Compressed nature : co-existing grazers in a small reserve in Kenya

Wildlife habitats in Kenya are getting more fragmented and isolated due to increasing human activities within them. This has resulted in the establishment of several small nature reserves where wildlife is protected from human interference. Grazers contribute a large proportion of total herbivore biomass in these reserves, and their populations are likely to increase due to stoppage of migration and reduction in their home range sizes (for large home range holders) caused by fencing, human settlements or cultivation on the periphery of the reserves. This might lead to changes in dynamics of grass-grazer or grazer-grazer interactions, which are useful to understand for successful management of these populations to take place. It was in view of this, that I conducted a study with the following hypotheses: that a decline in a grazer species biomass in an isolated small reserve is due to an increase in biomass of other co-existing grazers, and that there is a high resource overlap among co-existing grazers in an isolated small reserve. I considered an isolated grazer assemblage comprising of ten co-existing grazers: defassa waterbuck ( Kobus defassa ), African buffalo ( Syncerus caffer ), impala ( Aepyceros melampus ) , Grant's gazelle ( Gazella granti ), Thomson's gazelle ( Gazella thomsoni ), warthog ( Phacocoerus aethiopicus ), Burchell's zebra ( Equus burchelli ), eland ( Taurotragus oryx ), Chanler's reedbuck ( Redunca fulvorufula chanleri ), and Bohor reedbuck ( Redunca redunca ) in Lake Nakuru National Park. An analysis of the development of the assemblage over a 24-year period (1976-1999) showed that by 1999, its total biomass had reached 134 kg ha ^-1and that it was dominated by species above 300 kg (buffalo and eland). Population growth rate of waterbuck and warthog has declined, and Bohor reedbuck appears to have now disappeared from the system. The study shows that larger grazer species did not facilitate smaller ones despite having high habitat overlaps with them during the wet season when grass regrowth after cropping is possible. There were also no indications of habitat segregation among grazers, but competition was apparent through directional habitat use overlap indices. Large similarities in diet composition were found between grazers across all seasons. Niche breadth for diet was smallest during the dry season, and combined (diet + habitat use) overlap was high between all pairs of grazers during all seasons indicating that there is a high possibility of competitive interactions among grazers in Nakuru. However, despite utilising similar feeding sites, competitive interactions between impala and zebra might have been reduced by their foraging strategy where they feed at different plant structure (leaf/stem/flowering stalk) levels. In conclusion, it appears that the structuring of this assemblage is most likely due to competitive interactions among the grazers, which might increase in intensity during years of average or below average rainfall.