Study Area and Dataset
The study was carried out in nine protected areas in the grassland biome
in the summer rainfall region of central South Africa (supplementary
Fig. S1). This is an open, mostly treeless landscape, spread over a
generally flat topography (masl ~ 1 300m), although
three of the protected areas sampled are located in the eastern
escarpment (masl ~ 2 000m). Mean annual rainfall varies
between 300 and 700 mm per year, increasing from west to east. The
protected areas studied are fairly small (between 2 200 and 22 000 ha),
exclude large carnivores, and dispersal is limited to infrequent
translocation events, although animals are harvested in some years.
Resident herbivore populations live close to equilibrium, with average
population growth rates approaching 0 even after accounting for
harvesting events (Table 1), as is typical for many wildlife populations
(Fryxell et al. 2014).
Stable isotope data are from herbivore faeces collected during
excursions made over two years (2014 and 2015), during the dry season
months (May-October) when resources were more limiting and we expected
more pronounced patterns of niche partitioning. We collected faeces by
locating animals from within a vehicle and then following on foot, which
ensured that specimens were fresh and hence uncontaminated by soil and
insects, and had correct species designations. We aimed to collect
faeces from a minimum of 20 individuals per population (i.e. per species
per protected area), but ultimately included all populations for whichn ≥ 10. For simplicity, we treated both species of wildebeest
(Connochaetes taurinus and C. gnou ), and both species of
zebra (Equus quagga and E. zebra zebra ), as conspecifics,
because only one of each species was present in any one protected area.
The final dataset comprised 12 taxa, ranging in body size from
~40 to 600 kg, and included eight grazers, two browsers,
and two intermediate-feeders.
Faecal specimens were stored in brown paper bags, and air-dried indoors
to constant weight. Faeces were then ground in a coffee grinder, and
weighed separately in tin capsules. Specimens were combusted
individually in a Flash2000 elemental analyser and the resultant
CO2and N2gases introduced to a Delta V
Plus isotope ratio mass spectrometer via a Conflo IV gas control unit
(all instruments by Thermo Scientific, Bremen,
Germany).13C/12C and15N/14N ratios are presented in
standard delta (δ) notation relative to Vienna PeeDee Belemnite and
atmospheric N2 standards, respectively. Standard
deviations of repeated measurements of laboratory standards were less
than 0.2 ‰. Faecal δ13C data for most the populations
included here have previously been reported elsewhere (Malindie et
al. 2020). All data are included in supplementary Tables S1 and S2. The
analyses that follow were done in R v 3.5.2 (R_Core_Team 2015).