Are hippos Africa's most influential megaherbivore? A review of ecosystem engineering by the semi‐aquatic common hippopotamus

Abstract

Megaherbivores perform vital ecosystem engineering roles, and have their last remaining stronghold in Africa. Of Africa's remaining megaherbivores, the common hippopotamus (Hippopotamus amphibius) has received the least scientific and conservation attention, despite how influential their ecosystem engineering activities appear to be. Given the potentially crucial ecosystem engineering influence of hippos, as well as mounting conservation concerns threatening their long‐term persistence, a review of the evidence for hippos being ecosystem engineers, and the effects of their engineering, is both timely and necessary. In this review, we assess, (i) aspects of hippo biology that underlie their unique ecosystem engineering potential; (ii) evaluate hippo ecological impacts in terrestrial and aquatic environments; (iii) compare the ecosystem engineering influence of hippos to other extant African megaherbivores; (iv) evaluate factors most critical to hippo conservation and ecosystem engineering; and (v) highlight future research directions and challenges that may yield new insights into the ecological role of hippos, and of megaherbivores more broadly. We find that a variety of key life‐history traits determine the hippo's unique influence, including their semi‐aquatic lifestyle, large body size, specialised gut anatomy, muzzle structure, small and partially webbed feet, and highly gregarious nature. On land, hippos create grazing lawns that contain distinct plant communities and alter fire spatial extent, which shapes woody plant demographics and might assist in maintaining fire‐sensitive riverine vegetation. In water, hippos deposit nutrient‐rich dung, stimulating aquatic food chains and altering water chemistry and quality, impacting a host of different organisms. Hippo trampling and wallowing alters geomorphological processes, widening riverbanks, creating new river channels, and forming gullies along well‐utilised hippo paths. Taken together, we propose that these myriad impacts combine to make hippos Africa's most influential megaherbivore, specifically because of the high diversity and intensity of their ecological impacts compared with other megaherbivores, and because of their unique capacity to transfer nutrients across ecosystem boundaries, enriching both terrestrial and aquatic ecosystems. Nonetheless, water pollution and extraction for agriculture and industry, erratic rainfall patterns and human–hippo conflict, threaten hippo ecosystem engineering and persistence. Therefore, we encourage greater consideration of the unique role of hippos as ecosystem engineers when considering the functional importance of megafauna in African ecosystems, and increased attention to declining hippo habitat and populations, which if unchecked could change the way in which many African ecosystems function.

Small‐scale fires interact with herbivore feedbacks to create persistent grazing lawn environments

“…white rhino) in KNP and elsewhere (Nhleko et al., 2022) are likely to lead to the homogenization of grassland structure and an increase in tall grass (Cromsigt & te Beest, 2014), potentially limiting populations of short grass specialists. While other grazers consume short grass, megagrazers are uniquely capable of creating grazing lawns (Voysey et al., 2023; Waldram et al., 2008). Our findings suggest that the application of small‐scale fires, especially in the early dry season, could alleviate some of this homogenization and serve as an important management tool for the maintenance of diverse herbivore communities.…”

“…Grazing lawns in savanna ecosystems can span several square meters to hectares in extent (Archibald, 2008) and exhibit variable levels of persistence that can span from a few months (Cromsigt & Olff, 2008; Davies et al., 2016) to decades (Novellie & Gaylard, 2013), with lawn persistence dependent on a sufficient abundance of megagrazers, such as white rhinoceros ( Ceratotherium simum ) and common hippopotamus ( Hippopotamus amphibius ), on the landscape (Cromsigt & te Beest, 2014; Voysey et al., 2023; Waldram et al., 2008). In locations with low or decreasing megaherbivore abundance, expanding grazing lawns beyond hotspots of nutrient enrichment, such as riparian areas (Kanga et al., 2013), termite mounds (Davies et al., 2016), sodic sites (Grant & Scholes, 2006; Verweij et al., 2006) or abandoned cattle corrals (Porensky & Veblen, 2015), remains a challenge.…”

Human- common hippo (Hippopotamus amphibius)-conflict in the Dhidhessa Wildlife Sanctuary and its surrounding, Southwestern Ethiopia

“…Furthermore, the conservation strategy should also entail community participation in the coordination of local and district management offices, as well as community education, factory workers, sugarcane guardians, and awareness campaigns [55]. The primary subjects of these educational and communication programmes should be the cultural importance of hippos to communities, their role as ecosystem engineers [56], and their worth as a tourist attraction. We also suggest conducting surveys of the PLOS ONE population, ecology, and distribution of hippos, examining the role of hippos in disease transmission to cattle, conducting experimental tests on how hippos respond to deterrent methods in order to implement the most effective mitigation approaches, and quantifying the nature of anthropogenic-related threats to hippos and their habitats in the area.…”

Following the feeder: A global synthesis of disturbance‐based foraging associations of birds

“…With respect to disturber diet, predatory disturbers (insectivores, carnivores, piscivores and omnivores) and particularly insectivores were the most commonly followed by birds. Large herbivores such as hippopotamus (Hippopotamus amphibius) and elephants (Loxodonta africana) were responsible for relatively few disturber observations, despite being considered ecosystem engineers and hubs of a wide variety of other types of species interactions (Haynes, 2012;Pringle, 2008;Voysey et al, 2023). Heymann and Hsia (2015) argued that the proportion of wasted food, not necessarily diet, should be the strongest predictor of whether a disturber is followed.…”