De aantalsontwikkeling van de rosse vleermuis (Nyctalus noctula) in West-Nederland

Authors: Kees Mostert & Jan Piet Bekker

https://doi.org/10.63269/SJL5813

Abstract: Changes in numbers and distribution of the noctule in the west of the Netherlands
The occurrence of the noctule (Nyctalus noctula) in the west of the Netherlands, including the provinces of Noord-Holland (excluding Het Gooi), Zuid-Holland and Zeeland is for the most part limited to the wooded inner dune area. Until the late 1970s, data on the occurrence of this species were mainly based on casual finds and social calls and squeaks around colony trees, which alerted observers to the presence of the species. Since the early 1980s, the advent of the bat detector has made it possible to more effectively study areas for the occurrence of noctules, among other bats. The Dutch population in the 1990s was estimated at 6000 to 8000 animals, of which 810-1030 were recorded in the inner dune area. No maternity colonies of noctules were found outside the inner dune area. Since the 1980s, there have been three periods of monitoring of noctules in the western part of the Netherlands: 1986-1993 (period 1), 1998-2002 (2) and 2013-2018 (3). The area around The Hague, Rijswijk and Voorburg was studied in four separate periods: 1986-1993 (period A), 1998-2002 (B), 2008-2012 (C) and 2015-2018 (D). In that area in Zuid-Holland, systematic surveys of maternity colonies, summer roosts and foraging animals were conducted in suitable areas, including outside the inner dune area where the species was initially absent. The number of noctules emerging from their roost in the west of the Netherlands over periods 1 to 3 shows a definite, significant increase. Even so, the numbers of colonies increase over periods 1 to 3. Similarly, in The Hague, Rijswijk and Voorburg, both the numbers of emerging noctules and the number of colonies show an increase over periods A to D. Furthermore, between the periods of 1986-1993 and 2013-2018 the number of kilometre squares where noctules were heard in the west of the Netherlands increased significantly. The eventual establishment of a maternity colony is often preceded by years of observations of foraging animals and the establishment of roosts with one or a few animals. The population of noctules in the period 2013-2018 is estimated at 2600-2850 adult females compared to 800-1000 animals in the 1990s. This means a significant increase in the number of animals in recent decades. In addition to an increase in the number of maternity colonies and the number of foraging animals, an expansion and new settlements in areas outside the inner dune area were also observed. For example, new colony settlements were found in the vicinity of Zoetermeer, Rotterdam, Schiedam and ‘s-Gravenzande and also in the inner dune area of the former island of Voorne, areas in Zuid-Holland where the species was not found in the 1990s despite targeted efforts to find colonies. In Noord-Holland, new colonies were found in the Amsterdamse Bos and in a northerly direction in the vicinity of Alkmaar, north of Schoorl and in the Wildrijk. In Zeeland, however, the few formerly known colonies in the border area of Zeeuws-Vlaanderen and Flanders seem to have disappeared, while the area around Hulst has not been sufficiently investigated. The causes of the increase in the number of bats in the west of the Netherlands in the periods studied are related to both a general aging of the country estates and the forest, as well as to the greater availability of foraging areas, such as recreational forests and lakes. An increase of the number of recreational lakes was especially apparent in the western parts of the Netherlands, while the increase of recreational forest was greatest in Zuid-Holland, compared to other parts of the Netherlands. For these reasons, the observed increase in the number of noctules is not necessarily representative of other parts of the Netherlands. A changing forest management since the 1970s, which took place nationally, with a greater emphasis on the preservation of old and dead trees, can also be mentioned as a cause of the increase. Felling of old trees, however, remains a concern. Forests containing such trees are important: the noctule seems to mainly appear in forests that are older than 80 years. In addition to logging during World War II, urbanization, leading to loss of hunting grounds, had a major effect on the populations of noctules in the west of the Netherlands. The bans on insecticides in 1968 and 1973 may have had a positive effect on noctule numbers, although there is no prove to support this. Another factor in favour of the noctule may have been the scharp increase in numbers since the 1970s of the great spotted woodpecker (Dendrocopos major), being the main supplier of tree cavities suitable for roosting noctules. The expansion of the ring-necked parakeet (Psittacula krameri) may have a negative effect on noctules locally, but does not seem to be affecting the noctule population for the time being. Remarkably, the Eurasian nuthatch (Sitta
europaea) seems to have the same requirements as the noctule regarding age and size of trees and the required area with suitable trees. It is of great importance that new systematic area inventories are carried out on noctules throughout the Netherlands.