Rodents: Pest or pleasure?

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.

Linking economy to ecology and disease

Since I started as a PhD candidate in March 2012, I have read a lot about the relationship between biodiversity and disease. Actually, I am studying this relationship by looking at the effect of mammal diversity on Lyme disease risk in the Netherlands, where I define Lyme disease risk as a combination of the tick density and the percentage of infected ticks. If we go back in time, the current discussion about the effect of biodiversity on disease risk started about 10 years ago, with a number of publications by Richard Ostfeld and colleagues in which they proposed the dilution effect, which in short argues that disease risk increases when biodiversity decreases (I have to say here that Ostfeld and colleagues state it much more elaborate, as there are a number of criteria that have to be met before this statement is true). There has been a lot of debate about this, with people arguing both for and against the dilution effect. However, I recently noticed a paper published by an economist which drew my attention.

Bonds and colleagues argue (click here to read the full paper) that biodiversity is not only related to disease risk, but also to per capita income. This seems quite a bold statement only to increase the support for the protection of biodiversity, and perhaps it is, but it made me think. Bonds et al. argue that there is a strong link between the natural environment and economy, by pointing to the latitudinal gradient that is visible in all kinds of processes from animal body size, to biodiversity, human disease burden and economy. The relationship between economy and latitude could be caused by vector-borne and parasitic diseases (VBPDs), which are most common around the equator, and which are a major cause of morbidity and mortality in poor countries, influencing the economy in these countries.

In their analysis, they perform a multiple linear regression, where they show that biodiversity had a significant negative effect on VBPDs, when controlling for all other factors. Indicating that a loss in biodiversity would indeed increase disease risk. Furthermore, VBPDs had a significant negative effect on income, when controlling for all other factors. Could this mean that protecting biodiversity could increase or at least stabilize income in poor countries?

As the authors state, there are many complex mechanisms underlying diseases, and only by unraveling these, can we gain more insight into how things work, but it does make you think!