Livestock as vectors of organic matter and nutrient loading in aquatic ecosystems in African savannas

Abstract

Populations of large wildlife have declined in many landscapes around the world, and have been replaced or displaced by livestock. The consequences of these changes on the transfer of organic matter (OM) and nutrients from terrestrial to aquatic ecosystems are not well understood. We used behavioural data, excretion and egestion rates and C: N: P stoichiometry of dung and urine of zebu cattle, to develop a metabolism-based estimate of loading rates of OM (dung), C, N and P into the Mara River, Kenya. We also directly measured the deposition of OM and urine by cattle into the river during watering. Per head, zebu cattle excrete and/or egest 25.6 g dry matter (DM, 99.6 g wet mass; metabolism) - 27.7 g DM (direct input) of OM, 16.0–21.8 g C, 5.9–9.6 g N, and 0.3–0.5 g P per day into the river. To replace loading rates OM of an individual hippopotamus by cattle, around 100 individuals will be needed, but much less for different elements. In parts of the investigated sub-catchments loading rates by cattle were equivalent to or higher than that of the hippopotamus. The patterns of increased suspended materials and nutrients as a result of livestock activity fit into historical findings on nutrients concentrations, dissolved organic carbon and other variables in agricultural and livestock areas in the Mara River basin. Changing these patterns of carbon and nutrient transport and cycling are having significant effects on the structure and functioning of both terrestrial and aquatic ecosystems.

Drivers of water quality in Afromontane-savanna rivers

“…These data were collected from more than 150 sites spread out in the entire MRB in Kenya (Figure 1). Some of the data have been published in previous works, including Minaya et al (2013), Masese et al (2014aMasese et al ( , b, 2017Masese et al ( , 2022 and Iteba et al (2021). Although data were available from 2010 to 2018, most of the data used were collected in 2010, 2012, 2016 and 2018. To capture spatial and temporal variability in water quality across the entire river basin, sampling sites were in rivers draining a gradient of catchment land use from 100% forestry or grasslands to 100% agriculture.…”

“…In these landscapes, the abundance and biomass of large mammalian herbivores (both wildlife and livestock) also present a natural gradient characterized by low numbers in the forested uplands and high densities in the savanna grasslands in the lowlands. Rivers that drain these landscapes display a tight coupling with terrestrial ecosystems through the vectoring role of both livestock and large wildlife in transferring large amounts of organic matter and nutrients through egestion and excretion at watering and crossing points along the rivers (Subalusky et al, 2015;Iteba et al, 2021). African savanna landscapes are also emerging as new frontiers of land use change as human Frontiers in Environmental Science frontiersin.org 03 populations and their livestock migrate into marginal lands in semi-arid and arid areas to ease pressure on more productive and densely populated humid lands.…”

Water quality and ecology of Lake Kanyaboli, Kenya: Current status and historical changes

“…Excretory wastes from aquatic animals and livestock watering in the lake and feeder canal also contribute to the loading. Livestock grazing in the swamp also contribute to nutrient loadings from their excretion and egestion, and from runoff (Iteba et al, 2019). Yala Swamp is estimated to support an average of 1820 herds of cattle, 497 goats and 322 sheep daily (Mwaura et al, n.d.).…”

“…The cyanobacteria dominance is linked to the nutrient supply from the drainage basin (Wilfred et al, 2005). Further, livestock watering in such water bodies directly deposit large quantities of organic matter and nutrients from dung and urine during their excretion and egestion while watering (Iteba et al, 2019;Masese et al, 2020). These nutrients and carbon sources are fodder for the rapid proliferation of bacterio-and phytoplankton (Shevah, 2017).…”

Linking animal behavior to ecosystem change in disturbed environments

“…Likewise, changes in the activity of terrestrial herbivores can change the influx of nutrients to aquatic ecosystems and affect algae blooms and light penetration. For instance, the replacement of large wildlife with livestock on the African savanna has increased the transfer of organic matter and nutrient from terrestrial to aquatic ecosystems, with further consequences for these ecosystems (Iteba et al, 2021). The impact of consumers on vegetation can also alter the climate, given that the biomass and composition of vegetation influences humidity, temperature, and the release of water vapor and cloud formation (Green et al, 2017(Green et al, ).…”