I think that something that is important to mention is the kind of relation betwen the parasit an the patch, without this many organism that are accepted as parasits can be taken out of this group, i also think that a clasification based on the facility to grow the organisms is a little bit homocentric.
Now we have a problem because when you try to settle the kind of relation you dont know the exact limits betwen parasites, simbiontes, commensals, etc. This is not surprising, at less not in biology.
>>>>PARASITISM is a phenomenon in which a species' habitat consists of a
>>highly regulated, patchy environment, and in which the parasitic
>>species, probably in adaptive response to the highly regulated nature
>>of its habitat, has lost some functions essential for life outside its
>>patches. As in all catagories into which humans place plants and
>>animals, PARASITISM varies in the intensity of its manifestations.
>>However, the large and overwhelming majority of those animals we call
>>PARASITES cannot be easily cultured outside their patches, especially
>>if the term "culture" applies to all life cycle stages. And in those
>>cases in which culture is possible, we spend a rather substantial
>>amount of effort to duplicate certain qualities of the patch.
>>>>The patch, of course, is the host, and its "highly regulated" nature
>>is a product of its own genetic makeup, thus its evolutionary history.
>> A cow is not a parasite on the pasture. The pasture is not a patch,
>>and its physical and chemical makeup are not highly regulated, at
>>least in relative terms. If a pasture included only grasses, and was
>>distributed in patches throughout a matrix of coniferous forest, and a
>>calf could not complete development until it gained access to a patch
>>of pasture, then we might be correct in claiming that a cow was
>>parasitic upon its pasture. Indeed, the term PARASITISM is applied to
>>lepidopteran species dependent on their host plants, and the analogy
>>holds. A similar case could probably be made for the koala and
>>eucalyptus, and panda and bamboo. Both the koala and the panda are
>>ecological equivalents of lepidopterans dependent on a narrow range of
>>plant hosts. The rancher, of course, will claim that cows are not
>>easy to culture, either. The rancher is correct; cows are not easy to
>>culture. But compared to tapeworms, cows are cake.
>>>>The structure of parasite populations may well be a function of the
>>scale at which we measure their spatial distributions. Crofton's
>>observation that aggregation seems to be a property of parasitism may
>>be related to the fact that when we go looking for parasites in
>>nature, we depend on hosts to find them for us. The hosts are
>>generally very small patches in a large inhospitable matrix. Thus we
>>are bound by the nature of our sampling constraints (we must examine
>>hosts to find parasites) to count successful occupation of small
>>squares (hosts) in a very large grid (matrix). It is well known that
>>one can alter the conceptual picture of a distributed population
>>simply by altering the scale one uses to sample that population. But
>>when we go to the field to collect animals we call parasites, much of
>>our freedom to choose the scale is lost, primarily because we must
>>first choose a small scale (pick a host species). Crofton's
>>generality holds for macroparasites is, in my opinion, a valid one,
>>but it is also as much a generality about sampling as it is about
>>>>I believe that this mode of life we call PARASITISM is far and away
>>the most common mode of life on earth. That is, the vast majority of
>>species are adapted to highly regulated, patchy, environments, and
>>live in populations that are not evenly distributed among the patches.
>> That parasites are metabolic equivalents of predators is a fairly
>>trivial observation; "animals" (if you accept that classification of
>>convenience) are all heterotrophic and require their nitrogen in the
>>form of a mixture of amino acids. Thus all of these categories into
>>which we place plants and animals (predators, prey, commensals,
>>competitors, etc.) are really categories we use to legitimize and
>>communicate the suite of questions we're interested in. What's always
>>puzzled me is why biologists as a general rule don't seem to be
>>vitally interested in the most common way of life on Earth.
>>>>(I wrote this commentary in response to a posting asking for comments,
>>but I forgot the person who first posted, and eventually lost the
>>item. Sorry to the original writer, but here's one answer.)
>>>>John Janovy, Jr.
>>Lincoln, NE 68588-0118
>>jjanovy at unLinfo.unL.edu>>>>>>>>>>>>>