Hi all,
Ben Bolker suggested I float this by this group.
After 26 years of living and working the the Neotropics I have moved
back to the U.S.
One of the first issues for relative abundance of bats from acoustic
monitoring is how best to standardize data sets for comparison. It was
easy in the tropics with a straight forward application of my Acoustic
Activity Index (AI) [Miller, B. W. 2001. A method for determining
relative activity of free flying bats using a new activity index for
acoustic monitoring. Acta Chiropterologica. 3: 93-105].
This uses the occurrence of each species by 1-minute time blocks and is
then standardized by "Unit Effort" which is the total sample time each
night in hours. With a night length that was fairly standard with only a
small ± of 1 hr or so for seasonal variation and very small
night-to-night change this seem OK.
However looking at data from here in the northern latitudes, I am
beginning to think/ponder that I need something else in addition. The
night length not only changes rapidly night-to-night during the summer,
but has a very wide ±. So using my AI -occurrences per each 1-minute
time interval standardized by sampling time that I used in the tropics
may not reflect a realistic comparison measure up here in the north?
So my question is how to incorporate the length of each night into the
AI standardized by sample time?
How best to integrate the night length since that is one of the key
factors constraining bat activity?
So e.g. a standardized sample time of 8 hours with an AI may be high,
but during some times of the summer when the night is relatively short
however a similar standardized AI value would not be directly comparable
to seasons with longer nights when activity may be longer as well.
*/R/* seems awesome for running repetitive calculations once one has the
code line script. So I am trying to see how to develop such a new
standardized /R/ code for a DF with the simple AI values that have
already been standardized by unit effort that include the ever changing
sunset-sunrise data (that readily available and/or generated by altitude
for the sampling site) to use the night length in a way to further
adjust the data?
A suggestion on generating the sunset-sunrise then calculating the night
length for each night then using that value to further adjust the
"standardized AI value" is what I am trying to figure out. The
standardized AI is already coded into a master database as an output so
that is readily available by species and location.
For working out GGplot temporal activity plots by minute for each night
it took bit of hand holding by Hadley as R does not (or at lead did not)
"do time well". Not sure what can be done with this idea. The
crossover midnight may not be an issue for this calculation since we
just need a total night length in decimal hours.
Moon phase and % illumination is yet another issue, but not relevant at
the moment.:-)
Examples of what these DF would look like.
Then I need to have a way to generate the night length for each sampling
night and further adjust data for realistic comparisons of relative
activity that translates to relative abundance.
Any and all suggestions welcome. And the bats will depend on it.
Bruce
--
Bruce W. Miller, Ph.D.
Conservation Ecologist
Neotropical Bat Projects
office details
11384 Alpine Road
Stanwood, Mi. 49346
Phone (231) 679-6059
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