(* = Kurzbeitrag)
Herbivore Vögel und Säuger – auf der ewigen Suche nach genügender Stickstoffversorgung.
(von 1994 bis 2006 vergeben)
Verdauungsphysiologie, Nahrungsgrundlage, Körpergrösse, Raufusshühner, Huftierwanderungen, Antilopen, Savanne
Tetrao tetrix, Lagopus muta, Tetrao urogallus
Birkhuhn, Alpenschneehuhn, Auerhuhn
Herbivorous birds and mammals, and their eternal quest for nitrogen. This review links the question of how herbivores s. str., namely consumers of green plants, struggle to obtain a sufficiently nutritious diet, to the ongoing discussion of top-down versus bottom-up regulation of ecosystems. The top-down view, known as the «world is green» hypothesis, which states that the green biomass is not completely removed by herbivores because they are controlled by predators, has recently been backed by a number of studies demonstrating cascading effects. In such cascades, apex predators restrict populations of large herbivores and so produce responses among lower trophic levels, as the vegetation is released from heavy browsing pressure. However, closer scrutiny of long-term data has shown that both top-down and bottom-up mechanisms may be operating in a system at the same time. The bottom-up view has originally been explained by the «world is prickly and tastes bad» hypothesis, stating that the lack of the herbivores' ability to consume large parts of the green biomass is due to the self-defense mechanisms of the plants. There are, however, better reasons to assume that the main cause of bottom-up effects is the problem of the herbivores to secure enough nitrogen from their plant diet. Strictly herbivorous birds are few but goose populations do respond strongly to increased quality in their grass diet whereas grouse, being essentially browsers, do not seem to be limited by their often extremely low-quality winter diet. Ungulate grazers, on the other hand, are strongly driven by the nutritional quality of their diet. This aspect is illustrated by two examples, one involving red deer Cervus elaphus in the Swiss Alps, the other a community of antelopes living in a wet tallgrass savanna in coastal Tanzania. In both cases, the all-important role of highly nutritious grasses (with low fiber and thus relatively high crude protein concentrations) for diet and habitat selection could be clearly demonstrated. In the case of red deer, altitudinal migrations tracked the protein-richest forage on offer, providing strong support for the forage maturation hypothesis.
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