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The ecology of a truncated ecosystem. The Athi-Kapiti plains.

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posted on 19.11.2015, 08:52 by Helen Wanjiru. Gichohi
Many ecosystems in Kenya are undergoing severe alteration due to landuse changes and associated human impacts. In one such large ecosystem, the Athi-Kapiti plains to the south of the city of Nairobi (and including Nairobi National Park), relatively healthy populations and migrations of large herbivores have been maintained despite increasing concentrations of domestic stock, settled agriculture amongst former pastoralists and increasing industrialization. This study measured the factors that control the vegetation structure and the dynamics of the large herbivore communities. The effects of rainfall on vegetation quality, quantity and species composition was measured inside and outside exclosures, so that the effects of large herbivore grazing could be isolated. The distribution and movement patterns were established by aerial, ground and dung counts. Interactions between xi le herbivores and vegetation were studied by measuring patterns of vegetation use, species composition, chemical analyses for vegetation quality and dung quality. The results show that the quality of vegetation and its biomass reflect the gradient of total rainfall across the Athi-Kapiti plains. Biomass accumulation rates in the exclosures were directly proportional to rainfall. Vegetation biomass and quality changed seasonally and with this, the distribution and utilization patterns by large herbivores. In general, vegetation quality and biomass were highest during the wet months and lowest during the dry regardless of grazing intensity. However, during both seasons, the heavily grazed areas had the highest quality vegetation. The plains in the south experienced heavy grazing in the wet season compared to Nairobi National Park. The impact of heavy grazing in the wet season was to increase plant diversity and produce shorter, more prostrate growth forms of plants. Wildebeest and zebra migrated down the rainfall gradient from Nairobi National Park to the plains in the wet season and up the gradient on their return to the park in the dry season. Small ungulates tended to remain on the plains during the dry season while the large bulk feeders (e.g buffalo) remained in the park. The vegetation selection patterns shown by these herbivores were distinct: large grazers were negatively correlated with quality in contrast to small herbivores, which were strongly positively correlated. Medium sized grazers showed a positive relationship with vegetation quality but in the wet season only. Variation in dung protein between the various species also illustrated the differences in the quality of diet selected by the large herbivores in the ecosystem. Browser dung had the highest crude-protein content followed by small, medium and large grazers in that order. The data revealed significant declines in dung protein with increasing body weight. Zebra, the only non-ruminant in the group, was the only exception to the quality body weight trends. The conclusions drawn from the study show that despite being a highly altered ecosystem the large herbivore movements and diet selectivity patterns still follow rainfall and food quality pulses. This information can be used to make recommendations for the conservation of parts of the ecosystem which are critical to the migrations and the continued survival of this important wildlife population.


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College of Medicine, Biological Sciences and Psychology

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University of Leicester

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