Seasonal Variation in Browse Availability & Resource Utilisation by the eastern black rhinoceros
Lewa Wildlife Conservancy: Kenya
Background
The black rhino is a critically endangered species with just 5000 individuals left in the wild. Current conservation efforts rely heavily on secure, fenced reserves that prevent the free movement of animals into and out of the reserve. As a result, megaherbivore assemblages can quickly accumulate and factors such as, vegetation changes, overstocking and competition with other herbivores can significantly limit black rhino population growth.
The black rhino is a browsing species, ingesting large quantities of forbs, low-growing shrubs, succulents and woody vegetation. Long-term studies on Lewa have revealed changes in habitat structure and vegetation cover specifically from woodland to open grassland. Woody vegetation has decreased as a result of increased wildlife populations, a component that constitutes more than 63% of black rhino diet. Thus, declining coverage could have severe implications for population viability.
This study determined the amount of browse available to black rhino across the landscape and how those resources change seasonally. It also identified important food sources for the black rhino. Findings have direct applications for changes in the way that the black rhino population is managed on the Lewa Wildlife Conservancy and implications for the management of all fenced black rhino populations in Africa.
Publication is currently being worked towards.
Study Summary: Wet-Season
During the wet season of March-May 2018, browse availability and the utilisation of resources by black rhino were investigated. Key findings indicate that browse availability in forest habitats was 18% higher than hills and 20% higher than plains. In an effort to preserve some of the declining wooded area, Lewa established a number of exclusion zones. These are small, protected areas of woody vegetation surrounding by fences that allow black rhinos to pass underneath but exclude giraffes and elephants from entering. An assessment of the vegetation in the exclusion zones revealed that browse availability inside the zones was 22% higher than outside. However, higher quantities of browse availability were not correlated with the age of the exclusion zone, suggesting that the vegetation in some zones may have exceeded 2.0m in height and become inaccessible to black rhino.
Although exclusion zones are successfully increasing browse availability, they are not being managed in a way that optimises food quantities for black rhino. Selectivity indices revealed that black rhino select for Acacia drepanolobium, an important food source that has declined on the Lewa-Borana Landscape in recent years, this could have implications regarding resource availability and population viability. Finally, black rhino select for herbs and low growing succulent species during the wet season. This appears to be a behaviour that has evolved to avoid competition with other large herbivores, such as elephant, and has been documented in other black rhino populations. Overall, this study has resulted in several key findings that have implications for habitat manipulation, current management strategies and the conservation of fenced black rhino populations across Africa.
Study Summary: Dry-Season
Informed by the wet season study conducted in 2018, this follow-up dry season report (conducted between Aug-Oct 2019) had several key aims: (a) to understand how quantities of browse change seasonally across the landscape; (b) to determine whether black rhino are heavily reliant upon exclusion zones for additional food sources; (c) a balanced data set and understanding of how black rhino diet varies seasonally in order to implement a holistic approach to the management of a strategically important, fenced black rhino population.
Determining the location of our next vegetation plot after backtracking a black rhino.
Key findings indicate that browse availability was significantly lower during the dry season, even inside exclusion zones that are dominated by woody vegetation and year-round shrubs, suggesting that forbs and herbs greatly increase the number of food resources during the wet season. Heat maps generated using the historic sightings data (2016, 2017 and 2019) illustrate that whilst black rhino distribution remains concentrated in the same areas, individuals disperse a lot more during the wet season. This is indicative that black rhino are able to roam further away from areas of known vegetation thickets because their diet can be heavily supplemented by the growth of fresh herbs.
Black rhino do utilise the exclusion zones for foraging, however, during data collection only 32% of backtracks occurred inside the zones, indicating that a large portion of the population is not reliant upon the extra resources offered by these small protected areas of vegetation. To determine which individuals actively use the zones on a regular basis, an intensive camera trap survey should be conducted. This would provide a definitive answer to how reliant black rhino are on exclusion zones as it would provide information on the individuals that use them and the frequency of utilisation.
As predicted, fewer plant species were consumed by black rhino during the dry season, but dietary diversity was higher than the plant species available across the landscape suggesting that black rhino cannot afford to be as selective or ‘choosy’ for certain plant species due to the lower quantity of available browse in the dry season. Interestingly, black rhino actively avoid herbs during the dry season, instead selecting trees and shrubs – this could be due to higher nutritional content.
Acacia drepanolobium, a species that was flagged as important in the wet season diet of black rhino was not as represented during the dry season, but it was still selected for disproportionately to its availability. Plant species in the Fabaceae family (Acacia spp.) and Malvaceae family were important food sources across both seasons. It would be interesting to conduct a study on the chemical composition of black rhino forage to understand why they select certain species at different times of the year.
Lastly, black rhino are actively selecting low growing plants and Acacia drepanolobium seedlings, further corroborating the findings of the wet season study; black rhino are contributing to declines in woody vegetation and repressing of the growth of Acacia drepanolobium plants across the Lewa Wildlife Conservancy.
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