| Feedbacks between soil nutrients, plant communities, and large mammalian herbivores were studied at the Mpala Ranch and Research Centre in Laikipia, Kenya. The landscape consists of short-grass glades, typically 0.5–1.0 ha in size, dispersed throughout the dominant Acacia bushland vegetation. I examined (1) whether glades are created through the use and abandonment of overnight cattle corrals or ‘bomas’, (2) the importance of these glades as habitat for native and domestic ungulates, (3) feedback effects of herbivores on aboveground herbaceous productivity (ANPP) and soil nutrient dynamics, and (4) effects of climatic variability on herbivore abundance and soil-grass-grazer interactions.; All soil characteristics measured across a boma-glade chronosequence indicate glades are indeed derived from abandoned bomas. In particular, soil N, P and organic matter quality in the surface (0–15 cm) layer were similar for glades and 30–39 year old bomas, but were enriched relative to surrounding bushland. Soil texture was similar for bomas, glades, and bushland, indicating glades were not derived from a unique parent material. Cynodon leaves from bomas and glades were highly enriched in P, Ca and N relative to Cynodon from nearby bushland sites. Local abundance of impala, zebra and eland was closely tied to the distribution of nutrient-rich glades. Seasonal analyses of impala habitat selection suggested that selection for glade habitat was related both to predation risk and the availability of mineral-rich forage. In particular, P in boma and glade grass was above recommended levels for growing and lactating ruminants, while P content of bushland grass was lower than recommended levels.; Across a soil nutrient gradient, large herbivore consumption rates were linearly related to ANPP. A fertilization experiment and analyses of grass N:P ratios indicate that N and P co-limit productivity on the most nutrient-poor sites. Grazing pressure was consistently high (>60% of ANPP) at all but one site in a dry year (1999), and was greater in nutrient-rich glades (73.0 ± 4.2% of ANPP) compared to nutrient-poor bushland sites (42.7 ± 6.7% of ANPP) in a wet year (2001). Nitrogen budgets constructed for nutrient-rich and nutrient-poor sites showed that large herbivores themselves caused a net N input to the former and a net N loss from the latter. During short (1 month) growing seasons, grazers reduced aboveground productivity regardless of soil nutrient availability. (Abstract shortened by UMI.)... |
