This has a sound ring of validity about it. It is, in many places, very
difficult to find truly "pristine" ecosystems and their constituent
communities were like prior to disturbance/fragmentation, and even seemingly
"pristine" sites often have at least some history of anthropogenic
interference, but provided the control sites are accurately described, the
approach, as Crants wisely points out, should not be completely
invalidated--or invalidated at all on that basis. Some degree of
"disturbance" occurs in all ecosystems at all times, so TRENDS in diversity,
for example, should be useful in an accurately bounded context. Ups and
downs in communities are what reality is, not the snapshots provided by
single surveys. If one wants to get into fuzzy theory about it, one could
research collections (arthropods, fungi and other "insignificant" life forms
are commonly left out, so their influence, past and present, is not
reflected in such studies) and other sources that may reveal species not
found in the control site, note alien species, alien species present in the
past and present, and reconstruct a somewhat more "accurate" picture of what
the area was like before disturbance. Historical evidence such as timber
cruise data from the past, even photographs and art can be investigated, but
the latter can cross the line into conjecture. It would seem that the main
thing is to do the best one can to avoid any significant misrepresentation
by stating the conditional nature of the data, then move on. Come to think
of it, what would be wrong with an educated guess based on observations
provided that the purpose was not compromised? However, I must confess that
every time I have attempted an "educated" guess about even "simple"
community composition, the simplest sample always proved me wrong to some
extent, sometimes to a critical degree. As long as claims are not
exaggerated, it would seem that any study, within its limitations, can be
considered valid, even for single studies. After all, "anecdote is the
singular of data," eh? And "correlation is not necessarily causation," no?
WT
----- Original Message -----
From: "James Crants" <[email protected]>
To: <[email protected]>
Sent: Friday, January 30, 2009 8:15 AM
Subject: Re: [ECOLOG-L] Analysis of habitat specificity and circular logic
In attempting to explain why I don't find this reasoning circular, I've
managed to convince myself that it is. There is an implicit assumption
that
the control sites in a study are representative of the entire landscape
prior to fragmentation. Any difference between the control sites and the
fragments is attributed to anthropomorphic effects in the fragments.
However, the control site may never have had the same community as the
landscape that has become fragmented.
I can't speak on rainforests, but in southern Michigan, I found that
forest
fragments and preserves were mostly clustered around swamps, ponds,
streams,
and hills--sites where the topography is too rugged for the plow. Large
preserves tend to be on stabilized dune-lands and around wetlands, and I'm
certain that, before habitat fragmentation, the plant communities in such
sites were different from those in what are now farm fields.
I don't think this circularity completely invalidates the approach you
describe. Basically, people are going out and seeing what species they
find
in different habitats and categorizing the species based on what habitats
they find them in. They also try to figure out what biological
characteristics unite the species within each category and separate them
from species in other categories. Hopefully, they have a priori
hypotheses,
but it is also valid to propose new hypotheses based on your observations,
or to note that what you observed is consistent with hypotheses others
have
proposed.
The danger is in not recognizing and acknowledging that your control sites
may be unlike your fragments for reasons other than their relative lack of
disturbance. Unfortunately, the fundamental problem is that we don't
often
have good records of what communities were like before disturbance. Using
control sites assumes that the fragmented landscape once had a community
like that of the control sites. It seems to me that using indicator
scores
assumes that the fragmented landscape once had a community like those used
to produce the indicator scores.
On Thu, Jan 29, 2009 at 3:18 PM, Brian D. Campbell <
[email protected]> wrote:
Dear listserv members:
I've been reading a lot of literature recently on the effects of
fragmentation and land-use conversion from forests to agroforests and
have
been really troubled by what seems a pervasive issue (at least in my
mind);
defining the biodiversity value of a human-modified habitat type (e.g.
either fragment or agroforest). Almost all studies I've reviewed
partition
bird communities into categories of "forest species", "rainforest
specialists", "agricultural generalists", among others, and proceed to
compare these among different land-uses. This is not my issue per se,
but
rather, I find it very circular if one uses the data/observations they
collected in a study to define these groups; e.g. all species encountered
in
a "control" site of extensive forest were defined as forest species.
These
make useful and note-worthy observations but if one then proceed to
include
control sites in a statistical comparison then I think there is major
issue
with circularity. This also seems to me a very different approach than
having defined a priori (e.g. from distribution lists or other
literature)
a
set of forest-candidate species which may or may not be present in any
given
site surveyed. Have others here found similar issues when reviewing
papers
dealing with the biodiversity values of secondary forest and agricultural
habitats?
Brian Campbell
--
James Crants, PhD
Scientist, University of Minnesota
Agronomy and Plant Genetics
Cell: (734) 474-7478
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