Abstract
By defecating grasses into aquatic systems at massive scales and intensities, hippos can initiate complex changes to aquatic ecosystems. However, consequent effects on food webs are not well understood, particularly regarding shifts in basal resource contributions to consumer diets and their physiological condition. Here, we use fatty acid analysis to show that dense hippo aggregations and high dung loading are associated with (1) alterations to basal resource pools, (2) reduced quality of sediment organic matter and (3) increases in terrestrial and bacterial biomarker levels, but declines in those of diatoms in estuarine secondary consumers. While hippo defecation can increase boundary permeability between terrestrial and aquatic systems, our findings indicate that this may lead to a shift from a microphytobenthic food web base to one with increasing bacterial contributions to higher consumers. Our findings expand understanding of the mechanisms by which an iconic African megaherbivore indirectly structures aquatic ecosystems.
Highlights
By defecating grasses into aquatic systems at massive scales and intensities, hippos can initiate complex changes to aquatic ecosystems
These studies have challenged the notion that dung inputs act as subsidies in aquatic ecosystems by strengthening bottom-up trophic processes and facilitating increases in consumer biomass/abundance[13,14,15]
Essential fatty acid (EFA) levels provide information on the physiological health of consumers based on the premise that consumption of high quality trophic resources leads to greater essential fatty acid (EFA) quantities in c onsumers[22,34,36,37]
Summary
By defecating grasses into aquatic systems at massive scales and intensities, hippos can initiate complex changes to aquatic ecosystems. Hippo populations translocate roughly 36 tons of grass into the Mara River system daily[6], while annual inputs of 5,840 tons have been estimated for Lake Naivasha (Kenya)[7] Based on these estimates, it is doubtful whether other natural processes could replicate the scale of hippo-mediated resource transfers from terrestrial to aquatic ecosystems. Specific fatty acid biomarkers can provide critical information on the relative importance of dietary resources for consumers, pathways by which basal resources are incorporated into food webs and the physiological condition of consumers in relation to diet. Essential fatty acid (EFA) levels provide information on the physiological health of consumers based on the premise that consumption of high quality trophic resources leads to greater EFA quantities in c onsumers[22,34,36,37]. EFAs are obtained by animals almost exclusively through consumption of aquatic primary producers, directly or indirectly[34,38]
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