Exposure of non-target small mammals to rodenticides: short-term effects, recovery and implications for secondary poisoning

1. Monitoring of exposure to pesticides in many countries shows extensive exposure of predators to anticoagulant rodenticides, which are used to control rats. Many predators and scavengers are declining in numbers, and exposure to rodenticides might therefore be of importance in conservation biology.

2. Predators and scavengers of poisoned rats are at most risk of secondary poisoning. However, several predatory species of conservation concern rarely eat rats, implicating non-target small mammals as the major route of exposure. For the first time, this research investigated the importance of non-target small mammals as routes of exposure to rodenticide for predators and scavengers in the UK.

3. Exposure studies of non-target small mammals were carried out alongside routine rat control at five sites, around agricultural buildings (n = 2) and feed hoppers for game birds (n = 3).

4. Three non-target rodent species fed on rodenticide from bait boxes during routine rat control treatments. A large proportion (48·6%) of individuals in local populations ate the bait: woodmice Apodemus sylvaticus were most exposed, followed by bank voles Clethrionomys glareolus then field voles Microtus agrestis.

5. Local populations of non-target small mammals declined significantly following rodenticidal rat control but their relative proportions did not change significantly. Populations recovered partially after 3 months, depending on the time of the year relative to the breeding cycle.

6. Synthesis and applications. Our results clearly demonstrate that routine rat control reduced local populations of non-target small mammals. This may limit the food supply of some specialist predators. Most importantly, this demonstrates a significant route of exposure of predators and scavengers of small mammals to secondary poisoning. Rodenticides are applied on farms and game estates across the UK. Hence the results of this study are indicative of non-target rodenticide exposure nationally. Mitigation requires a shift from the current reliance on rodenticides to ecologically based rodent management, involving improvements in site management and the adoption of good farming practice.