Megaherbivore extinction, stage-dependent plant defenses, and landscape change in rangeland ecosystems
Elephants and other large ungulates are important agents of mortality for savanna trees, and often are regarded as critical in maintaining landscape heterogeneity. However, plant defenses may buffer against top-down control by ungulates, and thus might mediate herbivory as a driver of tree population dynamics. We have 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 exhibit population declines following increases in elephant numbers through time. Our findings demonstrate that ant symbionts deter catastrophic herbivory on trees, thereby decelerating landscape change following the extinction of elephants and other ungulates. Further, because adult A. drepanolobium are buffered from top-down control, tree recruitment inevitably assumes a critical role in driving population dynamics. As such, small mammals impose demographic filters by consuming tree seeds and seedlings, and have impacts on tree populations that rival those of their more conspicuous mammalian counterparts. Colleagues and I are beginning to test what we suspect is a more general phenomenon, through which herbivores force allocation trade-offs and variation in stage-dependent reproductive value to drive the ontogeny of plant defenses.
Interaction webs are pervasive features of ecological systems, and studies of these systems often attempt to reduce nature to pairs or subsets of species. However, the contextual nature of interaction webs merits closer inspection if we are to generate a mechanistic understanding of when and where the loss of species is likely to have consequences for community structure and ecosystem function in a world of increasing environmental change. 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 addressing a number of topical issues in rangeland conservation, 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.
Environmental perturbations and the resilience of vertebrate communities
The extent to which local communities are "filled" with species is a long-standing and important debate in community ecology. Using null models and time series data from a well-studied rodent community in the southwestern United States, colleagues and I have demonstrated that species richness is remarkably stable through time, despite considerable changes in species composition driven by long-term changes in vegetation structure. This occurs independently of the upper bound to richness imposed by the regional species pool, suggesting a role for species interactions in maintaining the local equilibrium. Indeed, the equilibrium seems to be upheld by compensatory colonization and extinction among granivores, the most speciose guild of consumers at the site. Our null models represent a complementary approach to testing for community saturation across space, and are a significant improvement over traditional Markov models which can suffer from low statistical power. Finally, along with colleagues from the University of British Columbia, I have applied the above statistical methods to demonstrate that outbreaks of mountain pine beetles constitute a resource pulse, effectively reducing interspecific competition and overriding local regulation of richness for guilds of forest birds that are able to exploit beetles as a temporary source of food.
Demography and movements of moose in northwestern Wyoming
Throughout western Wyoming, moose herds are exhibiting signs of poor population performance. Experts typically attribute these declines to some combination of predation, habitat degradation, climate change, and parasites. However, the relative importance of each factor has been elusive, in part because previous research typically has targeted a single potential factor in isolation, and then only within a single herd unit. In particular, we are entering a critical time for Sublette moose, which comprise the largest herd of Shiras moose in the lower 48 states, and which exist in a region of the Noble Basin proposed for energy development. Along with collaborators in the Wyoming Game and Fish Department, we are quantifying demographic rates and movements to 1) elucidate the mechanisms underlying poor population performance; 2) provide baseline information to help managers predict and mitigate the impacts of energy development on Sublette moose; and 3) compare with Jackson moose, which co-occur with significant numbers of wolves and grizzly bears.