|
Yes sure Michael, Your comments all entirely
reasonable.
Some points:
I'd suggest that changes in abundances over time do
broadly correlate with changes in distribution. This is consistent with what I
think you are describing. In the GBS context, this can be seen by graphing the A
and the F statistic. I have done several and it does work. Your comments also
hint at the W statistic, of which checking may be interesting.
As for trends, what made the GBS Report interesting
and worth doing is that trends occur but also change. As in this quote (from
page 17): "Year 1 of the GBS was not the start of bird
populations in Canberra and Year 21 was not the end. Many species show trends in
abundance in Canberra and the end point of these trends has not yet been
reached." Or as you say: "such trends and patterns are not set in stone but can
change". For example look at how the GBS Report shows that the detail of the
annual pattern for the Pied Currawong has changed. Even though we would describe
it the same in words, it is only because we have compiled numerical data that we
can show how much their occurrence pattern has changed.
Going beyond the general it is clear that some
species are fairly stable (notably the magpie) and some are variable. It is easy
to SEE this from the graphs. Unfortunately, rigorously statistically describing
this variability is far from simple and that is the main reason why I haven't
done it yet. Also it would have been too complex to include this in the GBS
Report (beyond quick comment).
Philip
| 