Effect of low rainfall and browsing by large herbivores on an enclosed savannah habitat in Kenya

Abstract

Savannah ecosystems in East Africa are rarely stable and can experience rapid local changes from dense woodlands to open plains. In this 3-year study there was a reduction of 16.3% in a height-stratified sample of nearly 1000 individually marked Acacia drepanolobium trees. The study was carried out in an enclosed fire-free wooded grassland habitat in the Laikipia region of Kenya. The trees were monitored from 1998 to 2001, a period that included 12 months when rainfall was 60% below average. Elephants were responsible for the loss of 40% of the trees, black rhinos 33% and 27% died from the effects of the drought. Low rainfall was correlated with increased damage as elephants switched diet from grass to trees. Heavy browsing by giraffes reduced tree growth rates and increased their susceptibility to drought. Hence the combination of low rainfall and heavy browsing by elephants, black rhinos and giraffes led to the rapid tree loss. These findings have implications for research into the causes of instability in savannah ecosystems and the management of enclosed reserves.

Habitat associations of birds at Mara Naboisho Conservancy, Kenya

“…As a result, the Serengeti has experienced a gradual opening up of its wooded habitats as trees get pushed over by elephants at a faster rate than they can regenerate. In Laikipia County of Kenya, Acacia drepanlobium trees were monitored from 1998 to 2001, a period that included 12 months when rainfall was 60% below average and elephants were responsible for the loss of 40% of the trees (Birkett and Stevens-Wood 2005). These findings have implications for research into the causes of instability in savanna ecosystems and the management of enclosed reserves.…”

“…Of these, only fire and herbivory are likely to change at a particular locality within the short term, and both are amenable to some degree of human interference and control (Scholes and Walker 1993). Savanna ecosystems in East Africa are rarely stable and can experience rapid local changes from dense woodlands to open plains (Birkett and Stevens-Wood 2005). Despites numerous studies showing associations between birds and vegetation (e.g.…”

Woody Plant-Herbivore Interactions in Semi-Arid Savanna Ecosystems

“…Rebrowsing means that the targeted trees suffer repeated damage and may eventually die or suffer reduced competitive ability relative to other woody plants (Skarpe & Hester, 2008). Heavy browsing by giraffe reduces tree growth rates increasing their susceptibility to drought (Birkett & Stevens-Wood, 2005). Fornara & du Toit (2008) reported high plant compensatory growth abilities of Acacia nigrescens as important for its persistence under heavy browsing in the Kruger National Park, South Africa.…”

Woody Vegetation Utilisation in Tembe Elephant Park, Kwazulu-Natal, South Africa

“…We consider that elephant utilisation assisted by the heavy smaller browser utilisation in Tembe's woodland conditions seenhere is likely to push succession from dense woodlands to sparse ones. This is likely to progressin much the same ways as happenedin East Africa (Western & Maitumo, 2004;Birkett & Stevens-Wood, 2005) unless a management action is taken to limit herbivore populations.…”