Abstract
Deforestation and forest management have resulted in massive habitat loss and habitat degradation for forest-dependent species, particularly for those associated with late successional stages. Whether integrative conservation measures, e.g., retaining deadwood and habitat trees in managed forests, are sufficient for the conservation of such species is unclear. In heterogeneous forest landscapes, habitat specialists with large area requirements may use a variety of forest stands to compensate for the lack of high-quality habitat or to meet different biological requirements. Investigating the space use of old-growth forest species in landscapes comprising stands of different management intensity may thus help to provide key insights into the potential of managed forests as habitat for such species and to better incorporate their requirements into forest management planning.
In this thesis, I examined the space use of the white-backed woodpecker Dendrocopos leucotos, a habitat specialist associated with old deciduous and mixed forests with high amounts of deadwood and a species of high conservation concern in Europe. I used data of adult white-backed woodpeckers radio-tracked in a region with high heterogeneity in landscape composition and forest management intensity combined with data on forest structure characteristics to examine variation in home range size and habitat selection in different seasons and at various spatial scales. The aims of this thesis were to close knowledge gaps in this species’ ecology and to find out how the occurrence of an old-growth forest specialist can be reconciled with forest management.
In Chapter 1, I computed seasonal and yearly home range sizes based on three home range estimators and examined variation in home range size in relation to season, year, sex, and body weight. Home range size varied depending on the used home range estimator and between seasons, with minimum convex polygons and autocorrelated kernel density estimation producing 1.6 – 1.8 and 2 – 3.3 times larger seasonal home ranges than traditional kernel density estimation, and summer, fall and winter home ranges being 1.6 to 3.3 times larger than those during the breeding season in spring. These results highlight the usefulness of using multiple methods for home range estimation and the importance of considering the full annual cycle for assessing home range sizes.
In Chapter 2, I evaluated three hypotheses proposed to explain variation in breeding and post-breeding home range sizes: the resource distribution hypothesis, the restricted habitat availability hypothesis, and the intraspecific competition hypothesis. I found support for the resource distribution and intraspecific competition hypotheses in the breeding season, whereas none of the hypotheses explained post-breeding home range size. In the breeding season, home range size decreased with increasing proportion of resource-rich habitat, particularly when forest cover was low. Moreover, breeding home range size decreased with increasing population density when the proportion of resource-rich habitat was high. These results suggest that the mechanisms determining breeding home range size varied with habitat quality. In regions with low forest cover and presumably low habitat quality, home range size appeared to be mainly determined by the distribution of resources. By contrast, home range size in regions with high habitat quality seemed to be restricted by intraspecific competition. I concluded that deadwood and old trees should ideally be clumped within stands with little or no forest management, and that these resource-rich forests should constitute a high proportion of areas at least the size of a breeding home range to improve the habitat quality for the white-backed woodpecker.
In Chapter 3, I analyzed seasonal habitat selection at three hierarchical levels. White-backed woodpeckers selected old deciduous and mixed stands for establishing seasonal home ranges within the annual home range in both the pre-breeding and breeding seasons, and deadwood and large-diameter trees at the level of foraging tree selection throughout the year and in all habitat types. By contrast, the proportions of all measured habitat types within post-breeding, summer/fall and winter home ranges corresponded to their availability in an area representing the annual home range, and within the seasonal home range, the habitat types were used according to their availability during all seasons. These findings suggest that coniferous and young deciduous forests can generally serve as non-breeding habitat for white-backed woodpeckers as long as suitable foraging trees and close-by (largely) unmanaged old deciduous or mixed stands are available as breeding habitat.
In Chapter 4, I investigated the occurrence of the white-backed woodpecker in 60 study sites in relation to local habitat (forest structure characteristics within the sites, i.e., at the spatial scale of the breeding home range) and landscape context (habitat amount, habitat fragmentation, and matrix quality at spatial scales ranging from 1 to 25 times the annual home range size). Habitat amount, habitat fragmentation and matrix quality at the landscape scale explained 2.5 times more of the variance in occurrence probability than local habitat (however, the amount of deadwood was high also in absence sites), showing that conservation measures for the white-backed woodpecker might be ineffective in areas with low forest cover at the landscape scale, even if enough high-quality habitat at the local scale is available. I concluded that conservation measures
should focus on the conservation of and habitat improvements in areas with enough forest to sustain multiple breeding pairs, and that these areas should ideally be connected via dispersed patches of old deciduous forest.
In summary, the results of the four chapters show that addressing multiple seasons, spatial scales, and levels of habitat selection might be necessary to gain a comprehensive understanding of a species’ space use. In accordance with previous studies on the white-backed woodpecker’s habitat requirements, the results of the thesis generally confirm the high importance of old deciduous or mixed forest with abundant deadwood as breeding habitat for this habitat specialist, underpinning its dependence on old forest stands with low management intensity and supporting the idea of land-sparing as useful conservation approach for a deadwood-dependent species. Complementing this existing knowledge, I found that white-backed woodpeckers appeared to be more flexible in their space use and less bound to old deciduous forests in the non-breeding season; as they selected deadwood and large-diameter trees in all forest types throughout the year, integrative measures such as the retention of deadwood and habitat trees in more intensively managed stands (including coniferous and young deciduous or mixed stands) may thus be a useful supplement in areas close to old (almost) unmanaged stands. Lastly, the high importance of the landscape context for the occurrence of the white-backed woodpecker showed that conservation measures might only be effective if the surrounding landscape is taken into account.
Ettwein, Antonia