Factors influencing the density of the brown rat (Rattus norvegicus) in and around houses in Amsterdam
The current strategy of the pest management department of the Public Health Service in Amsterdam is to identify causal factors in order to reduce the carrying capacity of pest populations and to minimise the use of pesticides. Rats have been controlled with rodenticides for decades, which has increased the survival of resistant rats. Rodenticide resistance has now been found in several rat populations in Europe. The main aim of this study was to establish the relationship between brown rat (Rattus norvegicus) occurrence in Amsterdam and a number of environmental and socio-economic factors. A second aim was to point out factors that can be managed by the local authorities as a next step towards prevention and pro-active, integrated pest management. The paper begins with a short overview of the biology of the brown rat, with an accent on diseases and habitat factors. The number of rat reports at the neighbourhood level during the years 2009-2012 is then related to 16 environmental and socioeconomic variables including availability of water, availability of urban green space, sewer type, construction year of houses, property tax value, number of inhabitants and population composition. A generalised linear model was used; it had a negative binomial distribution and all candidate models were fitted with a maximum of five terms. The most significant terms were number of inhabitants, percentage of area occupied by urban green space, the percentage of houses with a construction year before 1960, and either the length of foul water sewer (separated sewer) or the length of combined sewer. Rats have a short generation time and can produce a large number of offspring. A rat population is therefore able to recover quickly from a reduction in number. It is therefore important to change the carrying capacity of the habitat in which rats are unwanted. This can be achieved by changing the amount of food and cover they can find. The results of the regression analysis suggest that houses constructed before 1960 and their gardens could be evaluated to see if there may be general solutions that would make them less amenable to rats. Furthermore, the results suggest that the structure of urban green space may be adapted to make it less attractive to rats; an example of this would be to replace evergreen shrubs with deciduous shrubs and to mow high vegetation near buildings more frequently. Moreover, the influence of waste near and in urban green space should be investigated. Finally, we suggest that the inspection and maintenance of sewers be continued and that this should include the connection between properties and the public sewer.