It’s not only rhinos that are impacted by the rhino poaching crisis, the very landscape they inhabit could also be affected. New research published in the Journal of Ecology suggests that the reintroduction of white rhinos to South Africa’s Kruger national park has changed the structure and composition of the park’s savanna grasslands, indicating that rhinos have a key role in how the savanna functions. That role is now threatened by a poaching epidemic which could drive the species to extinction within 20 years.

White rhino in Kruger

Megaherbivores such as elephant and rhino, which as adults have no natural predators, can be considered as apex consumers, the vegetarian equivalent of apex predators. The idea that they have a significant impact on biodiversity isn’t new, but most of the research on their ecological impact has focused on elephants, which are predominantly browsers, whereas white rhino are grazers.

Rhinos were reintroduced to Kruger in the 1960s, and until the past three or four years had thrived, with numbers estimated to have reached as many as 10,000 or more. Researchers from the Swedish University of Agricultural Sciences wanted to investigate the impact of grazing rhinos on Kruger’s grassland. They carried out a ‘recolonisation experiment’, in which they identified landscapes with high rhino densities, that had been recolonised early, and others that were recolonised more recently and had lower densities of rhino. They then looked at the structure of grassland on 40 transects, totalling 30km. They found there was greater short grass cover in the high rhino impact landscapes, which also had around 20 times more ‘grazing lawns’, where the grass was cropped short.

Professor Joris Cromsigt, who carried out the Kruger research with colleague Mariska te Beest, told Project African Rhino this week that while the findings provided empirical evidence that white rhino had started to change the structure and composition of the park’s savanna grasslands, more research was needed on the ecological implications for other species.

‘We still know very little about this aspect of grazing lawns,’ he said. ‘The few studies that are available suggest that certain short-grass specialist birds, such as crowned lapwing, African pipit and Sabota lark, benefit, and perhaps invertebrates such as certain grasshopper species. It is quite likely that a whole range of species in fact benefit, but we just lack the empirical data to support this claim.’

Crowned lapwing: a short grass species that may benefit from rhino grazing

Professor Cromsigt also pointed out that while some species benefited from an increase in grazing lawns, others may suffer. ‘Medium-tall grass specialists, such as sable antelope and perhaps waterbuck, would suffer from very high proportions of lawn cover. Similarly, tall grass specialists among bird species would also struggle.’

Kruger sable: adversely impacted by rhino activity?

‘In short, a heterogeneous grass layer with grazing lawns interspersed with tall grass patches probably promotes biodiversity most. But what the optimal balance between short and tall grass is, is in fact one of the big remaining questions,’ he explained.

The impact of rhinos on grassland heterogeneity appears to have been much less in Kruger than in Kwazulu-Natal’s Hluhluwe iMfolozi Park (HiP), where Professor Cromsigt has been studying rhinos since 2002.

‘HiP is where the white rhino was saved from extinction, and from this park we know that rhino can have strong ecological effects. I was curious to learn about their effects in other parks, which brought me to Kruger around 2007,’ he explained. ‘At that time in Kruger, there was, ironically enough, concern about the strongly increasing white rhino population and their possible impact. By the time I started the study in 2010/2011, the concerns about high rhino numbers had of course drastically changed into concerns about poaching…’