By Josh Daskin, PhD Candidate, Princeton University
Over the last few decades ecologists have largely settled on the understanding that habitats are not static—habitats change over time, responding to disturbances like fires or floods, and to less intense, incremental processes. The extensive large mammal declines in Gorongosa, while human-caused, are another such disturbance with the potential to create cascading changes in the park’s habitats and the way species use them.
Tree cover in Gorongosa’s savanna from a small airplane in 2012.
As part of my doctoral dissertation, I’ve measured one such change by repurposing declassified American spy satellite images. I measured woody vegetation across Gorongosa before (in 1977) and after (in 2012) the war-driven mammal declines by training a computer mapping program to recognize trees and shrubs, and showed that the near-complete loss of large mammals led to a 34% increase in tree cover across the park; in some areas, woody vegetation more than doubled. Elephants have a habit of toppling trees—a lot. With many fewer elephants and fewer antelope eating woody plants, more trees sprouted and grew in the savanna.
Deforestation is among the major threats to tropical biodiversity (especially in tropical forests, but also in savannas); however, more trees is not necessarily a good thing in a savanna, where species are adapted to a mix of trees and grass. For instance, vegetation surveys I’ve conducted in Gorongosa show that shaded areas beneath trees that grew up since the war-driven mammal declines can reduce the number of grass and wildflower species in the understory by 20-50%!
The potential for reduced plant diversity is concerning in its own right, but the large mammals central to Gorongosa’s post-war recovery could also be affected. If areas that were once suitable savanna habitat are now too forest-like, antelope favoring savanna habitats may not have access to the areas they used to. To understand how mammals in Gorongosa’s recovering populations are selecting habitat in the changing landscape, in an ongoing collaboration with Professors Rob Pringle at Princeton University and Ryan Long at the University of Idaho, I have been studying the movements of three species of antelope—bushbuck, nyala, and kudu.
Bushbuck in a mixed stand of fever trees (yellow, Acacia xanthophloea) and winter-thorn acacia (gray, Faidherbia albida)
Ryan put GPS collars on about a dozen of each species and we automatically received each animal’s location every hour for about nine months (nearly 300,000 total locations!). Meanwhile, I used satellite images to make a very fine-scale map of woody versus open habitats. Now I’m comparing the locations used by the antelope with the full range of available habitats.
Preliminary results suggest that bushbuck and nyala prefer woodier areas while kudu prefer somewhat more open ones. This makes sense when you consider the most obvious difference among the three species—their size. The smaller bushbuck (40 kg for a full-grown adult) and nyala (90kg) use woody cover as protection from predators. Kudu, the largest of the three species (200 kg), is less vulnerable to Gorongosa’s lions. So far, it seems that increased tree cover may not be a problem for bushbuck and nyala, but might keep kudu out of some areas of the park.
Male kudu enjoying a lunch of Combretum imberbe.
In the short term, as Gorongosa’s mammal populations continue to grow towards their pre-war populations, habitat availability is unlikely to be the strongest limitation on their recovery, even given the increase in woody vegetation to date. Instead, the biggest bottleneck might be the natural pace of population growth; at the extreme slow end are elephants with a 22 month gestation period, while most of the large mammals are more in the range of a kudu’s 8 months. Meanwhile, the park’s rangers are continuing their anti-poaching efforts, which together with the park’s socio-economic programs aim to keep Gorongosa’s wildlife from being used as an expendable resource.
Vast expanses of Gorongosa remain wide-open plains (suitable for grazers like wildebeest, buffalo, and zebra) or sparsely-treed savanna (favored by bushbuck, nyala, and kudu, among others). That said, uncontrolled invasion by woody plants has had detrimental effects in other savannas. For instance, in South Africa’s Addo Elephant National Park, dense thickets developed where few elephants were present, restricting lion movements and predation.
Although the low density of elephants in the last 35 years led to increased tree cover, the future may hold yet more changes. Now that the elephant population is growing again, areas recently colonized by trees may be reopened. Keeping an eye on the future, I’ve initiated one more project monitoring several patches of trees that developed after the war. I’ve started following the fate of about 700 fever trees. According to my mapping work and the testimony of long-term Gorongosa rangers this species has become much more common since the war; it’s also a favorite food of elephants. By marking individual trees and returning to find them each year, I will measure what I expect to be an increasing impact of elephants on Gorongosa’s landscape. It’s possible the elephants and other browsers will recover in time to avoid strong impacts of the increased woody cover that has already developed.