Megaherbivory, plant defense, and landscape change in savanna ecosystems
Elephants and other ungulates are important agents of tree mortality in savannas, where browsing maintains tree-grass codominance. However, plant defenses--thorns, spines, chemicals, and ants--may protect trees from browsing and thereby shape tree population dynamics.
We combined time series of satellite images, free-choice feeding experiments, and long-term monitoring of tree growth to show that abundances of a myrmecophyte (Acacia drepanolobium) are decoupled from variation in elephant populations over a six-year period. Other acacia that are not protected by ant symbionts decline following increases in elephant numbers through time. So, tiny (3 mg) bodyguards are effectively protecting trees against 3,000,000,000 mg elephants.
Over the past two decades, the spread of an invasive ant (Pheidole megacephala) has disrupted the ant-acacia mutualism, with cascading effects for predator-prey dynamics. Currently, we are quantifying how quickly and faithfully the ant-acacia mutualism can be restored following targeted removal of big-headed ants.
Current and recent collaborators: Brandon Hays, Douglas Kamaru, Nelly Maiyo, Pat Milligan, Juliet Muteka, Todd Palmer, Alejandro Pietrek, Corinna Riginos
Representative papers: Goheen and Palmer 2010, Hays et al 2022, Kamaru et al 2024, Milligan et al 2026, Palmer et al 2020, Pietrek et al 2021
Outreach: DarkHorse Podcast, Science Friday, TED-Ed
Ungulate Herbivory Under Rainfall Uncertainty: the UHURU Experiment
Interaction webs are full of contingencies that affect how communities and ecosystems respond to the loss of species. Established in 2008 with colleagues Rob Pringle and Todd Palmer, the Ungulate Herbivory Under Rainfall Uncertainty (UHURU; the Kiswahili word for "freedom") experiment is unique both in size and in scope. Using a series of 36 1-ha plots situated across a pronounced rainfall gradient, UHURU differentially excludes or permits access by various combinations of wild herbivores to mimic extinction and climate change scenarios in East Africa.
Through UHURU, we are working on a number of topics in community ecology, including functional redundancy among mammalian grazers, effects of extinction on ecosystem processes (e.g., nutrient cycling, soil-water infiltration), and the extent to which herbivory can stabilize productivity under different levels of anthropogenic disturbance.
Current and recent collaborators: Bianca Brown, Tyler Coverdale, Finote Gijsman, Leo Khasoha, Todd Palmer, Rob Pringle, Courtney Reed
Representative papers: Brown et al 2023, Coverdale et al 2021, Gijsman et al 2026, Goheen et al 2013, Goheen et al 2018, Pringle et al 2014, Reed et al 2026
Outreach: Mongabay
Field Tests of Macroecological Patterns: Abundance-Occupancy Relationships
Abundance-occupancy relationships are among the most general patterns in community ecology, with species tending to be locally abundant and widespread, or rare with restricted distributions. The mechanisms underlying this pattern are rooted in long-standing debates over the relative importance of niche- versus dispersal-assembly.
Using a species-rich guild of small, herbivorous mammals, we are testing the hypothesis that abundant, widespread species are ecological generalists, capable of persisting on a variety of foods. In contrast, rare, restricted species may be ecological specialists, highly selective in their diets and habitats.
Current collaborators: Jesse Alston, Tyler Kartzinel, Leo Khasoha, Allison Louthan
Trophic dynamics and non-consumptive effects in mammal-dominated ecosystems
By consuming and scaring herbivores, carnivores have the potential to bolster the abundance and diversity of plants via trophic cascades. From highly publicized work in North America involving wolves, elk, and trees, trophic cascades have captured the attention of ecologists and the general public alike. Although the hypothesis of trophic cascades makes good sense, we lack a broader knowledge of if and how they work in other ecosystems, and how human activities such as livestock production and predator control might affect them.
Current and recent collaborators: Adam Ford, Annabella Helman, Sam Kiuna, Caroline Ng'weno
Representative papers: Ford et al 2014, Ford and Goheen 2015, Molina et al 2023, Ng'weno et al 2017, Ng'weno et al 2019, Oates et al 2019, Otieno et al 2019
Outreach: National Geographic Education Blog, Science Daily, Scientific American
Conservation, reintroduction, and restoration of mammal populations
Populations of wild mammals are dwindling in most areas, and are being reintroduced in others. We have worked on a diversity of species (rodents, carnivores, bats, ungulates) of conservation concern, mostly in human-occupied landscapes. Knowledge from this work has been used to guide conservation efforts at state, national, and international levels.
Current and recent collaborators: Abdullahi Ali, Saeideh Esmaeili, Douglas Kamaru, Lane Jeakle, Dedan Ngatia
Representative papers: Ali et al 2017, Ali et al 2019, Brito et al 2023, Esmaeili et al 2024, Jesmer et al 2018, Jesmer et al 2021
Outreach: National Public Radio, The Conversation, Western Confluence, Wyoming Public Media