260
THE ESSEX NATURALIST
RESULTS AS PRESENTED IN THE FORM OF A FOOD-WEB
While the histogram (Fig. 6) presents the relative abundance
and distribution of riffle organisms, it does not indicate how they
are inter-related. Since the riffle is a dynamic system, it should as
far as possible be presented in a dynamic way. I have chosen to
use a food-web to demonstrate the inter-relationship (Fig. 8).
In a central position are the Leeches, these being "related" to a
number of organisms in the habitat. In this way I believe the
presentation to be greatly simplified, reducing to a minimum the
number of lines vised in its construction. Two aspects of the food-
web which I consider important call for further comment. It
helps to explain the relative abundance of various organisms, for
example Glossophonia complanata (vf) may be correlated with the
mollusc Ancylus lacustris (vf) which is not at all obvious from the
histogram. It also helps to explain the presence of Stone Loaches
in large numbers, downstream of the riffle, where their main food
supply was present in abundance. From the dissection of the gut
of Stone Loaches, the presence of Gammarus was only very
frequent (vf). The only other predators on this group are Stickle-
backs and Helobdella stagnalis which were rare (r). Gammarus
therefore is the source of food of only a few members of the com-
munity and since it is a scavenger with an ample supply of food,
there will be no restriction on its multiplication and development.
This probably explains its very large number in the riffle.
The second aspect it helps to explain is that while there exists
an abundance of food for a particular parasite or predator, these
themselves are rare or entirely absent from the habitat. This
suggests that there are other factors which limit their occurrence,
one of which might well be the pollution state of the river. For
example Ephemeroptera, Sticklebacks, Helobdella and Asellus
were all rare, yet there was an ample food supply. Noted absentees
from the riffle habitat include the Stoneflies (Plecoptera) and the
Flatworms (Platyhelminthes).
The presentation of a particular habitat in the form of a food-
web helps to explain the relative abundance or absence of
organisms and their distribution as presented by the histogram.
It also defines, however inadequately, the habitat in a living state,
as it probably existed at the time of sampling. The results
obtained from a comparison with other riffles is made easier if
presented as a food-web.
PRACTICAL APPLICATIONS
A comparison of polluted and unpolluted riffles, whether the
pollutant be toxic as might arise from industrial premises, or
organic as from an overloaded sewage disposal works, will provide
much required information as to how the biological community
reacts to such discharges.
Effect of Chemical Pollution
Much work has been done on the reactions of specific organisms
to various toxic metals and compounds in laboratory controlled