THE ECOLOGICAL EFFECTS OF EXTERNAL METABOLITES
Authors: LUCAS C. E. 
Published: January 2008

1. Evidence is reviewed relating to the apparent ‘exclusion’ of some animals by some planktonic plants, together with the evidence that such plants may, when abundant, have an inhibitory or even lethal effect on associated animals. These relationships appear to be additional to those of prey and predator.

2. The extensive excretion and external secretion of metabolites by organisms of many types is now certain and the suggestion that the ‘exclusive’ relationships may be mediated by the external products of the plants is reconsidered in the light of recent work in marine and other fields.

3. The need of many organisms for certain substances which they themselves cannot manufacture, and which are not normal foods, has been shown. Such substances may be obtained from other members of the community, although, as is seen in recent work on antibiotics, by no means all such by‐products are beneficial for members of the community. They may either stimulate or inhibit various biological processes. Among others, substances of enzyme, hormone and vitamin nature appear to be concerned.

4. The most thoroughly studied examples of such relationships occur in microorganisms, but evidence of their occurrence among higher forms is growing. This evidence ranges from the stimulation of eggs, through sex‐determination by external secretions (e.g. carotenoids), to root secretions, and the extensive excretions of sterols of biological significance, etc.

5. It is believed that during the course of evolution many organisms have adapted themselves to tolerate or take advantage of the external metabolites of their neighbours, just as some organs of the individual have come to respond to the endocrine products of other, particularly vestigial, organs. Those organisms which failed to do so must have become extinct or have evolved some avoiding mechanisms. Many important ecological relationships appear to have arisen in this way, both stimulating or inhibiting; they appear to concern community integration, competition and succession, and probably symbiosis. The term ‘ectocrine’ is suggested for a substance mediating such processes.

6. Direct evidence for such relationships in the sea is still scanty, but it is now known that free by‐products of metabolism occur widely, and their influence on marine life has been shown in some cases, together with the comparable influence of a few synthetic compounds. It is suggested that the possible influence of carotenoids and sterols should be considered.

7. The probable significance of such relationships in the ecology of the marine plankton is dealt with, and it is considered that they might account in particular for Hardy's evidence of ‘animal exclusion’. This may be seen as one aspect of a more generalized biological exclusion in space and time which includes the phenomena of succession. Similar relationships may also be concerned in the initiation of the spring diatom crop and in the high productivity of coastal waters, as well as in that of boundaries between currents of diverse origin. The significance in these events of horizontal and vertical water movements is discussed. In the light of these processes a possible review of symbiotic relationships between animals and algae is suggested.