Veblen K.E.,University of California at Davis |
Veblen K.E.,Mpala Research Center |
Young T.P.,University of California at Davis |
Young T.P.,Mpala Research Center
Journal of Ecology | Year: 2010
1. Through their effects on plant communities, herbivores can exert strong direct and indirect effects on savanna ecosystems and have the potential to create and maintain savanna landscape heterogeneity. Throughout much of sub-Saharan Africa, periodic creation and abandonment of livestock corrals leads to landscape mosaics of long-term ecosystem hotspots that attract both cattle and large ungulate wildlife. 2. The development and maintenance of vegetation in these types of hotspots may be controlled in part by herbivory. Cattle and wildlife may have different, potentially contrasting effects on plant succession and plant-plant interactions. We ask how cattle and wild herbivores affect the maintenance and vegetation development of corral-derived landscape heterogeneity (0.25-1.0 ha treeless 'glades') in Laikipia, Kenya, through their effects on long-term successional and short-term plant-plant dynamics. 3. We used the Kenya Long-term Exclosure Experiment to exclude from glades different combinations of cattle, large ungulate wildlife (i.e. zebras, gazelles and other antelopes), and mega-herbivore wildlife (i.e. giraffes and elephants). We first assessed long-term changes in cover of the dominant grass species, Cynodon plectostachyus and Pennisetum stramineum (the early- and late-dominant species, respectively). We then used a neighbour removal experiment to test the effects of different herbivores on competition and facilitation between the two glade grass species. 4. In the long-term experiment, we found that large ungulate wildlife reinforced landscape heterogeneity over time by helping maintain glades in their early C. plectostachyus-dominated form. Cattle and mega-herbivore wildlife, on the other hand, appeared to reduce the positive effects through forage preference for C. plectostachyus. 5. In the neighbour removal experiment, we found that each grass species benefited from facilitation when it was the preferred forage for the dominant grazer. Facilitation of C. plectostachyus by P. stramineum was strongest when cattle co-occurred with wildlife, whereas facilitation of P. stramineum by C. plectostachyus was strongest when cattle were absent. 6. Synthesis. Our results demonstrate that different combinations of cattle and wildlife have different effects, largely via contrasting forage preferences, on the persistence of landscape heterogeneity in this savanna landscape. More generally, we provide evidence for contrasting effects of cattle and wildlife on short-term plant interactions (facilitation) and successional processes within the herbaceous plant community. © 2010 The Authors. Journal compilation © 2010 British Ecological Society.
Ogada D.L.,Mpala Research Center |
Keesing F.,Bard College
Journal of Raptor Research | Year: 2010
Raptors were monitored monthly over a three-year period in a protected area in central Kenya. The number of raptors declined more than 40 per year. Scavenging birds accounted for most of the decline; sightings decreased by 70 during our surveys, although these declines were not statistically significant. During the time of the study, the overall populations of large wild herbivores showed little change, whereas domestic herbivores, particularly sheep and goats, increased markedly, suggesting that food limitation was not the cause of the vulture declines at the study site. Possible causes of raptor decline include the consumption of poisoned baits, which are placed by pastoralists to kill large predators that attack livestock. Scavenging birds provide one of the most important yet underappreciated ecosystem services of any avian group. The rapid decline of scavenging birds, especially vultures, in central Kenya warrants additional population monitoring to understand whether declines are local or regional, and to elucidate causes of population decreases. © 2010 The Raptor Research Foundation, Inc.
Sensenig R.L.,University of California at Davis |
Sensenig R.L.,Mpala Research Center |
Sensenig R.L.,Goshen College |
Demment M.W.,University of California at Davis |
Laca E.A.,University of California at Davis
Ecology | Year: 2010
The high herbivore diversity in savanna systems has been attributed to the inherent spatial and temporal heterogeneity related to the quantity and quality of food resources. Allometric scaling predicts that smaller-bodied grazers rely on higher quality forage than larger-bodied grazers. We replicated burns at varying scales in an East African savanna and measured visitation by an entire guild of larger grazers ranging in size from hare to elephant. We found a strong negative relationship between burn preference and body mass with foregut fermenters preferring burns to a greater degree than hindgut fermenters. Burns with higher quality forage were preferred more than burns with lower quality forage by small- bodied grazers, while the opposite was true for large-bodied grazers. Our results represent some of the first experimental evidence demonstrating the importance of body size in predicting how large herbivores respond to fire-induced changes in plant quality and quantity. © 2010 by the Ecological Society of America.
