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Ecological niche differences are necessary for stable species coexistence
but are often difficult to discern. Models of dietary niche
differentiation in large mammalian herbivores invoke the quality,
quantity, and spatiotemporal distribution of plant tissues and
growth-forms but are agnostic towards food-plant species identity.
Empirical support for these models is variable, suggesting that additional
mechanisms of resource partitioning may be important in sustaining
large-herbivore diversity in African savannas. We used DNA metabarcoding
to conduct a taxonomically explicit analysis of large-herbivore diets
across southeastern Africa, analyzing ~4,000 fecal samples of 30 species
from 10 sites in 7 countries over 6 years. We detected 893 food-plant taxa
from 124 families, but just two families—grasses and legumes—accounted for
the majority of herbivore diets. Nonetheless, herbivore species almost
invariably partitioned food-plant taxa; diet composition differed
significantly in 97% of pairwise comparisons between sympatric species,
and dissimilarity was pronounced even between the strictest grazers (grass
eaters), strictest browsers (non-grass eaters), and closest relatives at
each site. Niche differentiation was weakest in an ecosystem recovering
from catastrophic defaunation, indicating that food-plant partitioning is
driven by species interactions, and stronger at low rainfall, as expected
if interspecific competition is a predominant driver. Diets differed more
between browsers than grazers, which predictably shaped community
organization: grazer-dominated trophic networks had higher nestedness and
lower modularity. That dietary differentiation is structured along
taxonomic lines complements prior work on how herbivores partition plant
parts and patches and suggests that common mechanisms govern herbivore
coexistence and community assembly in savannas.
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