Organic matter and nutrient inputs from large wildlife influence ecosystem function in the Mara River, Africa

Abstract

Animals can be important vectors for the movement of resources across ecosystem boundaries. Animals add resources to ecosystems primarily through egestion, excretion, and carcasses, and the stoichiometry and bioavailability of these inputs likely interact with characteristics of the recipient ecosystem to determine their effects on ecosystem function. We studied the influence of hippopotamus excretion/egestion and wildebeest carcasses, and their interactions with discharge, in the Mara River, Kenya. We measured nutrient dissolution and decomposition rates of wildlife inputs, the influence of inputs on nutrient concentrations and nutrient limitation in the river and the influence of inputs on biofilm growth and function in both experimental streams and along a gradient of inputs in the river. We found that hippopotamus excretion/egestion increases ammonium and coarse particulate organic matter in the river, and wildebeest carcasses increase ammonium, soluble reactive phosphorus, and total phosphorus. Concentrations of dissolved carbon and nutrients in the water column increased along a gradient of wildlife inputs and during low discharge, although concentrations of particulate carbon decreased during low discharge due to deposition on the river bottom. Autotrophs were nitrogen limited and heterotrophs were carbon limited and nitrogen and phosphorus colimited upstream of animal inputs but there was no nutrient limitation downstream of inputs. In experimental streams, hippo and wildebeest inputs together increased biofilm gross primary production (GPP) and respiration (R). These results differed in the river, where low concentrations of hippo inputs increased gross primary production (GPP) and respiration (R) of biofilms, but high concentrations of hippo inputs in conjunction with wildebeest inputs decreased GPP. Our research shows that inputs from large wildlife alleviate nutrient limitation and stimulate ecosystem metabolism in the Mara River and that the extent to which these inputs subsidize the ecosystem is mediated by the quantity and quality of inputs and discharge of the river ecosystem. Thus, animal inputs provide an important ecological subsidy to this river, and animal inputs were likely important in many other rivers prior to the widespread extirpation of large wildlife.

Context dependency of animal resource subsidies

“…A number of studies have examined the influence of animal excretion on ecosystem function (Atkinson et al, ). Far fewer have studied the influence of egestion, largely due to difficulties estimating it in the field, although research that has explicitly investigated egestion has shown it can play an important role in biogeochemical cycles and food webs (Joyce, Warren & Wotton, ; Halvorson et al, ; Masese et al, ; Atkinson et al, ; Stears et al, ; Subalusky et al, ). Animal tissue, primarily through gametes and carcasses, is typically high in nutrients as well as carbon, but also requires mineralization for uptake and may break down over a longer time, providing a high‐quality but potentially more complex resource than excretion and egestion (Wipfli et al, ).…”

“…carcasses) or are high in carbon (e.g. hippo faeces), mineralization of these subsidies could shift the ecosystem towards increased heterotrophy and a decline in NEP (Christenson et al, ; Marcarelli et al, ; Subalusky et al, ), although the effect of subsidies on heterotrophic communities has been studied much less frequently (Atkinson et al, ). The release of inorganic nutrients after mineralization may then stimulate either autotrophs or heterotrophs, and the resulting outcome may be mediated by other factors, including temperature, light and nutrient availability and community composition (Roberts & Howarth, ).…”

“…By travelling further inland in response to lower donor ecosystem productivity, hippos spend a longer time feeding in the grasslands, which leads to less time spent in the river and therefore less excretion/egestion in the aquatic ecosystem (Subalusky et al, ). The magnitude of hippo inputs entering the recipient ecosystem appears to drive large differences in effects on ecosystem function, with low levels of input increasing primary and secondary production, and high levels decreasing production, particularly during low discharge when inputs concentrate on the river bottom (Masese et al, , ; McCauley et al, ; Dawson et al, ; Stears et al, ; Dutton et al, ; Subalusky et al, ). Hippo inputs can become so concentrated they lead to hypoxic conditions in the hippo pools and hypoxic floods and fish kills downstream, a condition Dutton et al .…”

Potential ecological and socio-economic effects of a novel megaherbivore introduction: the hippopotamus in Colombia

“…An individual can transport 750 kg dry mass per year of carbon and nutrients from terrestrial to aquatic ecosystems via defecation and excretion (Subalusky et al, 2015). These resource subsidies can influence aquatic productivity in complex ways (Subalusky et al, 2018). Low to moderate levels of resource input may stimulate productivity and provide food resources for aquatic insects and fish (Masese et al, 2015).…”

Ecosystem effects of the world’s largest invasive animal

“…The fertilizing effect of nutrients imported on aquatic ecosystems by hippos in Africa also varies seasonally. The ecosystem effects of hippos have been studied in savannah habitats of East Africa, where animals become concentrated in isolated pools during dry seasons of low river discharge (McCauley et al , Subalusky et al , Dutton et al , Stears et al , Subalusky et al , Schoelynck et al ). The impact of organic matter imported by hippos on aquatic ecosystems is often greatest in dry seasons when their densities are high and export through river flow is low (McCauley et al , Stears et al ).…”