IntroductionTsessebe antelope are one of the fastest declining species in the Okavango Delta. This study aims to identify the key factors leading to this decline, by furthering our understanding of tsessebe ecology. The study also aims to develop methods of slowing or reversing the decline.
This will be achieved through the collaring of tsessebe, and the analysis of their behaviour, grazing sites and genetic diversity, to see whether any of these are factors leading to the decline of the species in the Moremi Game Reserve.
Researcher: Harriet ReevesCountry: Moremi Game Reserve, Botswana
Partner Organisations: Okavango Research Institute
BackgroundSince 1996, aerial surveys of northern Botswana have recorded significant tsessebe population declines, with an 87% decline in the Moremi Game Reserve in particular. This makes tsessebe one of the fastest declining species in the Okavango Delta.
With the majority of studies focussing on the Kruger National Park, little is known about the species in Botswana – the last Botswana-based study took place in 1972. This project will address the need for a better understanding of the species resource requirements so that management may be undertaken to reverse this population decline.
ObjectivesThe study is considering the following hypothesises:
1. Spatial ecology – Are habitat changes causing the decline of tsessebe at the landscape and patch scale?
To determine tsessebe spatial behaviour and how they utilise their habitat, information is being gathered on habitat use, preference, seasonal movement and home-range size using GPS collars. Intergroup spatial behaviour at the landscape scale is also being determined where possible.
2. Nutritional ecology – Are declines caused by reductions in food sources?
Nutrition affects most aspects of herbivore ecology, such as the timing and success of reproductive efforts, individual growth rates, susceptibility to predation and disease. Information is being collected on feeding-site characteristics to determine forage quality.
3. Behaviour – Are vital behaviours prevented due to a loss of habitat?
To identify priority habitats for the species, focal behavioural analysis will provide an insight into which behaviours are performed when, where and for how long. The study is investigating whether certain habitat types and attributes within those habitats are preferred for different behaviours, identifying which characteristics are required for certain behaviours, such as breeding, to occur.
4. Genetic diversity – Are the significant declines leading to a lack of genetic diversity and risk of inbreeding?
Habitat loss and fragmentation can cause severe reductions in genetic diversity by restricting gene flow between isolated populations and reducing habitat carrying capacity. Genetic analyses of individuals and relatedness studies will allow us to determine whether a lack of genetic diversity is contributing to the species decline.
5. Adaptability to environmental change: Are landscape-wide changes observed over the past 20 years correlated with tsessebe declines?
Global wilderness areas are under rising threat from increasing anthropogenic (human caused) activities and climate change. Historical data spanning two decades from aerial surveys and satellite imagery will give insight into the fluctuation of tsessebe populations in the Delta in relation to shifting environmental pressures and identify the drivers that caused historical declines in the region.
Eight store-on-board GPS collars are being fitted to individual females in different herds of a period of one year. Following the procurement of permits from Botswana’s Department of Wildlife and National Park (DWNP), darting operations were undertaken to fit the collars and obtain tissue samples. GPS coordinates are being recorded every five minutes, providing detailed spatial data, downloaded via VHF on a monthly basis. Data are being mapped in ArcGIS against different habitat layers and the distances moved determined.
Females are tracked by CHF using conventional telemetry equipment to observe habitat preference at the patch scale, behaviour and dietary selection. The collars have a programmable drop-off feature, removing the need to dart the individuals to recover the collars. Funds allowing, a further eight collars will be fitted in the second year to increase the sample and prevent potential bias by sampling in only one year.
GPS positions of collared females will identify grazing sites. Data on available forage biomass, sward roughness, species diversity, tuft density and sward height will be collected at each sampling site. A total of 20 different grazing sites per habitat will be sampled during each season for a period of two years, yielding 60 grazing sites per habitat per year. These will be compared to pre-determined reference sites.
Observational ethograms (catalogues of behaviours) are being conducted on collared individuals from a stationary vehicle using a pair of binoculars. Point samples will be conducted every two minutes for a period of one hour per individuals and all observed behaviours will be recorded. Observations will be conducted thrice-daily during one of three time-blocks to identify behaviour patterns, and will be repeated every season.
A minimum of 50 tissue samples will be collected from collared and uncollared individuals. Collared individuals will be sampled during the collaring procedure, while uncollared individuals will be sampled through the use of biopsy darts. Genetic markers for tsessebe are unavailable, so microsatellites will be obtained from the closely related topic to determine relatedness estimates, census and effective population size estimates, and the level of genetic polymorphism and differentiation within sample herds.
In September 2015, GPS-enabled collars were deployed onto eight female tsessebe in different herds between Black Pools and Second Bridge, in the Moremi Game Reserve. These collars were programmed to drop off in October 2016 and four of them did so on time. One was recovered from the ground in March 2017. Of the remaining three, two were removed in November 2016.
Several attempts were made to dart the last collared individual, but these were all unsuccessful. The high rainfall that has occurred this rainy season has made it very difficult to track any animals, let alone approach them for darting. Since the animals that had been darted showed no signs of hair loss or any other discomfort often associated with collars, it was decided that the last individual would not suffer any adverse effects if she carried her collar until the end of the rainy season, at which point another attempt to remove it would take place.
Data gathered from tsessebe and other species that were collared concurrently (wildebeest, zebra and impala) has been analysed. Spatial analysis and remote sensing images are being used to quantify tsessebe resource requirements and compare current environmental conditions to those over the last two decades, which will provide an indication of the factors causing the current population decline. Resource requirements for each of the herbivore species will be compared, so as to identify factors that could affect the species’ resilience to climatic variation.
This project is in collaboration with the Royal Veterinary College of London, who developed and provided eight GPS-enabled collars that were fitted to female tsessebe in the Moremi Game Reserve, Botswana, in September 2015. Since then, we have attempted to locate each animal on a monthly basis. Three individuals disappeared for several months, but a tracking flight allowed us to locate them in wooded habitats to the east of Moremi Game Reserve. Most of the collared animals have spent the majority of their time in open habitats, primarily grassland, but mopane woodlands have also been selected during the rainy season.
We have been using data from GPS-enabled collars to identify foraging grounds and record data on the forage characteristics in those sites. These will be compared to foraging sites selected by wildebeest (Connochaetes taurinus) and zebra (Equus quagga), which have also been collared in the same area, as part of a sympatric grazing study.
LINK to PDF: Quarterly Report August 2016
PDF download: Quarterly Report April 2016
In September 2015, GPS-enabled collars were deployed onto eight female tsessebe in different herds between Black Pools and Second Bridge, in the Moremi Game Reserve. Barring two individuals in December, we have located these animals once a month since to check on their condition, record any changes in population demographics, and download data from the collars.
Maps and more details of each tsessebe can be found on the PDF here: Quarterly Report January 2016
On the 25th August, 2015, we were issued with a darting permit to collar eight (8) female tsessebe in the Moremi Game Reserve. The darting team comprised Dr Hattie Bartlam-Brooks, Dr Emily Bennitt, Prof. Alan Wilson (a Botswana-registered veterinarian), Dr Tatjana Hubel, Mr Richard James Harvey and Mr Mosimane Nkape.
All individuals bar one have been observed since the dartings, and preliminary data show that they suffered no ill effects. They have not moved from the area where they were first located and are showing no signs of being disturbed by the collars in any way.
PDF Download: Quarterly Report September 2015