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Increasingly, the restoration of large carnivores is proposed as a means through which to restore community structure and ecosystem function via trophic cascades. After a decades-long absence, African wild dogs (Lycaon pictus) recolonized the Laikipia Plateau in central Kenya, which we hypothesized would trigger a trophic cascade via suppression of their primary prey (dik-dik, Madoqua guentheri ) and the subsequent relaxation of browsing pressure on trees. We tested the trophic-cascade hypothesis using (1) a 14-year time series of wild dog abundance; (2) surveys of dik-dik population densities conducted before and after wild dog recovery; and (3) two separate, replicated, herbivore-exclusion experiments initiated before and after wild dog recovery. The dik-dik population declined by 33% following wild dog recovery, which is best explained by wild dog predation. Dik-dik browsing suppressed tree abundance, but the strength of suppression did not differ between before and after wild dog recovery. Despite strong, top-down limitation between adjacent trophic levels (carnivore- herbivore and herbivore-plant), a trophic cascade did not occur, possibly because of a time lag in indirect effects, variation in rainfall, and foraging by herbivores other than dik-dik. Our ability to reject the trophic-cascade hypothesis required two important approaches: (1) temporally replicated herbivore exclusions, separately established before and after wild dog recovery; and (2) evaluating multiple drivers of variation in the abundance of dik-dik and trees. While the restoration of large carnivores is often a conservation priority, our results suggest that indirect effects are mediated by ecological context, and that trophic cascades are not a foregone conclusion of such recoveries.

The majority of research on savanna vegetation dynamics has focused on the coexistence of woody and herbaceous vegetation. Interactions among woody plants in savannas are relatively poorly understood. We present data from a 10-year longitudinal study of spatially explicit growth patterns of woody vegetation in an East African savanna following exclusion of large herbivores and in the absence of fire. We examined plant spatial patterns and quantified the degree of competition among woody individuals. Woody plants in this semi-arid savanna exhibit strongly clumped spatial distributions at scales of 1 – 5 m. However, analysis of woody plant growth rates relative to their conspecific and heterospecific neighbors revealed evidence for strong competitive interactions at neighborhood scales of up to 5 m for most woody plant species. Thus, woody plants were aggregated in clumps despite significantly decreased growth rates in close proximity to neighbors, indicating that the spatial distribution of woody plants in this region depends on dispersal and establishment processes rather than on competitive, density-dependent mortality. However, our documentation of suppressive effects of woody plants on neighbors also suggests a potentially important role for tree-tree competition in controlling vegetation structure and indicates that the balanced-competition hypothesis may contribute to well-known patterns in maximum tree cover across rainfall gradients in Africa. This article is protected by copyright. All rights reserved.

Ecosystem-level studies of producer–decomposer interactions have focused primarily on plant production and soil texture as regulators of decomposer abundance but have rarely considered the role of grazers in mediating such interactions. Here, we conducted replicated exclosure experiments at both high and low levels of soil fertility to investigate the effects of large, mammalian grazers on decomposer biomass and activity patterns in a semiarid grazing ecosystem in Kenya. Within only two years of grazer exclusion, microbial biomass was greater in soil of fenced grassland across all levels of soil fertility. This consistent negative effect of grazers on microbial biomass occurred despite the fact that grazers stimulated aboveground plant production in nutrient-rich sites and depressed it in nutrient-poor sites. A consideration of all the potential pathways by which grazers influence decomposer populations suggests that observed grazing-induced reductions in microbial biomass were predominantly associated with a depression in the amount of plant carbon inputs to soils. Finally, across all study sites, microbial biomass was highly correlated with soil carbon content, suggesting that landscape-scale constraints on soil organic matter content and plant production overarch grazer effects on microbial abundance. Our results support previous ecosystem-level studies showing that microbial biomass and growth are constrained by plant production and soil C availability. In addition, our findings demonstrate that decomposer abundance can be influenced by an ecosystem's trophic structure, with significant reductions in microbial biomass occurring as a result of herbivores diverting plant carbon away from soils.

Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts and more intense rainfall events predicted with climate change. The early 21st-century drought in the southwestern United States resulted in hydroclimatic conditions that are similar to those expected with future climate change. We investigated the impact of the early 21st-century drought on aboveground net primary production (ANPP) of six desert and plains grasslands dominated by C4 (warm season) grasses in terms of significant deviations between observed and expected ANPP. In desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (PT), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in plains grasslands exhibited a strong linear function of the current-year PT and the previous-year ANPP, despite prolonged warm drought. We used these results to disentangle the impacts of interannual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus generalize the functional response of C4 grasslands to predicted climate change. This will allow managers to plan for predictable shifts in resources associated with climate change related to fire risk, loss of forage, and ecosystem services. © 2014 by the Ecological Society of America.

Large herbivores are known to respond to spatial heterogeneity in the distribution of forage resources, but how important herbivores are in creating those spatial patterns, how their influence may be scale dependent, and how herbivore-induced patterns affect ecosystem processes remain unclear. We examined the effects of native migratory ungulates on the spatial distribution of soil nitrogen and N-mineralization potential at scales ranging from 0.1–30 m in grasslands of Yellowstone National Park using geostatistical analyses of soils collected inside and outside long-term (36+-yr) exclosures. At small spatial scales (0.1–2 m), ungrazed grasslands showed a high degree of patchiness in the distribution of soil N and N-mineralization potential (94% and 77% of sample variation explained by small-scale patchiness, respectively). For both variables, patches occurred at a consistent mean size of ∼40 cm. In contrast, grazed grassland exhibited minimal spatial structure in the distribution of soil N and N-mineralization rates (