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
biology.
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.
UN-L SBS
Lincoln, NE 68588-0118
jjanovy at unLinfo.unL.edu
