Okavango Delta Management Plan - Ramsar Convention on Wetlands

15000 in 1988 and 35000 in 2002 (Bonyongo, 2004). Similarly, the numbers of two other
large herbivores, Hippopotamus and Buffalo, have increased remarkably during the last
fifteen years, while most small and medium sized herbivores have declined (op. cit.). The
abundance of elephants is now so high that they may constitute a threat to woodlands,
especially when considering that about 25% of the <strong>Delta</strong> is permanently wet and not used
by elephants. The Hippopotamus numbers given at about 2500 are likely to be a minimum,
due to the problems in aerial counting of these often submerged animals. For similar
reasons the numbers based on aerial counts given in Table 2-5 for Sitatunga and Kudu, are
likely to be under-estimated.
The total mammal biomass for the Moremi Game Reserve (7000 km 2) in the <strong>Okavango</strong> <strong>Delta</strong>
has been estimated as being 12 000 kg/km 2 (Bonyongo, 2004), which is much higher than
for most wildlife areas in southern Africa and comparable with the rich savannas in the
East African Rift valley. The regression models between rainfall (Coe, Cumming and
Phillipson, 1976) and rainfall + nutrient level (East, 1984) estimates the <strong>Okavango</strong>
mammalian biomass to be 4-8 times lower than what direct calculations generated for a
system like the <strong>Okavango</strong> <strong>Delta</strong>. The high biomass densities of the <strong>Okavango</strong> <strong>Delta</strong>
compared to the rich savannas is attributed to high numbers of elephants and buffalo who
jointly contribute 73% of the total mammalian biomass in <strong>Delta</strong> (Bonyongo 2004). The two
moisture regimes, from the annual rainfall and the annual seasonal flooding results in two
distinct growing seasons. This results to high productivity of foraging resources enabling
the <strong>Delta</strong> to support high numbers of mammalian herbivores.
Table 2-5: Common species and numbers
Species Total number
Elephant, Loxodonta africana 35 000
Zebra, Equus burchelli 14 000
Warthog, Phacochoerus aethiopicus 2 000
Hippopotamus, Hippopotamus amphibius 2 500
Giraffe, Giraffa camelopardalis 5000
Wildebeest, Connochaetes taurinus 8 000
Tsessebe, Damaliscus lunatus 3 000
Impala, Aepyceros melampus 140 000
Buffalo, Syncerus caffer 60 000
Kudu, Tragelaphus strepsiceros 300
Sitatunga, Tragelaphus spekei 500
Red Lechwe, Kobus leche 60 000
Number of common large mammals in the <strong>Okavango</strong> <strong>Delta</strong> in 2002, calculated for an area of 20 000 km 2 , based on 10 aerial counts done
1988 – 2002 by the Department of Wildlife and National Parks, Government of Botswana (from Bonyongo, 2004). For Impala the numbers
have been corrected based on ground counts
The extended productive period caused by the annual flood is certainly one of the causes
for this. On the other hand the generally low nutrient levels in the <strong>Delta</strong> should limit
biological production (op. cit), but the dynamic vegetation successions caused by flooding
with periodically and locally high mobilization of nutrients may cause high nutrient levels
for forbs and hence favourable production conditions for grazing mammals.
Habitat assemblages
All the common species mentioned above, except the Buffalo, depend on more than one
habitat. For instance, the Impala inhabits floodplains and grasslands adjacent to riparian
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