Piecewise Disassembly of a Large-Herbivore Community across a Rainfall Gradient: The UHURU Experiment

Abstract

Large mammalian herbivores (LMH) strongly influence plant communities, and these effects can propagate indirectly throughout food webs. Most existing large-scale manipulations of LMH presence/absence consist of a single exclusion treatment, and few are replicated across environmental gradients. Thus, important questions remain about the functional roles of different LMH, and how these roles depend on abiotic context. In September 2008, we constructed a series of 1-ha herbivore-exclusion plots across a 20-km rainfall gradient in central Kenya. Dubbed "UHURU" (Ungulate Herbivory Under Rainfall Uncertainty), this experiment aims to illuminate the ecological effects of three size classes of LMH, and how rainfall regimes shape the direction and magnitude of these effects. UHURU consists of four treatments: total-exclusion (all ungulate herbivores), mesoherbivore-exclusion (LMH >120-cm tall), megaherbivore-exclusion (elephants and giraffes), and unfenced open plots. Each treatment is replicated three times at three locations (“sites”) along the rainfall gradient: low (440 mm/year), intermediate (580 mm/year), and high (640 mm/year). There was limited variation across sites in soil attributes and LMH activity levels. Understory-plant cover was greater in plots without mesoherbivores, but did not respond strongly to the exclusion of megaherbivores, or to the additional exclusion of dik-dik and warthog. Eleven of the thirteen understory plant species that responded significantly to exclusion treatment were more common in exclusion plots than open ones. Significant interactions between site and treatment on plant communities, although uncommon, suggested that differences between treatments may be greater at sites with lower rainfall. Browsers reduced densities of several common overstory species, along with growth rates of the three dominant Acacia species. Small-mammal densities were 2–3 times greater in total-exclusion than in open plots at all sites. Although we expect patterns to become clearer with time, results from 2008–2012 show that the effects of excluding successively smaller-bodied subsets of the LMH community are generally non-additive for a given response variable, and inconsistent across response variables, indicating that the different LMH size classes are not functionally redundant. Several response variables showed significant treatment-by-site interactions, suggesting that the nature of plant-herbivore interactions can vary across restricted spatial scales.

Declines in large wildlife increase landscape-level prevalence of rodent-borne disease in Africa

“…More generally, however, given the ubiquity of large wildlife loss; the pervasive and well-substantiated observations of rodent increases following such defaunation globally (35,58,63,64); and the fact that rodents are one of the most frequent hosts of zoonotic diseases (65), particularly flea-borne diseases (58), these results suggest that size-selective animal loss may have a major impact on global risk of rodent-borne diseases. This study also adds to a growing body of evidence from both the Lyme and hantavirus systems that pathogen regulation through susceptible host regulation of rodent-borne diseases may be equally or more important than transmission interference in some cases (31,38).…”

Mechanisms of plant–plant interactions: concealment from herbivores is more important than abiotic-stress mediation in an African savannah

“…Each treatment is replicated three times at either end of a 22-km rainfall gradient, making three 'blocks' of total exclosure/control pairs at each end. From 2009 through 2011, mean annual precipitation increased more than 45% from the North (arid) to the South (mesic) site (440 mm yr 21 in arid, 640 mm yr 21 in mesic, [21]). We conducted our experiment between June 2011 and August 2012, during a comparatively wet year (1034 mm yr 21 in mesic and 757 mm yr 21 in arid).…”

Seasonal patterns in decomposition and nutrient release from East African savanna grasses grown under contrasting nutrient conditions

“…The soils are red sandy loams (Typic Haplustalfs in Soil Taxonomy) derived from metamorphic basement rock (Ahn and Geiger, 1987;Goheen et al, 2013). Prior to fertilization, soil pH was 6.3 (measured in water), total soil P was 230 mg kg −1 and available P was 11 mg kg −1 (determined by resin bags as outlined by Kouno et al (1995)).…”

“…Total soil C and N contents were 10 and 1.1 g kg −1 , respectively and the mean annual rainfall was approximately 640 mm over a 13 years period (Fig. 1a) (Goheen et al, 2013). Monthly maximum temperatures range from 25 to 33 • C, while minimum temperatures range from 12 to 17 • C (Young et al, 1998).…”

“…This study was conducted in the Ungulate Herbivory Under Rainfall Uncertainty (UHURU) experiment, established in 2008 at the Mpala Research Centre (Goheen et al, 2013). The study was conducted in the southern (wettest) site of the UHURU experiment, which received an average of 638 mm rain year −1 from 2009 to 2011.…”