NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/environm/12914

Internet Message Format | 1992-11-20 | 5.7 KB

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From: [e-mail] (Steinn Sigurdsson)
Newsgroups: sci.environment
Subject: Re: rainforests and diversity
Message-ID: <[e-mail]>
Date: 21 Nov 92 01:09:40 GMT
References: <[e-mail]> <[e-mail]>
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In-reply-to: [e-mail]'s message of Mon, 16 Nov 1992 05:08:14 GMT

In article <[e-mail]> [e-mail] (Andrew Taylor) writes:

   In article <[e-mail]> [e-mail] (Steinn Sigurdsson) writes:
   >Not necessarily true, there was a recent paper in Nature I believe
   >that argued that rainforest diversity was overestimated and that
   >grasslands were actually more diverse.

   This seems unlikely. Certainly, in terms of raw species diversity rainforests
   are much richer. I would think they would be also richer in most other
   interesting measures of diversity. Particular groups, such as reptiles,
   may well be more diverse in grasslands. Do you have a ref. for the paper?

Ref is Science, _255_ 976 1992, I was a bit off, the paper dealt
specifically with mammals, I'd remembered it as more broad.
The point was though that the emphasis on rainforest preservation
was not necessarily providing the optimal priority for conservation.

   >Most ecosystems vary considerably on evolutionary time
   >scales and the species within experience a range of conditions
   >to adapt to, further, the set of species which is critically
   >dependent on a particular feature of an ecosystem is small (although
   >they do make very nice examples for PBS specials), most adaptation
   >is incidentally appropriate to a range of conditions and species
   >can often occupy different niches as conditions change, sometimes
   >sub-optimally in some limited sense, sometimes they thrive, indeed may
   >do better than in the niche they evolved in.

   Your assertion is too vague and sweeping to evaluate but if you are trying
   to suggest that most *species* occupy niches with only a few critical
   variables and that hence we are under-estimating their ability to
   adapt to anthropogenic change, I think you are wrong.

Could well be.

   It is difficult to make statements about the set of all species.
   We neither know (to an order of a magnitude) how many species there are
   nor what proportions various groups are of these species. However, the work

Worse than that, the species we know of are self-selected to not be
restricted to very narrow niches, by observational selection.

   of Terry Erwin and others suggest there are a great number of tropical
   arthropod species each occurring only on a single species of tree.
   Erwin suggests there could be 30 million such arthropods, others
   believe 5-10 million may be more accurate.

or there could be as few as a million, it's very difficult to
estimate the unknown.

   Whatever there number, these arthropods certainly constitute a significant
   fraction, if not the majority, of the world's species and clearly they are
   critically dependent on at least one eco-system feature, their tree.

But, if a tree of different species nearby were denuded and
not allowed to be recolonised by "its" species would different species
from another tree species occupy the niche (initially at least with a
suboptimal population)? I don't know the answer and don't know anyone
who does.

   Arthropods specific to a tree species will only constitute a small
   fraction of the arthropod species that occur in that tree species but it is
   in this small fraction that the diversity lies.

   I think this will apply to other groups of species and other eco-systems.
   Most species in a particular location may occupy niches constrained in only
   a few variables but it will tend to be the few species (in that location)
   occupying heavily-constrained niches which are important to global diversity.

Ok, let's play devil's advocate: there have been many reasons bandied
about as to _why_ global (and local) biodiversity is important, both
for humans and "intrinsically". Now, should the "species count"
approach to biodiversity be weighed by extrinsic considerations in
considering biodiversity (note they are already, de facto, photogenic
and anthromorphic species are given disproportionate weight in
conservation priorities). In particular, if a species is restricted to
a very narrow niche and will not occupy other niches
opportunistically, is it relevant as far as ecosystem "robustness"
and biodiversity? It seems to me such species are dead ends, unlikely
to branch into new species and can't provide replacement function lost when
adjacent micro-ecologies fail?

   The adaptability of tropical forest species is somewhat moot as the usual
   anthropogenic change is complete destruction of the forest.  The burning
   question is how long can diversity persist in the islands of forest that
   remain (and the prognosis seems dim).  In a few areas even this is a moot
   point as no forest at all remains.

Yeah, the situation is grim, but there are some indications it is not
catastrophic, in particular the rate of loss is decreasing - at least
in South Americe, I understand, and I believe Puerto Rico provides and
example of how rainforests actually can recover from small islands,
at least in some cases...

*  Steinn Sigurdsson               Lick Observatory          *
*  [e-mail]        "standard disclaimer"      *
*  I know people whose idea of fun                *
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*                  - BB 1986            *