News - April 3, 2008

Wildebeest threatened by loss of trees

The current decline in numbers of large trees in African savannas has consequences for large herbivores due to changes in forage quality. Researchers of Wageningen University published this discovery in the March edition of the journal Oecologia.

Wildebeest grazing on the savanna in Tanzania.
Cover of large trees in African savannas is rapidly declining due to elephant pressure, frequent fires and charcoal production. Dr Fulco Ludwig of the department of Environmental Sciences studied the impact of large savanna trees on forage quality for wildebeest by collecting samples of dominant grass species in open grassland and under and around large Acacia tortilis trees, in Tarangire National Park, Northern Tanzania.

He learned that grasses growing under trees were of a much higher forage quality than grasses from the open field. The grasses beneath the acacias had higher protein and lower fibre concentrations. With fewer trees growing in the field, herbivores only have low quality food.

Ludwig writes that his analysis of the data indicated that large savanna trees could be essential for the survival of wildebeest, the dominant herbivore in Tarangire. ‘Due to the high fibre content and low nutrient and protein concentrations of grasses from the open field, maximum fibre intake is reached before nutrient requirements are satisfied.’ A combination of foraging from open grassland with either forage from under or around tree canopies is therefore essential for satisfying all requirements.

Forage quality does not decline immediately after trees die: grass quality was also higher around dead trees than in the open field. This explains why negative effects of reduced tree numbers initially go unnoticed. ‘The results suggest that continued destruction of large trees could affect future numbers of large herbivores in African savannas and better protection of large trees is probably necessary to sustain high animal densities in these ecosystems,’ Ludwig and his co-authors conclude.