In un messaggio del Thursday 05 February 2009, hai scritto: > Hi Damiano, > > > This is a first point: I've used Nams' FRACTAL on several species up to > > now, that is Brown bear, the Alpine Ibex, Leisler's noctule and a > > putativa invasive bird species (Paradoxornis webbianus). > > I found the vFractal estimator rather useful to have an idea not just of > > the 'path tortuosity', but of the scale level at which 'search patterns' > > occur. The results achieved are quite interesting, i.e. Brown bears > > released in NE Italy showed significant differences in D calculated in > > the release year and in subsequent years, Ibexes showed a 'random walk > > peak' in the D-vs-scale diagram ad about 1 km (which does make perfect > > sense for an Ibex) and P. webbianus, already known as a short distamce > > mover, showed a 300 to 500 m search range. > > I found this very useful to have an idea of the scale of habitat > > perceprtion by a species, and since I run all my analyses in R (what > > else? :) I'm annoyed to go back and forth from FRACTAL to adehabitat. > > > > This could be an example of fractal dimension use (...hope so...) > > Many thanks for this answer, you made a very interesting point. > Actually, my point of view is anchored in the field of statistical tools > development for animal movement analysis, and I sometimes overlook the > need of biologists in practical studies. I of course acknowledge that > this approach could be used presently with benefits as a heuristic tool, > to explore scale-related patterns in the trajectory.
Glad it worked... perhaps this is the result of being the GIS & statistics
person in a field-oriented reseach group... contamination is always friutful,
isnt'it? :) We rely on people that 'just writes software tools', but this
kind of feedback IMHO is necessary to have better and really usable tools!
> However, this leads to an interesting question from a theoretical point
> of view: the fractal D has a clear mathematical meaning only when
> objects are self-similar (same properties at large and small scale), and
> from an "applied studies" point of view, this measure is mainly used on
> objects which are not self-similar, precisely to study this deviation
> from self-similarity, and the scale at which this deviation occurs (but
> IMHO, we should not call it "fractal" D in this context). What I do not
> understand is why a parameter measuring a property on one category of
> objects would necessarily be a meaningful measure of the deviation of
> one object from this category?
>
> The main argument of Nams (2005; Oecologia 143: 179-188), a strong one
> actually, is an empirical observation: all the simulations he did seem
> to confirm that D changes continuously within domains, and that
> discontinuity occur at domain changes (under the hypothesis of spatial
> homogeneity of the trajectory). Both your successes in using this
> approach and this empirical argument make the approach interesting for
> multiscale analysis (together with the present lack of alternative
> approaches in the ecological literature).
Good point... I agree with Nams' conclusions, and the same speculations were
already discused in Millspaugh & Marzluff 2001 (chap 6).
As I wrote above, I'm half a theoretician (my, 25%?) and half an applied
biologist, so I try to adapt the existing theoretical frameworks to our daily
chores...
> This is a bit off-topic, so I do not pursue further, but I think that
> more theoretical research may be needed here to establish clearly the
> theoretical foundations of this approach, which would define the context
> on which it relies (hypotheses on which it relies, required properties
> for the trajectories, etc.; but I may have missed such work in the
> literature)...
Not of topic at all, IMHO... just about off-topics, what about proposing a
Google Summer of Code project either for a vFractal implementation in
adehabitat?
all the best
--
Se i fatti e la teoria non concordano, cambia i fatti.
-- Albert Einstein
-----------------------------------------------------------
Damiano G. Preatoni, PhD
Unità di Analisi e Gestione delle Risorse Ambientali
Dipartimento Ambiente-Salute-Sicurezza
Università degli Studi dell'Insubria
Via J.H. Dunant, 3 - 21100 Varese (ITALY)
tel +39 0332421538 fax +39 0332421446
http://biocenosi.dipbsf.uninsubria.it/
ICQ: 78690321 jabber: [email protected] skype: prea.net
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