Journal für Ornithologie

, Volume 118, Issue 2, pp 156–174 | Cite as

Bird communities ofBrachystegia andAcacia woodlands in Zambia

A quantitative study with special reference to the significance of habitat modification for the Palaearctic migrants
  • Staffan Ulfstrand
  • Thomas Alerstam
Article

Summary

The bird communities of fourBrachystegia and twoAcacia woodlands near Lusaka, Republic of Zambia, were quantitatively assessed in the rainy season (February 1976). The number of species and the species diversity values were rather similar in all areas, and so was also the total bird density (with one exception) in spite of a sevenfold difference of foliage quantity between the poorest and richest habitats investigated. Foliage-gleaning birds exploited foliage-richer habitats less intensively than foliage-poorer ones. The density of Palaearctic migrants, among whichPhylloscopus trochilus andMusicapa striata were most abundant, was low throughout; in fact practically none was recorded in relatively untouched habitats, but in much disturbed areas, for example, thinned-outBrachystegia or heavily grazedAcacia woodlands, they were locally more numerous. Everywhere they made up only a minor portion of all birds present, and this holds also if their density is related to the guild of ecologically corresponding African species; for example, the density ofPh. trochilus in comparison with that of the whole guild of foliage-gleaning passerines.

The total bird density of the Zambian habitats was compared to that of temperate broadleaved woodlands, with which especially theBrachystegia woods bear a superficial resemblance, and was found to be about similar. This was unexpected considering the twice higher primary production measured for tropicalBrachystegia. The different degree of seasonality is pointed out as an important circumstance in this context. It was also unexpected that the total bird density was about the same in theBrachystegia andAcacia habitats, respectively, since the primary production, to judge from the difference in foliage volume, was much higher in the former.

The species richness and diversity were much higher in the Zambian study plots than in temperate areas of the same size. Diversity indices were calculated and compared to expected ones on the assumption of MacArthur's broken-stick distribution and the canonical lognormal distribution, respectively. It was found that the abundance distributions of the Zambian communities fell in the range between the values expected for these models; perhaps somewhat closer to the broken-stick distribution. This accords to expectation for closely integrated bird communities.

The Palaearctic migrants in the study areas occurred in much lower densities than they do in moderately favourable habitats in their European breeding areas. At leastPhylloscopus trochilus, and maybe also the other Palaearctic species encountered, probably inhabit other areas in tropical Africa in higher density than that found by us. That Palaearctic migrants are more abundant in modified habitats than in more undisturbed ones has been asserted before on qualitative grounds and was quantitatively confirmed by the present study. Their capacity to exploit modified habitats may be a significant factor for their future destiny, considering the profound landscape alteration that has commenced and is now accelerating in much of Africa.

Keywords

Bird Community European Breeding Palaearctic Species Modify Habitat Landscape Alteration 

Vogelgesellschaften derBrachystegia- undAcacia-Savannen in Zambia

Zusammenfassung

Die Vogelgesellschaften auf 4 Kontrollflächen derBrachystegia- und auf 2 derAcacia-Baumsavanne wurden während der Regenzeit (Februar 1976) in der Umgebung von Lusaka, Zambia, quantitativ erfaßt. Artenzahl und Artendiversität waren auf allen 6 Flächen fast gleich, obwohl das Laubvolumen der am reichsten strukturierten Fläche etwa den siebenfachen Betrag jenes der ärmsten ausmachte. In der Laubschicht nahrungssuchende Vögel beuteten das Gebiet mit dem geringsten Laubvolumen am stärksten aus. Die Dichte der paläarktischen Zugvögel, unter denen Fitis und Grauschnäpper am häufigsten waren, war niedrig; fast keine wurden im unberührten Wald angetroffen. In den vom Menschen stärker beeinflußten Abschnitten kamen sie dagegen häufiger vor. Veränderungen der Baumsavanne treten durch Ausholzung der Wälder oder auch durch intensives Beweiden der offenen Savannenflächen ein. Paläarktische Zugvögel machten stets nur einen sehr geringen Teil der Vogelgesellschaften aus, auch innerhalb des relativ begrenzten Spektrums afrikanischer Vögel gleichen Nahrungsverhaltens.

Die Vogeldichte der zambischen Wälder ist im Vergleich zu Laubwaldbiotopen der gemäßigten Zone eher niedriger, obwohl z. B. die Primärproduktion derBrachystegia-Wälder ungefähr zweimal so hoch ist. In diesem Zusammenhang ist natürlich der stärkere Jahreswechsel in der gemäßigten Zone von Bedeutung. Auch fällt auf, daß die Vogeldichte in allen unseren Untersuchungen ziemlich ähnlich war, ungeachtet offensichtlich stark unterschiedlicher Primärproduktion ausgedrückt durch die Laubmasse.

Artendichte und -diversität waren in den zambischen Untersuchungsgebieten beträchtlich höher als in entsprechend großen Flächen der gemäßigten Laubwälder. Verschiedene Diversitätsindices wurden berechnet, die verglichen mit der Artenzahl dafür sprechen, daß die zambischen Vogelgesellschaften zwischen einer „broken-stick“- bzw. lognormalen Verteilung der Arten liegen. Die erstere ist bei gut integrierten und stabileren Gesellschaften zu erwarten, letztere bei instabileren.

Die paläarktischen Zugvögel traten in viel geringerer Dichte auf als in mäßig günstigen Brutbiotopen ihrer Heimat. Wahrscheinlich besitzen die festgestellten Arten, vor allem der Fitis, andere Winterquartiere. Die Tatsache, daß Paläarkten in vom Menschen beeinflußten Biotopen in beträchtlich größerer Dichte auftreten als in weniger gestörten, war zwar schon früher aufgrund qualitativer Beobachtungen behauptet worden, wurde aber hier erstmals quantitativ bestätigt. Daß Zugvögel sich besser in der „Kulturlandschaft” im weitesten Sinne behaupten können als im unberührten Wald, kann für ihr weiteres Schicksal sehr bedeutsam sein, da zunehmend größere Teile Afrikas einer Landschaftsumgestaltung verbunden mit Zerstörung ursprünglicher Vegetationseinheiten unterworfen werden.

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Copyright information

© Verlag der Deutschen Ornithologen-Gesellschaft 1977

Authors and Affiliations

  • Staffan Ulfstrand
    • 1
  • Thomas Alerstam
    • 1
  1. 1.Department of Animal EcologyLundSweden

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