supports my argument. Since the units that comprise an
ecosystem are unrelated by genetics (as opposed to a species), their only
important characteristics are the functional ones. So as long as species B
is functionally equivalent to species C, the ecosystem will work equally
well with either one.



--
Yours, Bill Morse

.

Relevant Pages

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