Population dynamics of sewer rats

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Abstract
1. The effects of poison treatments on the rat populations of two sample districts of heavily infested sewers in a London borough were studied.2. Each district had a maximum rat population represented by a census figure of about 10,000 g. wheat eaten per day, corresponding to at least 400 rats.3. Poison treatments after prebaiting, at intervals of 6 months, reduced the rat population of each district to less than 10% of the maximum. A rapid restoration of the population followed, and the level reached in 6 months was near the maximum.4. In both areas, direct poisoning with sodium monofluoroacetate ‘1080’ was as effective as a prebaiting treatment. Direct poisoning with zinc phosphide was relatively unsuccessful.5. When both areas received double treatments, consisting of direct poisoning with sodium monofluoroacetate followed by a prebaiting treatment, the estimated population of one district was reduced to about 3% of the maximum. In the other area, rain made the second poisoning ineffective.6. The rate at which the population was restored from the 3% level was even higher than the rates previously observed, and was probably too high to be accounted for solely by breeding. Evidently, invasion from the surface played a part.7. No relationship could be established between the estimated sewer rat population densities within each district, and the known sites of surface infestation.We are grateful to the Council and officers of the Borough in which the work was done, for unstinted help throughout the inquiry. We also owe thanks to H. V. Thompson for help in the early stages; to him and to Miss M. D. A. Lawrence for discussing the work during its progress; to Miss M. M. Spencer for help in the analysis of the records and in checking calculations; and to A. G. Jenson for mapping the districts and for drawing the figures.
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