Introduction. Alternative Methods
101
For success in research on trophodynamics and in the following model
studies, it is important to carry out experiments and gather comprehensive
data on all these parameters of nutrition and related matters. Estimation of
only one or two of them, which often happens, is of as little value as an
unfinished portrait painting. Therefore, when planning a nutrition study of any
animal species, the researcher must:
1. take all the measurements listed above, as components of the trophic
certificate, and
2. use the radioisotopic method, as a unique way to obtain the necessary data.
Without the use of radioisotopes, it is possible to make semiquantitative estimations of food rations in filtering animals, or to obtain approximations of
the spectrum of feeding and food rations in larger animals; but these are often
invalid or confusing. The alternative (nonradioisotopic) methods of determination of food spectra and food selectivity, based on investigation of gut
contents, give no real information because of the difficulty in identifying semidigested objects in the guts, as the organisms which represent the real food of
the consumer are digested first. As a result, indigestible remains of food objects
swallowed accidentally or with prey remain in the gut. For example, in the gut
of most predators who do not digest plant material, are found quantities of
algae and also a mass of detritus of unknown origin, ingested together with
herbivorous preys (Monakov 1976).
Even having estimated the spectrum of feeding and food rations according to data on gut contents or on decrease in prey concentration in an experimental vessel, without knowing the real assimilability of all kinds of ingested
food, it is easy to draw false conclusions about the availability and provision
of food for a given grazer in a native water body, because this food, although
ingested, may be indigestible for the animal. A classical example is the conclusion that planktonic algae are food for cyclopids derived from observing
their predomination in the guts of these predatory plankters. Experiments
using 14C labeled algae proved that the cyclopids indeed ingest the algae
together with prey, but the latter are rapidly digested, while the for them indigestible algae remain safe in the guts, thus giving the false impression that they
are a main source of food (Monakov and Sorokin 1972). Equally deceiving are
quantitative data on nutrition of filtering animals obtained in experiments on
the clearing rate of suspended food. The clearing rate cannot serve as a real
measure even of the rate of ingestion, because in most filtering animals the
function of filtration is combined with respiration. At a high food concentration (which is often the case in experiments), they form pseudofeces, simply
precipitating the suspended food, but not ingesting it. In many cases, the
filtering animals passively ingest the suspended matter, which they are not able
to digest (so-called superfluous feeding). Therefore it is impossible to obtain
realistic information about nutrition even in filter feeders without an estimation of the assimilation rate and assimilability of the suspended food, because
these are the real evidence of feeding rate and nutrition.
101
For success in research on trophodynamics and in the following model
studies, it is important to carry out experiments and gather comprehensive
data on all these parameters of nutrition and related matters. Estimation of
only one or two of them, which often happens, is of as little value as an
unfinished portrait painting. Therefore, when planning a nutrition study of any
animal species, the researcher must:
1. take all the measurements listed above, as components of the trophic
certificate, and
2. use the radioisotopic method, as a unique way to obtain the necessary data.
Without the use of radioisotopes, it is possible to make semiquantitative estimations of food rations in filtering animals, or to obtain approximations of
the spectrum of feeding and food rations in larger animals; but these are often
invalid or confusing. The alternative (nonradioisotopic) methods of determination of food spectra and food selectivity, based on investigation of gut
contents, give no real information because of the difficulty in identifying semidigested objects in the guts, as the organisms which represent the real food of
the consumer are digested first. As a result, indigestible remains of food objects
swallowed accidentally or with prey remain in the gut. For example, in the gut
of most predators who do not digest plant material, are found quantities of
algae and also a mass of detritus of unknown origin, ingested together with
herbivorous preys (Monakov 1976).
Even having estimated the spectrum of feeding and food rations according to data on gut contents or on decrease in prey concentration in an experimental vessel, without knowing the real assimilability of all kinds of ingested
food, it is easy to draw false conclusions about the availability and provision
of food for a given grazer in a native water body, because this food, although
ingested, may be indigestible for the animal. A classical example is the conclusion that planktonic algae are food for cyclopids derived from observing
their predomination in the guts of these predatory plankters. Experiments
using 14C labeled algae proved that the cyclopids indeed ingest the algae
together with prey, but the latter are rapidly digested, while the for them indigestible algae remain safe in the guts, thus giving the false impression that they
are a main source of food (Monakov and Sorokin 1972). Equally deceiving are
quantitative data on nutrition of filtering animals obtained in experiments on
the clearing rate of suspended food. The clearing rate cannot serve as a real
measure even of the rate of ingestion, because in most filtering animals the
function of filtration is combined with respiration. At a high food concentration (which is often the case in experiments), they form pseudofeces, simply
precipitating the suspended food, but not ingesting it. In many cases, the
filtering animals passively ingest the suspended matter, which they are not able
to digest (so-called superfluous feeding). Therefore it is impossible to obtain
realistic information about nutrition even in filter feeders without an estimation of the assimilation rate and assimilability of the suspended food, because
these are the real evidence of feeding rate and nutrition.