Seed dispersal effectiveness by greater one-horned rhinos and domestic bovids of a megafaunal fruit

Abstract

The extinction of megafauna, such as rhinos, from tropical Asian forests may have detrimental effects on the regeneration of plant species that rely heavily on their dispersal services. Understanding the potential of other animals to substitute the ecological function of megafauna is important for predicting ecosystem change following declines or extirpation of megafauna. Anthropogenic activities have led to the presence of domestic bovids in many tropical habitats, and these animals could be capable of substituting seed dispersal roles of megafauna. Here, we conducted feeding trials, germination tests, and collected movement data on greater one-horned rhino (Rhinoceros unicornis) and domestic bovids (Bos primigenius; Bubalus bubalis) in the national parks of Chitwan and Shuklaphanta, Nepal. Our aim was to investigate whether domestic bovids can serve as effective substitutes for rhinos in dispersing the megafaunal-fruited Mallotus nudiflorus, which are specialized for rhino dispersal. Specifically, we assessed the long-distance seed dispersal capabilities of rhinos and bovids by combining gut passage times and movement patterns. Additionally, we compared the seed germination success rates among rhinos, bovids and other dispersal modes (e.g., deer and water dispersal). We found that rhinos dispersed 80 percent of seeds over distances greater than 1 km, with potential dispersal distances of up to approximately 5.4 km within their maximum gut passage time. In contrast, cattle dispersed 20 percent of seeds over 1 km, with a maximum distance of around 2.2 km, while buffaloes dispersed 15 percent of seeds over 1 km, with a maximum distance of around 1.4 km. Seeds consumed by rhinos exhibited similar or better germination rates compared to seeds retrieved from cattle and buffalo dung, as well as other dispersal modes. These findings indicate that rhinos provide a functionally unique dispersal service for this megafaunal fruit, especially in long-distance dispersal. While domestic bovids can partially substitute the seed dispersal functions of rhinos, they cannot fully compensate for the loss of seed dispersal that occurs with the disappearance of rhinos.