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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Quantification of the integrative effect of ecotones as exemplified by the habitat choice of Blackcap and Whitethroat (Sylvia atricapilla and S-communis, Sylviidae)
Auteurs :	O. Elle, Auteur
Type de document :	Article
Dans :	Journal of Ornithology (144(3) JUL 2003)
Article en page(s) :	271-283
Langues:	Anglais
Résumé :	Owing to different preferences of habitat structures, Blackcaps (BC) and Whitethroats (WH) do not occur syntopically in most biotopes. The WH is most likely to be found in scrub, whereas the BC is more of a woodland species. However, how much do these two species, which are supposed to be so different, have in common when they encounter in a transitional landscape (ecotone), which offers suitable vegetation structures for both species on a small scale? The study area is characterised by a habitat mosaic of orchards, small woods, hedgerows and shrub in different successional stages. This study quantifies how much ecotone-like biotopes allow interspecific approaches with respect to both the choice of vegetation structures and the vicinity of the territories. Over three breeding periods the vegetation structures of 162 territories of the BC and 93 territories of the WH were subjected to a discriminant analysis. Wrong classifications of species membership by the discriminant models reflect interspecific similarities of the selected vegetation structures. If the complete study period is analysed, the species membership of 26.3 % of territories is classified incorrectly by a discriminant analysis of their structural features. The single year analysis resulted in proportions of misclassified territories between 22.8 % and 35.4 %. The Garden Warbler (S. borin ) was additionally investigated because of its habitat preferences for woodland edge and tall scrub, which lie intermediately between the preferences of the BC and the WH. This interspecific constellation of habitat preferences was quantitatively confirmed by the model. This is a strong argument in favour of the conclusion that the postulated partial similarities between the BC and the WH are real and not merely the result of low discriminating qualities of the model owing to inadequate structural variables. Furthermore the validity of the model is supported by a considerable interspecific spatial overlap of the territories. To summarise the complete study period, 23 % of the area that is selected at least in one year by either the BC or the WH is used commonly by both species. Thus interspecific structural similarities in habitat choice also result in propinquity of territories. In contrast, 46 % of the area is used exclusively by the BC and 31 % by the WH. These parts of the ecotone are considered strictly BC-habitat and WH-habitat. Probably the interspecific encounter happens predominantly in vegetation structures that are sub-optimal for one of the two species. Clear boundaries and sudden transitions between different, homogeneous biotopes are characteristic of vast parts of our man-made landscape, which results in a polarisation of the avifauna (i. e. wood species/shrub species). The lack of ecotones impedes interspecific encounters of bird species from different biotopes in the landscape and increases the observers' impression of interspecific habitat segregation. Negative population trends of the WH in recent decades and concurrent positive trends of the

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