Current Research at Mpala

Click on the boxes on the map to learn more about projects happening on Mpala.

Ungulate Herbivory Under Rainfall Uncertainty (UHURU)

PI’s: Jake Goheen (U. of British Columbia), Rob Pringle (Stanford U.), Todd Palmer (U. of Florida)

Human activities are causing extinctions to occur at unprecedented rates worldwide. Although we often possess detailed knowledge about their causes, a dearth of information exists regarding the consequences of extinction for ecological systems. Our research team works to understand how extinctions of large herbivores affect ecosystem dynamics of African savannas. The Mpala Ranch and associated environs in Laikipia, Kenya comprise the ideal system in which to study the ramifications of such extinctions for ecosystems. Laikipia consists of a 10,000 km2 mosaic of private properties, where each property is associated with one of four scenarios reflecting the progressive, size-biased extinction typical of animal communities: 1) megaherbivores (i.e., elephant and giraffe; >900 kg) + impala (ca. 60 kg) + dik-dik (ca. 6 kg); 2) impala + dik-dik; 3) dik-dik only; and 4) no large mammalian herbivores. Using a series of semi-permeable, electrified fences dubbed UHURU (Ungulate Herbivory Under Rainfall Uncertainty), our team is simulating the above extinction scenarios to better understand how large mammalian herbivores influence plant, insect, and small mammal populations, as well as soil chemistry and rates of nutrient cycling.

Through UHURU, we are the first to simulate size-biased extinctions in a large-scale, long-term experimental framework. Our experiment consists of 36 1-ha plots arranged along a marked gradient in precipitation, permitting an unprecedented chance to disentangle complex interactions among extinction and variation in rainfall. Specifically, we are working to answer the following questions: 1) how do extinctions of large mammalian herbivores change population dynamics of members of the genus Acacia, trees of fundamental importance in driving patterns of biodiversity?; 2) how important are large mammalian herbivores relative to rainfall in controlling ecosystem dynamics?; and 3) can results from UHURU be used to generate robust predictions about landscape change (i.e., tree encroachment) in African savannas under different scenarios of herbivore extinction and climate change? Our team’s research will contribute significantly to the conservation and management efforts, as well as to our understanding of how, when, and where extinction is manifested in ecosystems.

Recent Activities:

• Plant Survey Protocol with trees
• ANPP & consumption
• Grass and Herb Community: % cover, species composition, and approximated relative biomass
• dung surveys
• small mammal surveys
• soil sampling
• Ongoing maintenance of UHURU exclosure experiment
• Insect sampling for stable isotope analyses in Corinna Riginos's tree-clearing/thinning experiment (in collaboration with Corinna Riginos and Kena Fox-Dobbs)
• Continuing efforts to understand why termite mounds are overdispersed in the black cotton (in collaboration with Alison Brody, others)

Recent Findings:

From Field, and Recent Publications: The establishment of this experiment was finished in September 2008. As such, our response variables (understory abundance, tree cover, small mammals, etc.) have not yet responded to our manipulations. But an exciting recent finding from the black cotton is that termite mounds enhance the densities of arboreal insects, spiders, and lizards, both locally (because mounds are highly productive) and at the landscape scale (because mounds are overdispersed). This project is in collaboration with Dan Doak, Alison Brody, and Todd Palmer.

Recent Publications:

Palmer, TM, ML Stanton, TP Young, JR Goheen, RM Pringle, and R Karban. 2008. Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna. Science 319:192-195.

Pringle, RM. 2008. Elephants as agents of habitat creation for small vertebrates at the patch scale. Ecology 89:26-33. Palmer, TM, ML Stanton, TP Young, JR Goheen, RM Pringle, and R Karban. 2008. Putting ant-acacia mutualisms to the fire. Science 319: 1760-1761.

Pringle, RM and K Fox-Dobbs. 2008. Coupling of canopy and understory food webs by ground-dwelling predators. Ecology Letters 11:1328-1337.

The evolution of sociality in African starlings

PI: Dustin Rubenstein

Our research on cooperative breeding in African starlings examines the causes and consequences of vertebrate sociality. We conduct intensive behavioral, endocrine, immunological, and molecular studies using field observations, field manipulations, and captive experiments. We have shown that environmental variability drives much of the social behavior, physiology, and life history in superb starlings; temporal variability in rainfall influences reproductive conflict, stress hormones, immune function, breeding roles, and even offspring sex ratio. We have also been using game theory modeling and empirical data to tease apart the relative importance of social and environmental stressors in influencing conflict and reproductive roles in superb starlings and other social vertebrates. For the past few years, we have been studying two additional species of co-occurring starlings from the same genus that have different social systems (simple cooperative and non-cooperative) and life history traits. Despite many similarities among these sister species, they have dramatic differences in their social behavior and reproductive life histories. Finally, we examine the causes and consequences of sociality in a comparative framework using the entire group of African starlings (Sturnidae). We have sampled dozens of species of starlings from across Kenya, and built a molecular phylogeny for the entire family. We have used this evolutionary tree to study how environmental variability influences the evolution of complex social systems. We have shown that cooperatively breeding species tend to be found in temporally variable environments where rainfall patterns are unpredictable. We are also examining the roles of natural and sexual selection on behavioral, morphological, and physiological variation in social and non-social starling species.

