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Whilst Andrew's comments are correct, it is more
practical to do a graph of cumulative species recorded over time, as well as a
graph of number of species that have been recorded on every survey as at the end
of each survey. These together give a measure of both the likely potential and
the number of species regularly present. My (soon to be available) report on
COG's GBS demonstrates and describes these aspects in detail, using yearly
blocks. Even after 18 years (1151 observer years of data), the graphs have not
stopped changing. However after about ten years, the rate of change slowed down
distinctly, so by now we are very close to a stable state to be able to estimate
the number of species potentially present and the number of species that can be
reliably found every year (as well as the the number of species can any one year
is likely to record).
-----Original Message-----
From:
Andrew Taylor <>
To:
<>
Date: Wednesday, 30 May 2001 17:18
Subject: Re:
[BIRDING-AUS] Bird Numbers
On Fri, 25 May 2001, Wynton
wrote:
> I would be interested to see if this ratio is consistent with
other =
> surveys as it could be used for conservation and other
reasons to =
> estimate the total number of species based on only a
few visits.
You may find it difficult to estimate the size of the
eventual bird list
of an area, because the number doesn't exist.
I
believe typically the list of birds recorded from an area will
continue
to grow, with continuing observation, without bound. Over tens
and
hundreds of years the growth will come from observation of
species
(vagrants) which occur only very infrequently in the area.
Theoretically,
over tens or hundreds of thousands of years growth will
come from
speciation - the birth of new species. Admittedly the
extinction of
class Aves would end the growth.
In other words, if
you lived forever, I'm saying that your local list
would never reach a
limit and stop growing. Although after a few
centuries the growth
might be very slow.
Its actually fortunate that birdwatchers do not
live thousands of years
- species boundaries already provoke enough
arguments without having to
consider the much more problematic notion of
chronospecies.
You could, of course, estimate the number of
species present within
an area at a given time or within a given
period.
If you are really interested in this stuff try looking at
ecology
textbooks - biologists have been considering methods to estimate
or
compare species richness and related concepts from limited samples
for
at least 60 years - and still seem to be inventing new
ones.
Andrew Taylor
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