Goheen J.R.,University of Wyoming |
Goheen J.R.,University of British Columbia |
Goheen J.R.,Mpala Research Center |
Palmer T.M.,Mpala Research Center |
Palmer T.M.,University of Florida
Current Biology | Year: 2010
Tree cover in savanna ecosystems is usually regarded as unstable, varying with rainfall, fire, and herbivory [1-4]. In sub-Saharan Africa, elephants (Loxodonta africana) suppress tree cover, thereby maintaining landscape heterogeneity by promoting tree-grass coexistence. In the absence of elephants, tree encroachment may convert savannas into closed-canopy woodlands [5, 6]; when elephants increase in abundance, intensified browsing pressure can transform savannas into open grasslands [5-8]. We show that symbiotic ants stabilize tree cover across landscapes in Kenya by protecting a dominant tree from elephants. In feeding trials, elephants avoided plants with ants and did not distinguish between a myrmecophyte (the whistling-thorn tree [Acacia drepanolobium]) from which ants had been removed and a highly palatable, nonmyrmecophytic congener. In field experiments, elephants inflicted severe damage on whistling-thorn trees from which ants had been removed. Across two properties on which elephants increased between 2003 and 2008, cover of whistling-thorn did not change significantly inside versus outside large-scale elephant exclusion fences; over the same period of time, cover of nonmyrmecophytes differed profoundly inside versus outside exclusion fences. These results highlight the powerful role that symbioses and plant defense play in driving tree growth and survival in savannas, ecosystems of global economic and ecological importance. PaperFlick: © 2010 Elsevier Ltd.
Stanton M.L.,University of California at Davis |
Stanton M.L.,Mpala Research Center |
Palmer T.M.,University of California at Davis |
Palmer T.M.,Mpala Research Center |
Palmer T.M.,University of Florida
Ecology | Year: 2011
Three recent meta-analyses of protective plant-ant mutualisms report a surprisingly weak relationship between herbivore protection and measured demographic benefits to ant-plants, suggesting high tolerance for herbivory, substantial costs of antmediated defense, and/or benefits that are realized episodically rather than continuously. Experimental manipulations of protective ant-plant associations typically last for less than a year, yet virtually all specialized myrmecophytes are long-lived perennials for which the costs and benefits of maintaining ant symbionts could accrue at different rates over the host's lifetime. To complement long-term monitoring studies, we experimentally excluded each of four ant symbionts from their long-lived myrmecophyte host trees (Acacia drepanolobium) for 4.5 years. Ant species varied in their effectiveness against herbivores and in their effects on intermediate-term growth and reproduction, but the level of herbivore protection provided was a poor predictor of the net impact they had on host trees. Removal of the three Crematogaster species resulted in cumulative gains in host tree growth and/or reproduction over the course of the experiment, despite the fact that two of those species significantly reduce chronic herbivore damage. In contrast, although T. penzigi is a relatively poor defender, the low cost of maintaining this ant symbiont apparently eliminated negative impacts on overall tree growth and reproduction, resulting in enhanced allocation to new branch growth by the final census. Acacia drepanolobium is evidently highly tolerant of herbivory by insects and small browsers, and the costs of maintaining Crematogaster colonies exceeded the benefits received during the study. No experimental trees were killed by elephants, but elephant damage was uniquely associated with reduced tree growth, and at least one ant species (C. mimosae) strongly deterred elephant browsing. We hypothesize that rare but catastrophic damage by elephants may be more important than chronic herbivory in maintaining the costly myrmecophyte habit in this system. © 2011 by the Ecological Society of America.
Odadi W.O.,Mpala Research Center |
Karachi M.K.,Egerton University |
Abdulrazak S.A.,National Council for Science and Technology |
Young T.P.,Mpala Research Center |
Young T.P.,University of California at Davis
Science | Year: 2011
Savannas worldwide are vital for both socioeconomic and biodiversity values. In these ecosystems, management decisions are based on the perception that wildlife and livestock compete for food, yet there are virtually no experimental data to support this assumption. We examined the effects of wild African ungulates on cattle performance, food intake, and diet quality. Wild ungulates depressed cattle food intake and performance during the dry season (competition) but enhanced cattle diet quality and performance during the wet season (facilitation). These results extend our understanding of the context-dependent-competition-facilitation balance, in general, and are critical for better understanding and managing wildlife-livestock coexistence in human-occupied savanna landscapes.
Kinnaird M.F.,Mpala Research Center |
Kinnaird M.F.,Wildlife Conservation Society |
O'brien T.G.,Mpala Research Center |
O'brien T.G.,Wildlife Conservation Society
Conservation Biology | Year: 2012
Successful conservation of large terrestrial mammals (wildlife) on private lands requires that landowners be empowered to manage wildlife so that benefits outweigh the costs. Laikipia County, Kenya, is predominantly unfenced, and the land uses in the area allow wide-ranging wildlife to move freely between different management systems on private land. We used camera traps to sample large mammals associated with 4 different management systems (rhinoceros sanctuaries, no livestock; conservancies, intermediate stocking level; fenced ranches, high stocking level; and group ranches, high stocking level, no fencing, pastoralist clan ownership) to examine whether management and stocking levels affect wildlife. We deployed cameras at 522 locations across 8 properties from January 2008 through October 2010 and used the photographs taken during this period to estimate richness, occupancy, and relative abundance of species. Species richness was highest in conservancies and sanctuaries and lowest on fenced and group ranches. Occupancy estimates were, on average, 2 and 5 times higher in sanctuaries and conservancies as on fenced and group ranches, respectively. Nineteen species on fenced ranches and 25 species on group ranches were considered uncommon (occupancy < 0.1). The relative abundance of most species was highest or second highest in sanctuaries and conservancies. Lack of rights to manage and utilize wildlife and uncertain land tenure dampen many owners' incentives to tolerate wildlife. We suggest national conservation strategies consider landscape-level approaches to land-use planning that aim to increase conserved areas by providing landowners with incentives to tolerate wildlife. Possible incentives include improving access to ecotourism benefits, forging agreements to maintain wildlife habitat and corridors, resolving land-ownership conflicts, restoring degraded rangelands, expanding opportunities for grazing leases, and allowing direct benefits to landowners through wildlife harvesting. © 2012 Society for Conservation Biology.
