¿Tuvieron efecto los tratamientos químicos contra los topillos?: inferencia a partir del estudio de la dieta de la lechuza común y censos de rapaces diurnas?

The Junta de Castilla y León carried out several campaigns of control of common vole (Microtus arvalis) from 2007 to 2009, which coincided with a demographic explosion of the species. Anticoagulants products like chlorophacinone and bromadiolone were used during these campaigns. Information on the treatment campaigns which used rodenticides was compared with two studies which had been carried out to estimate the abundance of common vole, so as to evaluate the effectiveness of such treatments on the control of Microtus arvalis populations. The first study is based on the percentage of Microtus arvalis in barn-owl (Tyto alba) diet and the second by making road transects to find the abundance of diurnal birds of prey. Sixteen batches of barn-owl pellets were analysed –one each trimester, between January 2006 and december 2009– and 3,964 items of prey were identified. 96.3% of them were micromammals of seven species, and 63.7% were common vole. The percentages of Microtus arvalis / trimester varied from 18.0% –after the collapse of a demographic explosion– to 96.4% at the demographic explosion’s peak. These results made it possible to detect two demographic explosions, one starting at the beginning of the second trimester of 2006 and finishing in the second trimester of 2008. The second started at the second semester of 2009 and it seemed to enter a collapse stage at the end of the same year. On the road, 5,387 birds of prey of fifteen different species were counted from January 2005 to December 2009. This is a monthly Kilometric Abundance Index average of 69.0 individuals/100 km. The data showed that peaks of abundance of birds of prey coincided in time with both common vole demographic explosions. Nevertheless, the period considered for each demographic explosion was significantly shortened. According to the bird data, the first cycle started in July 2006 and continued until February 2007. The second cycle started in May 2009 and continued until December 2009. Both studies reveal the ineffectiveness of the chemical treatments, which were carried out some months after the beginning of the demographic explosion, which was already at its highest peak or at collapse stage. The effect of these treatments on non-target species is also discussed.