Entwicklung von Bestand und Verbreitung des Mittelspechts Dendrocopos medius 1978-2002 im Kanton Zürich: Analyse der Veränderungen und Folgerungen für den Artenschutz

Changes in population size and range of the Middle Spotted Woodpecker Dendrocopos medius 197 8– 2002 in the canton of Zurich: analyses of trends and consequences for species conservation. – The population of the Middle Spotted Woodpecker Dendrocopos medius was surveyed in 2002 for the third time after 1978 and 1988. In total, 106 «pairs» were found in 27 of the 69 potentially suitable oak wood areas. Population size was 22 % lower than in 1978 (148 «pairs»). Heavy losses occurred in three of the four «large» woodland areas. The local range decreased to about half the size of 1978. Our observations partly confirm the predictions of Muller (1982) that extinction probability of the Middle Spotted Woodpecker is linked to the degree of isolation of a suitable woodland plot. The loss of old oak trees in the last 24 years is likely to have led to a deterioration of many Middle Spotted Woodpecker habitats within the study area, although quantitative data on changes of factors influencing population size (density of old oaks, potential cavity trees and exact size of oak forests) are lacking. Shade-tolerant tree species growing up into the oak crowns and oak wood areas destroyed by gales are further problems. Broad-scale strategies to enhance habitat suitability for Middle Spotted Woodpeckers are urgently needed for the four core populations. We suggest a moratorium for harvesting oaks, until oak regeneration areas of at least 10 ha in size have reached an age suitable for colonization by Middle Spotted Woodpeckers. Such regeneration areas should be situated adjacent to existing old oak woods.