Sometimes when going through databases of publications, you find something interesting on a topic you were not looking for in the first place. It can be something related to the topic you were looking for, or something totally different. This recently happened to me. While going through all the publications in Lutra (the scientific magazine of the Dutch mammal society) looking for information on polecats for a publications I am working on together with Jasja Dekker, I saw a title which caught my interest. ‘Why do some rodents become a pest, while others barely survive?’ A question that I had never thought about, but now that I saw it, I wanted to know the answer.
I love working with rodents as they are relatively easy to study (for a mammal) and because they are so interesting in many ways. Rodents are on the lower end of the food chain, as they are the staple food for many raptors and predators. They are themselves predators of plants and seeds, working as dispersers and gardeners. They have very interesting population dynamics. And last but not least, rodents play an important role in many disease systems. All of these reasons make rodents a pleasure for me as an animal ecologist, while they make them a nuisance or pest for farmers and public health institutes, and this contradiction, I hoped, would be addressed in the paper with the interesting title.
The paper starts by setting a definition for when a rodent is a pest as “a rodent that, at a given time and a given place, causes damage, or constitutes a risk for such damage, that is unacceptable to humans”. This definition includes the transmission of diseases from rodents to animals and humans, as well as the perceived risk of the occurrence of burrowing rodents near a dyke. It does not include a strict damage threshold above which the rodents become a pest, as many other definitions of pests do. It does, however, include a subjective damage threshold, which is defined as “unacceptable”. To emphasize this subjectivity, the author gives a personal account of a situation which he encountered during his work. He was working on black rats (Rattus rattus) in Zanzibar, of which he caught a lot in rural houses. When he asked how the local leaders perceived this problem they answered that they “were aware of the large numbers of rats, that the rats now and then would bite the fingers of small babies, but that they did not cause real problems”! So as you can see, a pest can only be defined in a subjective way, because it is humans that perceive a rodent as a pest or not, irrelevant of any economic or epidemiological data.
According to the author, only five out of 61 European rodents are generally considered pests, which is a very small number. Also there are no clear characteristics that describe these species, apart from the fact that they are numerous and occur in environments where the chance of doing damage to humans is large (agricultural and urban/sub-urban areas). Also, one species can be considered a pest in one place, while it is the subject of many conservation efforts in another place. A nice example of this is the European hamster (Cricetus cricetus), which is strictly protected in Western Europe, while it is considered a pest in Eastern Europe (although there are signs that the Eastern European population is declining). The author shows that it is environmental conditions and often man initiated changes to the environment that cause rodents to become pests, and that it is not innate characteristics of the species that make them pests. In the end, it is the subjective opinion of the observer that determines if a species is a pest or not. So it is entirely up to you, if rodents are a pest or a pleasure!
Want to know more?
Leirs H. (2002) Why do some rodents become a pest, while others barely survive? Lutra, 45(2), 75-82.