Bonachela J.A.,University of Strathclyde |
Pringle R.M.,Princeton University |
Pringle R.M.,Mpala Research Center |
Sheffer E.,Princeton University |
And 7 more authors.
Science | Year: 2015
Self-organized spatial vegetation patterning is widespread and has been described using models of scale-dependent feedback between plants and water on homogeneous substrates. As rainfall decreases, these models yield a characteristic sequence of patterns with increasingly sparse vegetation, followed by sudden collapse to desert. Thus, the final, spot-like pattern may provide early warning for such catastrophic shifts. In many arid ecosystems, however, termite nests impart substrate heterogeneity by altering soil properties, thereby enhancing plant growth.We show that termite-induced heterogeneity interacts with scale-dependent feedbacks to produce vegetation patterns at different spatial grains. Although the coarse-grained patterning resembles that created by scale-dependent feedback alone, it does not indicate imminent desertification. Rather, mound-field landscapes are more robust to aridity, suggesting that termites may help stabilize ecosystems under global change.
Porensky L.M.,University of California at Davis |
Porensky L.M.,Mpala Research Center
Journal of Ecology | Year: 2011
1.Ecological edges (zones separating ecosystems or land cover types) can function as active boundaries, unique habitats and dynamic transition zones. Abiotic factors, species and species interactions exhibit strong responses to edges, and these responses - edge effects - can profoundly impact ecosystem structure and function. 2.Edge effects may be altered by the presence or proximity of other nearby edges. This phenomenon - edge interaction - is poorly understood, though its importance is increasingly recognized. Edge interactions are likely in fragmented or patchy landscapes that contain many edges. In such landscapes, understanding how nearby edges interact may be critical for effective conservation and management. 3.I examined edge interactions in an East African savanna. In this landscape, abandoned cattle corrals develop into treeless, nutrient-rich 'glades' that persist as preferentially grazed areas for decades to centuries. Glades represent important sources of structural and functional landscape heterogeneity and have major impacts on distributional patterns of plant and animals. 4.I used existing variation in inter-glade distance to investigate the importance and strength of glade edge interactions for plants, Acacia ants and large herbivores. Specifically, I compared response patterns obtained from transects that extended outward from isolated glades (>250m from another glade) and non-isolated glades (<150m from another glade). 5.Edge effect patterns between nearby glades differed significantly from patterns around isolated glades. When compared to areas outside isolated glades, areas between glades had almost twice the density of trees, half as much large herbivore use, reduced cover of glade-dominant grasses, and different Acacia ant communities. Many of the edge effects observed between non-isolated glades could not be inferred from effects around isolated glades. 6. Synthesis. These findings suggest that edge interactions can alter plant and animal distributions in patchy landscapes. Edge effects near multiple edges can be stronger, weaker or qualitatively different from those near isolated edges. Such edge interactions can increase or decrease structural and functional continuity between nearby patches. Appropriate extrapolation of local edge effects in complex and fragmented landscapes will require greater understanding of edge interactions. © 2011 The Author. Journal of Ecology © 2011 British Ecological Society.
Veblen K.E.,University of California at Davis |
Veblen K.E.,Mpala Research Center
Journal of Arid Environments | Year: 2012
In African savannas, abandonment of traditional livestock corrals (bomas) creates long-term mosaics of nutrient hotspots embedded in a lower-nutrient matrix. It is unclear how plant communities develop over time on these sites in clay-rich "black cotton" soils or how herbivores attracted to these sites affect vegetation development. I first examined whether treeless "glades", derived from abandoned bomas, function as nutrient and herbivore hotspots. Soil, vegetation, and herbivore data were collected on glades of varying ages. The results indicated that glades persist as long-term (≥four decades) patches (0.25-1.0 ha) of improved soil texture and increased nutrient levels, palatable grasses, and herbivore use. Glade vegetation also appears to undergo succession from Cynodon plectostachyus to Pennisetum stramineum dominance. Based on these patterns, exclusion cages were used to test herbivore effects on glade vegetation development. I found that large herbivores may retard succession by suppressing invasion of P. stramineum into C. plectostachyus-dominated areas. These results provide evidence that abundant anthropogenic glades function as long-term nutrient and wildlife hotspots in black cotton soils, distinct from similar hotspots in other soil types. The findings provide evidence that large herbivores can exert control over development and persistence of glades through their effects on plant community dynamics. © 2011 Elsevier Ltd.