Author: Rob Pringle, PhD, Princeton University
Each member of my research group has a specific objective in Gorongosa National Park, and their independent projects are linked by a common desire to understand the role played by large mammals in creating and sustaining savanna ecosystems.
Jen Guyton has a passion for hippos, which (like most species) suffered greatly during Mozambique’s civil war and are now recovering. Hippos function as architects of aquatic and waterfront ecosystems like those of Gorongosa’s Lake Urema, yet surprisingly little is known about the effects of their engineering activities. Jen suspects that by mowing the lawn at night, hippos may promote the recovery of other species that prefer to nibble shorter grasses (short grass having become a rare commodity in Gorongosa following the decline of bulk grazers such as zebra and buffalo). It is even possible that hippos play a role in maintaining the very existence of Lake Urema: by moving around and stirring up sediment, hippos may prevent the lake from filling in with sand and silt.
Hippos aren’t the only large mammals that need water, and savanna watering holes are hotspots of biological activity. Josh Daskin is studying the ecology of these “pans,” which are miniature ecosystems in and of themselves: in addition to attracting herbivores in search of water and lions in search of prey, pans support diverse communities of frogs, fishes, and aquatic plants. But how are these essential landscape elements formed, and what keeps them filling up with water year after year? Josh is testing the hypothesis that pans form when herbivores browse around the edges of termite mounds, gradually creating shallow depressions, and that they are maintained by species such as elephants, warthog, and buffalo that come to drink bathe, and wallow in the mud. In this way, enormous elephants may be indispensable allies of tiny tadpoles.
Dr. Ryan Long, a post-doctoral researcher, came to my lab from the University of Idaho, where he studied the spatial ecology and physiology of elk. In Gorongosa, he’ll be tracking the movements of three closely related herbivore species, bushbuck, nyala, and kudu. Do they indeed frequent the margins of termite mounds, as Josh’s hypothesis holds? If so, why? Ryan will use his expertise in the movement behavior and nutritional ecology of ungulates to ascertain what rules herbivores use in deciding when and where to forage, and whether the observed patterns match theoretical predictions. Are these animals efficient in obtaining food, or do other concerns – such as the possibility of becoming food – trump the quest for a perfectly balanced diet?
Of course, answering these questions (and many others) requires understanding exactly what the animals are eating. Until now, this has been quite difficult to do – try identifying which species of grass a waterbuck is eating 100 meters away . . . and then try to do it in the dark. Dr. Tyler Kartzinel will be applying his expertise in molecular ecology to assemble dietary profiles for different herbivore species by sequencing the plant DNA present in herbivore dung. It might sound like a crappy job, but we’re actually pretty excited about it! For example, this information will provide insight about why some herbivores have recovered particularly well in Gorongosa, and why others are lagging behind.
It might seem like bad news for a plant when a herbivore comes calling, and sometimes it is – but not always. Herbivores can sometimes benefit plants by dispersing seeds and stimulating new growth. Likewise, plant species can compete with one another, but they can also protect one another from herbivores; just ask the vulnerable new shoot sprouting beneath the spiny Acacia tree. Tyler Coverdale is interested in understanding this spectrum of species interactions, and what can tip an interaction from antagonism and exploitation towards facilitation and mutual benefit. As Gorongosa’s large herbivores recover, how are they altering the balance sheet of positive and negative interactions, and with what implications for the structure and functioning of the ecosystem?
As for myself, I’m interested in regime change. Specifically, what factors maintain the balance of trees and grasses that defines the savanna biome? What prevents the development of a forest instead – or, alternatively, a grassland? We know that fire plays a role in stabilizing savannas, but what about elephants, which are capable of taking down all but the very biggest of trees? Gorongosa’s elephant population is on the mend, but elephant numbers were low for many years, during which time dense woodland has developed in some areas. Can the recovery of elephants reopen these densely wooded areas? Or have those areas passed the threshold into a new stable state, one that might take centuries to revert to open savanna? Knowing this will help the Park’s conservation department evaluate whether or not an intervention, such as the selective removal of certain trees, is warranted as part of the ongoing restoration effort.
Our investigations are just beginning. Some of these questions will take years to answer, and we will have to work closely with other scientists, such as Marc Stalmans and Alan Short from Gorongosa’s Department of Scientific Services, and Paola Bouley of the Gorongosa Lion project. But visitors to the park can help too, by recording any unusual observations and by taking (and sharing!) high-quality photographs of study species such as hippos, elephants, and lions.