DNA metabarcoding illuminates dietary niche partitioning by African large herbivores

Abstract

Niche partitioning facilitates species coexistence in a world of limited resources, thereby enriching biodiversity. For decades, biologists have sought to understand how diverse assemblages of large mammalian herbivores (LMH) partition food resources. Several complementary mechanisms have been identified, including differential consumption of grasses versus nongrasses and spatiotemporal stratification in use of different parts of the same plant. However, the extent to which LMH partition food-plant species is largely unknown because comprehensive species-level identification is prohibitively difficult with traditional methods. We used DNA metabarcoding to quantify diet breadth, composition, and overlap for seven abundant LMH species (six wild, one domestic) in semiarid African savanna. These species ranged from almost-exclusive grazers to almost-exclusive browsers: Grass consumption inferred from mean sequence relative read abundance (RRA) ranged from >99% (plains zebra) to <1% (dik-dik). Grass RRA was highly correlated with isotopic estimates of % grass consumption, indicating that RRA conveys reliable quantitative information about consumption. Dietary overlap was greatest between species that were similar in body size and proportional grass consumption. Nonetheless, diet composition differed between all species-even pairs of grazers matched in size, digestive physiology, and location-and dietary similarity was sometimes greater across grazing and browsing guilds than within them. Such taxonomically fine-grained diet partitioning suggests that coarse trophic categorizations may generate misleading conclusions about competition and coexistence in LMH assemblages, and that LMH diversity may be more tightly linked to plant diversity than is currently recognized.African savannas | body size | competition | coexistence | ungulates D ietary niche partitioning contributes to the origin and maintenance of biodiversity by alleviating competition and allowing ecologically similar consumers to coexist (1-3). Of the many faunas in which this mechanism is thought to play a major structuring role, few have inspired as much research and debate as the diverse assemblages of large mammalian herbivores (LMH) (≥5 kg) that occur in African savannas and that were globally widespread before the Pleistocene extinctions (4-7). These assemblages often comprise 10-25 species (8, 9) that forage within the same areas. How can so many apparently generalist consumer species coexist on a limited range of resource types?Attempts to address this question have yielded several key insights. One is that sympatric LMH vary in their proportional consumption of grasses versus browse (i.e., all nongrasses, including trees, shrubs, and forbs) (10). Thus, LMH can achieve dietary separation along a spectrum from pure grazers to pure browsers, and species can be categorized as predominantly grazers, browsers, or mixed feeders. In recent decades, stable-isotope analysis of C 3 (browse) versus C 4 (grass) consumption has been used to quantify this continu...

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