Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs?

Abstract

The tropical savannas of Africa have witnessed a dramatic reduction in native large mammalian herbivore populations. The consequences of these changes for terrestrial-aquatic food-web linkages are poorly documented. We used natural abundances of stable carbon and nitrogen isotopes (d 13 C, d 15 N) to determine spatial and temporal patterns in the importance of herbivore-mediated subsidies for consumers in the Mara River, Kenya. Potential primary producers (terrestrial C3 and C4 producers and periphyton) and consumers (invertebrates and fish) were collected during dry and wet seasons from different sites along the river, representing a gradient from forested highlands to natural savanna grasslands with high herbivore densities across mixed agricultural and livestock-dominated zones. Bayesian mixing models were used to estimate the relative contributions of terrestrial and algal sources of organic carbon supporting consumer trophic groups. Organic carbon sources differed for consumer groups and sites and with season. Overall, periphyton was the major energy source for most consumer groups during the dry season, but with wide 95% confidence intervals. During the wet season, the importance of terrestrial-derived carbon for consumers increased. The importance of C3 producers declined from 40 and 41% at the forested upper reaches to 20 and 8% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. The reciprocal increase in the importance of C4 producers was higher than expected based on areal cover of riparian vegetation that was mainly C3. The importance of C4 producers notably increased from 18 and 10% at the forested upper reaches to 33 and 58% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. This study highlights the importance of large herbivores to the functioning of riverine ecosystems and the potential implications of their loss from savanna landscapes that currently harbor remnant populations. Although the importance of C4 terrestrial carbon in most river systems has been reported to be negligible, this study shows that its importance can be mediated by large herbivores as vectors, which enhance energetic terrestrial-aquatic linkages in rivers in savanna landscapes.

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Moreover, the presence of beavers (Castor canadensis) enhanced cross-boundary resource subsidies by impounding streams which resulted in higher terrestrially derived organic material in habitats [105], or created suitable habitats for many other species such as bats through the habitat management [72]. Thus, mammals can enhance trophic and energetic aquatic-terrestrial linkage and affect ecosystem functioning in aquatic systems [86,106,107].…”

Declines in benthic macroinvertebrate community metrics and microphytobenthic biomass in an estuarine lake following enrichment by hippo dung

“…However, despite dominating gravimetrically and ecologically, hippos rank amongst the most understudied faunistic components on the continent1. The roles hippos play in physically engineering aquatic ecosystems, and the feedbacks transmitted to sympatric species is a particular gap in knowledge, though these have frequently been alluded to in the literature2345. The direct transfer of trophic resources from terrestrial to aquatic ecosystems is one of the most significant functions provided by hippos across landscapes in which they occur3678.…”

“…The roles hippos play in physically engineering aquatic ecosystems, and the feedbacks transmitted to sympatric species is a particular gap in knowledge, though these have frequently been alluded to in the literature2345. The direct transfer of trophic resources from terrestrial to aquatic ecosystems is one of the most significant functions provided by hippos across landscapes in which they occur3678. By consuming large quantities of terrestrial grasses by night and excreting this material into aquatic systems by day35910, hippos translocate trophic resources across terrestrial-aquatic boundaries at rates and magnitudes that are unlikely to be replicated by other natural phenomena.…”

“…The direct transfer of trophic resources from terrestrial to aquatic ecosystems is one of the most significant functions provided by hippos across landscapes in which they occur3678. By consuming large quantities of terrestrial grasses by night and excreting this material into aquatic systems by day35910, hippos translocate trophic resources across terrestrial-aquatic boundaries at rates and magnitudes that are unlikely to be replicated by other natural phenomena. In the Mara River, for example, it has been estimated that individual hippos defecate approximately 8.7 kg of grasses (wet weight) into the river per day, and that the entire population transfers 36 tonnes of grass into the river daily10.…”

A comprehensive biogeochemical record and annual flux estimates for the Sabaki River (Kenya)

“…Athi and Galana rivers) and Tsavo West (i.e. Tsavo River) 29 National Parks, considerably less than the ~4000 reported from the Masai Mara National Reserve where the research of 30 Masese et al (2015) was conducted. Supplementary Figure 1 highlights the high density of other large mammals 31 congregating around the Athi and Galana rivers, and though a smaller proportion of their total excrement will be released 32 directly to the river relative to hippopotami, the combined quantity may be a significant contribution to the riverine OC pool 33 under low flow conditions.…”