Researcher: Dr. Hattie BartlamRegion: Okavango Delta, Botswana
BackgroundDuring a three-year study investigating herbivore distribution and movement patterns within the Okavango Delta researcher Hattie Bartlam and colleagues discovered a zebra migration that moved between the Delta and the Makgadikgadi National Park. At 580km it is the second longest intact zebra migration in the world after the Serengeti-Mara migration.
Historically terrestrial mammal
migrations occurred in most grassland ecosystems around the world, often numbering millions of animals. Over the last century or more these declined dramatically and those remaining are under threat due to human encroachment and habitat fragmentation. In southern Africa the erection of fences across ancient migratory routes resulted in the cessation of migrations and decline of migratory populations. Between the 1950s-1980s Botswana erected veterinary cordon fences across much of the country, to prevent the transmission of diseases between wildlife and cattle. These fences were put up with little investigation into how they would affect the movement or home ranges of wildlife populations in the area and whilst the deployment of fences was balanced by increased protection for wildlife in specified areas, many migratory movements were disrupted and presumed lost as routes between seasonal home ranges were blocked.
The migrating zebras found by the researchers spend the dry season in the Delta moving to the Makgadikgadi grasslands for the rainy season. As seen in the Serengeti, they may be moving to take advantage of higher quality resources rather than differences in absolute abundance.
Perhaps the most interesting factor is that between 1968 and 2003 the migration route was blocked by the 100km Northern Nxai Pan Fence so that any zebra attempting to migrate would have either had to walk around the fence (a difficult 400km diversion) or stopped at the fence and ceased to migrate altogether. Whilst at present it is not possible to determine which occurred, it is of great topical interest to conservationists that the migration re-established itself to the present, highly directed, route as it had been presumed that ungulate migratory routes were learnt through following their parents.
This is of interest in the context of wildlife corridors, an avenue that is increasingly being seen as a way to reduce area specific mammal overcrowding and ensure genetic dispersal by reconnecting discrete protected areas that would have previously been part of the same ecosystem. The success of such schemes relies heavily on animals utilising the newly accessible areas, such that larger mammals move either permanently or seasonally between the connected areas. Thus the ability of adult zebra to re-establish old migratory routes once physical barriers have been removed, even if one or more generation has not used it, is of great importance.
ObjectivesThe aim of this project is to increase the understanding of the recently discovered Okavango-Makgadikgadi zebra migration; the second longest zebra migration in Africa.
- Determine how the migratory route varies annually.
- Estimate of the size of the migratory population.
- Investigate energetic implications of migratory zebra as compared to Delta and Makgadikgadi resident zebra.
MethodologyTen GPS collars will be fitted to migratory zebra to record animal movement: five onto animals already fitted with collars and five onto new animals. Eight collars are satellite-downloadable, meaning that waypoints are sent by email, allowing the animal’s position to be remotely monitored.
A population estimate will be conducted by two ground surveys: one of the dry-season home range of migratory zebra and one immediately after the first heavy rain as the animals begin the migration (as determined by positions of satellite-downloadable animals).
Foraging efficiency observations and preferred resource structure samples will be used to further investigate the migration’s energetic determinants.
Funding from the Wilderness Trust is being used for the refurbishment of previously used GPS collars, veterinary costs and basic operational costs.
UpdateAnnual Report 2011
Whilst 2010 was a relatively quiet year in the field, research continues to go well, with the team focusing on publishing results as well continuing to collect continuous data on this poorly understood migration.
In August I was awarded a PhD by the University of Bristol, UK, for my work on herbivore ecology in the Okavango Delta, Botswana (Entitled: Spatial heterogeneity in a dynamic wetland: determinants of herbivore distribution in the Okavango Delta and their relevance to conservation). The results from this study, which the Wilderness Trust also supported, were numerous, but perhaps the most relevant for the long-term conservation of this fragile Delta system was the movement and vegetation data that quantitatively illustrated the importance of flooding regime on herbivore assemblage patterns in the Delta, through their effect on landscape heterogeneity and resource quality. The spatially contained nature of the Delta within the Kalahari basin means a decrease in flood extent could affect herbivore abundance by increasing home range size and thus inter- and intra-specific competition. Long-term conservation of herbivore populations within the Delta is primarily dependent on the success of trans-boundary water use policies within the Okavango River system. These results have now been submitted for publication in international peer-reviewed journals.
GPS and VHF collars continued to record the movement patterns of Okavango-Makgadikgadi migratory zebra. With another year of good rains the zebra have spent the majority of the year on the Makgadikgadi grasslands, interspersed by a relatively short dry season in the south-eastern Delta. Collaborative work with academics in the USA is ongoing, allowing us to utilise highly detailed daily satellite data to remotely link zebra movement patterns with vegetation quality and localised rainfall. Working across such a large and remote area means the use of such methodology is both cost- and time-efficient and we hope to publish the results from this work this year. The first academic paper detailing the migratory route has already been accepted for publication and will be published in early 2011.
It has been a confusing year for the zebra! The zebra left their dry season home range in the Okavango Delta at the onset of the rainy season in November 2008. All those hypothesised as migratory did indeed migrate and all survived the 280km trek to the Makgadikgadi grasslands.
In May, as the last waterholes dried up in the Makgadikgadi, all the zebra returned to the Delta, taking approximately 14 days to make the journey. However, unlike in previous years when they remained in the Delta until the end of the year, unseasonal storms in June saw them return the Makgadikgadi. Apart from another short trip back to the Delta they have spent the vast majority of the year in the Makgadikgadi grasslands, utilising the high-quality resources and benefiting from relatively low predator densities.
Overall, the zebra undertook five migratory movements this year, covering an impressive 1400km in long distance movements. The collars collected excellent data on their daily movement patterns, allowing researchers to begin quantifying the energetic benefits gained by migrating rather than remaining in the apparently productive Delta year round.
The zebras’ absence from the Delta has, however, caused a few problems by disrupting planned collaring sessions; Okavango-Makgadikgadi migrating zebra that are not collared can only be distinguished from Makgadikgadi resident animals when they return to their Okavango dry season home range. The remaining reconditioned collars, along with additional new GPS collars, will be deployed next year as soon as the animals return to the Okavango.
Next year, we will continue fieldwork focusing on movement strategies, energetic observations, resource analysis and body condition scores in our effort to increase understanding of how migratory routes vary and to quantify the physiological costs and benefits that migrating incurs.