The ups and downs of a Tree Frog population on a military airbase In 1985, a small population of the Common Tree Frog (Hyla arborea) was discovered on the military airbase Gilze-Rijen in the province of Noord- Brabant. From 1986 onwards, the size of the population was estimated annually. Despite numerous measures to improve the habitat, the population remained small for many years (mostly less than 20 males), numbers only increasing significantly after 2000. Since then, in several years more than 100 juveniles were counted and up to 250 males. However in 2011, a genetic study revealed that the population had an alarmingly narrow genetic base. During this investigation, two males were found to have reached an age of 9 years!