Endozoochory by Black Rhinoceroses Enhances Germination of a Key Arid Savanna Tree Species

Megaherbivores are typically regarded as agents of top-down control, limiting woody encroachment through destructive foraging. Yet they also possess traits and engage in behaviours that facilitate plant success. For example, megaherbivores can act as effective endozoochorous seed dispersers. However, studies on facilitative roles are heavily biased towards the African savanna elephant (Loxodonta africana), with little attention paid to other species or to effects beyond germination, across early ontogenic stages. The African black rhinoceros (Diceros bicornis), an obligate browser that exhibits frugivory and defecates in fixed dung middens, may offer ecologically distinct dispersal services. We conducted controlled experiments to test whether black rhino interactions with Vachellia erioloba, a leguminous tree of ecological importance in arid savannas, enhance germination, early seedling development or seedling resilience to herbivory. Germination was compared among dung-derived seeds, untreated controls and chemically scarified seeds. Seedling growth was assessed in dung versus sand and under simulated black rhino herbivory. Dung-derived seeds germinated most steadily and produced the highest cumulative germination (+40%) over the longest period (+13 days). Growth trials revealed that dung substrates did not enhance initial growth. Rather, seedlings being older conferred greater resilience to biomass loss than exposure to different substrate conditions. Our results provide the first experimental evidence of an apparent mutualism between black rhino and V. erioloba. This relationship is not driven by enhanced seedling development through legacy effects of gut passage, nor by dung conditions, as expected. Instead, it stems from gut passage effects on germination. In addition to increasing total germination, gut passage accelerates germination and extends the germination period, producing a seedling cohort with both older individuals and greater age variation-a population structure that may enhance persistence beyond the germination bottleneck. This research supports a more nuanced view of megaherbivores as both disturbance agents and mutualists in arid ecosystems.