Quantifying vegetation responses to elephant carcasses in a semi-arid savanna
Plant communities form the structural and functional basis for most terrestrial ecosystems. For this reason, an improved understanding of plant community ecology, or the co-occurrence of species across time and space, is important for improved management and conservation of biodiversity.
The plant structure of semi-arid savannas is mainly determined by natural disturbances such as water and nutrient availability, fire, and herbivory. Although we know a lot about how herbivores affect the vegetation structure of semi-arid species while they are still alive, we know less about how these herbivores would affect vegetation structure and function after they die.
This is particularly true for the largest land mammals – the African elephant. Elephants are considered important ecosystem engineers in savannas by maintaining open wooded grasslands. They induce landscape-scale changes which in general favours grasses over trees. Elephant diet includes both life forms found in semi-arid herbaceous layers – i.e., forbs and grasses. Even though elephants are thought to have strong effects on the broader landscape, they can also alter which species are present in the landscape at a smaller scale.
Forbs are the less studied and understood component of herbaceous layers (plants). They are generally known as ‘wildflowers’ and are functionally important in semi-arid savannas. We are excited to see how this life form responds to a major change in the system (i.e., a megacarcass), and how these changes will impact the productivity and biodiversity of the savanna in the Lowveld region of South Africa.
What, might you ask, is a forb?
Decomposing carcasses represents another, and perhaps less studied and understood, natural disturbance in semi-arid savannas. We know and understand a lot more about smaller herbivore carcasses and their effects. But observations from megacarcasses (the carcasses of megaherbivores such as elephants) are few. Carcasses can have strong impacts on ecosystems by affecting soil properties and the community dynamics of plants and animals. Megacarcasses can create nutrient hotspots, or areas in the landscape with higher nutrients than their surroundings. These nutrient hotspots can cause long-term changes to the landscape. The Megacarcass Ecology Project provides a unique opportunity to determine what these changes are, and to quantify megacarcass effects on vegetation structure, function, and dynamics.
Examples of semi-arid savanna forbs:
1. Do carcass sites support different plant communities to the surrounding matrix? What life forms (grass vs forb), species and species traits are associated with carcass sites? What species drive these differences?
2. Is plant loss at carcass sites driven by nutrient toxicity and/or physical trampling?
3. Do recovering plant communities at carcass sites show a pattern of succession over 1) time and 2) increasing distance from the carcass centre point?
4. What drives the recovery of vegetation change – existing seed banks, immigration of seed from the matrix, below-ground bud banks, and vegetative colonization from surviving/peripheral individuals?
5. Are plant communities enriched (nutritionally) relative to matrix communities?
6. Do plant communities at carcass sites provide different ecosystem functions (e.g., provision of high-quality forage) relative to the surrounding matrix?
7. How do plant community dynamics at carcass sites compare to other local nutrient hotspots?
During a field work campaign in March of 2022 and 2023, ten elephant carcasses were sampled, of which four were on granitic, and six on basaltic soil respectively.
In the Field:
At each carcass, transects were laid out in the four cardinal directions (see the sampling design below). Vegetation was sampled within 1 meter squared subplots placed at various distances from the carcass (i.e., at 1 m – 2 m, 2.5 m – 3.5 m, 5 m – 6 m, 10 m – 11 m, and 14 m – 15 m). All plant life forms (i.e., forbs, grasses, sedges, dwarf shrubs, shrubs and, trees) were identified up to species level. Total percentage cover of vegetation, bare soil, bone, rock, litter/wood, and dung were recorded within each of the 1 meter squared plots at the various distances. Sampling will be repeated annually.
Our project aims to enhance our understanding of the effects of mega carcasses on:
1. Plant productivity, diversity, and forage quality
2. Vegetation dynamics and ecology, particularly of the forb component