Recent Activities:

• Habitat quality surveys
• Telemetry studies of mating system
• Molecular studies of kinship

Recent Findings from the Field:

1. Temporal variability in rainfall influences reproductive conflict, stress hormones, immune function, breeding roles, and even offspring sex ratio
2. We have shown that cooperatively breeding species tend to be found in temporally variable environments where rainfall patterns are unpredictable.
3. Although extrapair fertilizations are relatively rare in superb starlings, females seek out extrapair males for both direct and indirect benefits.

Recent Publications:

Rubenstein, D.R. and S.-F. Shen. 2009. Reproductive conflict and the costs of social status in cooperatively breeding vertebrates. The American Naturalist 173:650-661.

Rubenstein, D.R., A.F. Parlow, C.R. Hutch, and L.B. Martin. 2008. Environmental and hormonal correlates of immune activity in a cooperatively breeding tropical bird. General and Comparative Endocrinology 159:10-15.

Lovette, I.J., B.V. McCleery, A.L. Talba, and D.R. Rubenstein. 2008. A complete species-level molecular phylogeny for the “Eurasian” starlings (Sturnidae: Sturnus, Acridotheres, and allies): recent diversification in a highly social and dispersive avian group. Molecular Phylogenetics and Evolution 47:251-260.

Martin, L.B. and D.R. Rubenstein. 2008. Stress hormones in tropical birds: patterns and future directions. Ornitologia Neotropical 19 (Suppl.):207-218. (Invited Review)

Rubenstein, D.R. and I.J. Lovette. 2007. Temporal environmental variability drives the evolution of cooperative breeding in birds. Current Biology 17:1414-1419.

Rubenstein, D.R. 2007. Territory quality drives intraspecific patterns in extrapair paternity. Behavioral Ecology 18:1058-1064.

Rubenstein, D.R. 2007. Female extrapair mate choice in a cooperative breeder: trading sex for help and increasing offspring heterozygosity. Proceedings of the Royal Society of London B 274:1895-1903.

Rubenstein, D.R. 2007. Temporal but not spatial environmental variation drives adaptive offspring sex allocation in a plural cooperative breeder. The American Naturalist 170:155-165.

Lovette, I.J. and D.R. Rubenstein. 2007. A comprehensive molecular phylogeny of the starlings (Aves: Sturnidae) and mockingbirds (Aves: Mimidae): congruent mtDNA and nuclear trees for a cosmopolitan avian radiation. Molecular Phylogenetics and Evolution 44:1031-1056.

Sachs, J.L. and D.R. Rubenstein. 2007. The evolution of cooperative breeding; is there cheating? Behavioural Processes 76:131-137. (Invited Commentary)

Rubenstein, D.R. 2007. Stress hormones and sociality: integrating social and environmental stressors. Proceedings of the Royal Society of London B 274:967-975.

Interactions among keystone species: effects of termites and ungulates on biodiversity in East African savannas.

PI: Alison K. Brody (U. of Vermont)
Co-PI’s: Dan F. Doak (U. of Washington, Seattle) and Todd M. Palmer (U. of Florida)

Understanding what governs the distribution, diversity and abundance of species is a central theme of ecology. Such an understanding is of paramount importance as habitats are lost, fragmented, and modified through agricultural use. We are examining the interactions between termites and large vertebrates on biodiversity in East African savannas. Termites have been identified as both keystone species and ecosystem engineers for a variety of systems. In East Africa, their mounds are a common feature of the landscape where they enrich soils and may be critical to plant and invertebrate diversity and abundance. Vertebrates have also been implicated as keystone herbivores in savannas. Through observations, and experiments, we are testing three hypotheses: I) Termite mound building leads to patterns of density and diversity in plant, invertebrate and vertebrate communities by creating a mosaic of soil fertility and soil structure. At the landscape level, these islands of fertility enhance biodiversity. II) High nutrient levels of termite mound soils are maintained and/or enhanced by preferential use of mounds by large herbivores that deposit nutrients in dung and urine. III) Heavy cattle grazing will reduce termite populations and result in a net exportation of nutrients from mounds. These negative effects, in turn, cascade throughout the plant and invertebrate communities to ultimately impact the diversity of savanna ecosystems. Our work is providing novel data on the interactions between termites and vertebrates in providing habitat and resources that structure plant and animal communities. These data are critical to preserving biodiversity and managing savanna ecosystems.

Recent Activities:

• Ongoing censuses of plants and invertebrates across 40 mounds. Half of these mounds are fenced to exclude large vertebrate herbivores.
• Sampling of soils on and off mounds: analysis of static and dynamic nutrients.
• Grazing and dung transects to establish patterns of herbivore use on and off termite mounds.

Recent Findings:

• Proximity to termite-enriched soils is positively correlated with growth and reproduction of Acacia drepanolobium trees.
• A. drepanolobium trees fix less atmospheric nitrogen when growing close to mounds than when further away.
• Invertebrate density is higher on termite mounds, however directly indices are higher from off-mound collections.

Recent Publications:

Brody, A. K., T. Palmer, K. Fox-Dobbs, & D. Doak. 2009. Termites, vertebrate herbivores, and the fruiting success of Acacia drepanolobium. Ecology, in press.

Fox-Dobbs, K., D. F. Doak, A. K. Brody and T. M. Palmer. 2009. Termites create spatial structure and govern ecosystem function in an East African savannah. Ecology, in review.