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Bühlmann, J. & G. Pasinelli

Analyse des Bestandsrückgangs beim Mittelspecht Dendrocopos medius von 1978–2002 im Kanton Zürich: Grundlagen für den nachhaltigen Schutz einer gefährdeten Waldvogelart

2012

109

73–94

Key words
(von 1994 bis 2006 vergeben)

Eichenwälder, Vernetzung, Isolation

Schlagwort_Vogelart
(wissenschaftlich)

Dendrocopos medius

Schlagwort_Vogelart
(deutsch)

Mittelspecht

Zürich

deutsch

Abhandlung

Analysis of the population decline of the Middle Spotted Woodpecker Dendrocopos medius from 1978-2002 in the canton of Zurich: basic information for the sustained protection of a threatened forest bird species. – The size of the Middle Spotted Woodpecker population in the canton of Zurich, Switzerland, declined by 22 % from 1978 to 2002 (Buehlmann et al. 2003). To find possible reasons for this decline, changes in oak forests in terms of area and age structure over the study period were assessed on the basis of forestry plans and aerial photographs. Since the 1970s, oak area of 50 forests with an oak share of at least 10 % declined by 27 %, oak volume by 11 % and the number of stems by 25 %. An average of 1045 oaks with a diameter at breast height from 8-16 cm and larger were found in a Middle Spotted Woodpecker territory, corresponding to an average oak volume of 1477 m3. Although only just about 15 % of the forests examined had an oak share of at least 50 %, these forests held 30 % of all woodpecker territories in 2002. In contrast, 57 % of the forests had an oak share below 30 %, but hosted only 34 % of all territories. In a second step, we analysed the relationships between changes in numbers of Middle Spotted Woodpeckers and changes in area, quality and connectivity of oak forests. The decrease in population size in a given oak forest was most strongly related to an increase in isolation of the forest. Both from 1978 to 1988 and from 1988 to 2002, we found a positive relationship between the reduction in the sizes of occupied oak forests within 3 km of a given local oak forest and the population size within that local oak forest. Furthermore, the reduction in Middle Spotted Woodpecker numbers in a given oak forest was associated with the reduction in area of the respective local oak forest, but only for the period 1988 to 2002. Changes in woodpecker population size were neither related to changes in the quality of an oak forest, with quality expressed as the number of old oaks per ha and the oak volume per ha, respectively, nor to the amount of oak forest (irrespective of occupation by the Middle Spotted Woodpecker) in the vicinity of that local population. Our findings suggest that the oak area of both a local forest and of occupied forests within 3 km of this forest should be the primary targets of Middle Spotted Woodpecker conservation. In addition, oak share in a forest should be above 30 %. Finally, large oaks should not be removed to avoid an increasing concentration of the overall oak volume in a smaller and smaller number of trees.