Feedbacks Between Soil Nutrients and Large Herbivores in a Managed Savanna Ecosystem

Abstract

Small-scale fertilization experiments have shown that soil nutrients limit plant productivity in many semiarid grasslands and savannas, but linkages among nutrients, grasses, and grazers are rarely studied in an ecosystem context. We used hectare-scale heterogeneity in soil nutrients created by cattle management practices within a geologically homogeneous savanna to examine relationships among soil nitrogen and phosphorus, aboveground net primary production (ANPP), grass nutrient content, and a mixed community of native and domestic herbivores on central Kenyan rangeland. Increasing soil N and P content was consistently associated with increasing plant productivity and rainfall use efficiency in wet, dry, and drought years. A fertilization experiment and analyses of grass N:P ratios across sites indicated that N is the primary limiting nutrient on nutrient-rich glades, whereas N and P co-limit productivity on nutrient-poor bushland sites. Variation in ANPP among patches within the landscape was linearly correlated with consumption rates of large herbivores. Grazing pressure was consistently high (Ͼ60% of ANPP) at all but one site in a dry year (1999), and was greater in nutrient-rich glades (73 Ϯ 4% of ANPP) than in nutrientpoor bushland sites (43 Ϯ 7% of ANPP) in a wet year (2001). Grasses of nutrient-rich sites contained sufficient P concentrations to meet requirements for pregnant and lactating ungulates, whereas grasses in nutrient-poor swards were P deficient. Even though native and domestic herbivores selectively used and intensively grazed nutrient-rich sites, productivity on these sites remained high throughout the study. Analyses of nitrogen budgets for nutrientrich 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. Thus, large herbivores not only respond to heterogeneity in soil and plant nutrients across the landscape, but also play a role in maintaining the N-enriched status of highly productive and intensively grazed sites.

Pathways for Positive Cattle–Wildlife Interactions in Semiarid Rangelands

“…First, nutrient-enriched patches created by abandoned cattle bomas can persist for decades to centuries and provide a key source of nutrient-rich forage for both native and domestic grazers. Nutrient-rich glades that develop from abandoned bomas in the red sands habitat not only sustain local impala abundance but are also maintained as glades by nitrogen inputs from impala (Augustine et al, 2003). Such feedbacks and interactions among cattle bomas, soil and plant nutrients, and wild ungulates indicate that ranch managers can influence the long-term distribution and abundance of wild ungulates though the placement and rotation of current bomas.…”

“…Soil nitrogen can be lost from glades over time via several pathways, including leaching, denitrification, and volatilization, such that long-term glade persistence likely depends upon nitrogen inputs to replenish these losses. Detailed analysis of glade nitrogen budgets at MRC showed that during dry seasons, impala bed within glades (Figure 6) while foraging in adjacent bushland, causing a substantial net input of nitrogen via dung and urine deposition (Augustine et al, 2003). This N input may facilitate long-term glade persistence in the Cynodon-dominated state and represents a pathway by which impala benefit all ungulate grazers that make use of glades, including livestock.…”

“…Long-term persistence of glades may also be contingent on use by wild ungulates. In bushlands on red soils, where shrub-free glades attract herbivores year-round, dung and urine inputs from native ungulates such as impala can prevent long-term nitrogen loss from glades and hence can maintain them in a shortgrass, Cynodon-dominated state (Augustine et al, 2003). In purely grassland habitats, soil and plant nitrogen in glades may decline, and Pennisetum cover may increase over time.…”

Herbivore-initiated interaction cascades and their modulation by productivity in an African savanna

“…Herbaceous species were clearly edible to arthropods in highproductivity blocks, as shown by the increase in arthropod abundance with increasing herbaceous productivity and percent cover. Moreover, there is a well established relationship between primary productivity and ungulate consumption rates in rangelands (29), which argues against the hypothesis that plants overall were less palatable in the high-productivity sites. However, we cannot conclusively rule out the latter scenario for the direct effect of ungulates on tree density (and hence for the treedensity-mediated component of the indirect effect of ungulates on lizards).…”

Plant coexistence depends on ecosystem nutrient cycles: Extension of the resource-ratio theory

“…ref. 28, dissolved organic nitrogen pump) or net nitrogen transfer by large herbivores from nutrient-poor to nutrient-rich grasslands in the Serengeti (30). Abiotic factors can also induce plantunavailable paths of nutrient loss.…”