Hello Gianluca,
There are a couple of ways to answer your question. First, of course it is
"feasible" to use a volumetric approach, as the ratio of relative volume
present in gut to relative volume present in environment is a measure of
"selectivity."
However, there's a reason why you don't see this often done. Foods such as
detritous, CPOM, FPOM etc are rarely "rare" in the environment, so if that is
what a consumer specializes in, that is what it will get as barriers to
detritous typically don't exist. Hence, its electivity index will be 100%
detritous. It is a very rare case when a "carnivour" switches to feeding lower
on the food chain (from hetertrophs to autotrophs) as they simply lack the
physiology to do so. For an omnivour, the typical question is where does the
energy come from - i.e, the caloric content of the items in the gut. A single
small fish in the gut will likely contribute much more energy to the total diet
that a large volume of detritous or plant matter or comprarable food. For this
reason, if a consumer has access to multiple types of accpetable foods at a
variety of trophic levels, it will always choose the high energy food (if a
barrier to access exists then it is not a potential food source!
and it is not a matter of selectivity).
These 2 points demonstrate why electivity indicies are typically only
used on high trophic level consumers selecting from a variety of similar prey
types.
You do not provide details of your project so perhaps there are special
circumstances, but based on what you wrote, I don't see why you are doing
electivity analysis.
Sent from (and mis-spelled on) my iPhone.
On Aug 11, 2011, at 2:10 PM, "Gianluca Polgar" <[email protected]>
wrote:
> Dear all,
>
> thank you to all of you who replied on food selectivity index of
> non-countable items.
> Unfortunately, my methodological problems are still largely unsolved.
>
> If anyone is interested in this topic and would like to contribute, please
> continue to send suggestions, comments etc.!
>
> Thank you!
>
> Gianluca
>
>
> -----Original Message-----
> From: Gianluca Polgar <[email protected]>
> To: ECOLOG-L <[email protected]>
> Sent: Tue, Aug 2, 2011 11:40 pm
> Subject: [ECOLOG-L] food selectivity index
>
> Dear all,
>
> I'm going to estimate the food selectivity of some fishes and I'd like
> to adopt the food selectivity index of Berg (1979).
>
> This index utilises the numerical abundance of food items, measured both
> in the gut and in the habitat.
> Nonetheless, a considerable percentage of food items is NOT countable
> (e.g. detritus, plant material)...
>
> My question is: would it be a feasible option to utilise a measure of
> bulk of food items (e.g. volume) instead of amounts?
> Is anyone aware of any other indexes of food selectivity that could be
> adopted with non-countable items?
>
> Any suggestion will be more than welcome!
>
> All the best,
>
>
> Gianluca
>
>
> --
> Gianluca Polgar Ph.D.
> Senior lecturer
> Institute of Biological Sciences
> Institute of Ocean and Earth Sciences
> Faculty of Science, University of Malaya
> 50603 Kuala Lumpur, Malaysia
> Tel.: 017-6223549
> Fax (ISB): 03-79674178
> e-mail: [email protected]
> www.themudskipper.org
>
> O___!||||__/////__
> {__)_\
