Discussion
Niche partitioning is probably the most commonly cited mechanism to
explain coexistence in diverse communities, including herbivores (Prins
& Olff 1998; Arsenault & Owen-Smith 2011; Macandza et al.2012a; Pansu et al. 2019). We showed that the stabilizing effect
of partitioning is sufficient to maintain diversity of herbivore
communities, even over-compensating for differences in species’ fitness.
The majority of populations we studied experienced niche partitioning
that averaged around 70% more than necessary to satisfy coexistence
conditions. Thus, while coexistence is not possible in its absence, the
amount of partitioning that occurs is excessive given the relatively
small fitness differences between species.
An excess of niche partitioning has previously been demonstrated for
some plant communities (Adler et al. 2010), and our results for
mammalian herbivores suggest that over-stabilization may be a general
ecological phenomenon. High levels of resource niche partitioning have
been found in a number of herbivore communities (Gordon & Illius 1989;
Hopcraft et al. 2012; Kartzinel et al. 2015; and others),
although there are also instances where such niche structure is absent
(Prins et al. 2006; de Iongh et al. 2011). Our study
considers multiple assemblages simultaneously, and thus confirms that
widespread partitioning is the norm, even if there are cases where it is
less pronounced. Moreover, our results probably underestimate the full
extent of resource partitioning that exists, because isotopic niches do
not capture the complete array of herbivore trophic niches. Herbivores
utilize different plant species, plant organs of varying nutritional
value, sward or canopy heights, or microhabitats (Bell 1971; Demment &
Van Soest 1985; du Toit 1990; Cromsigt & Olff 2006; Arsenault &
Owen-Smith 2008; Kleynhans et al. 2011; Macandza et al.2012b; Kartzinel et al. 2015), but these factors do not vary
systematically in isotopic compositions in our study systems (Codronet al. 2013).
Over-stabilization implies that niche partitioning has other ecological
benefits besides facilitating coexistence. Elevated population growth
rates could act as a buffer against competitive exclusion under variable
environmental conditions. Even species with positive population growth
rates can go locally extinct due to demographic and environmental
stochasticity (Pande et al. ; Lande 1993; Adler & Drake 2008).
So, rather than simply being a condition for coexistence, niche
partitioning could confer demographic resilience to populations to avoid
stochastic extinctions. Here, we aimed only to quantify stabilizing
effects, and so our simulations are based on models that are purely
deterministic. Incorporating stochasticity, especially for herbivores
whose life cycles are longer than the timescales of most environmental
variations, appears to be a promising step towards resolving how an
excess of niches evolves.
Fitness equivalence seems to affect herbivore coexistence more than
niche partitioning. In our study, species’ intrinsic reproductive rates
were broadly similar over the range of body sizes considered
(~40 to 600 kg), ensuring that coexistence is possible
over virtually the full range of niche overlaps that occurred. We
propose that evolutionary constraints linked to herbivory act to
equalize fitness. One thing all herbivore species do better than other
animals is exploit vegetation as a resource. Their unique morphological
and physiological characteristics related to perception, ingestion, and
digestion, enable them to extract sufficient energy from this low
quality resource in ways that other animals cannot (Foose 1982; Van
Soest 1994; Clauss et al. 2008). Further, many of these traits
have been linked not only with differences in botanical composition of
the diet (reviewed in Codron et al. 2019), but also to species’
competitive abilities within particular diet niches (Murray & Illius
2000; Codron & Clauss 2010; Pansu et al. 2019). Such life
history traits, along with reproductive parameters, are by definition
the ultimate proxies for fitness. In the ecophysiological literature,
however, most traits are considered in a functional perspective that
assumes a certain interplay between form and function will result in a
fitness advantage (Janis & Ehrhardt 1988; Murray & Illius 2000; Codronet al. 2008; Fraser & Theodor 2011). But, it is becoming
increasingly clear that i) morphophysiological traits do not necessarily
preclude particular diet niches, and ii) specific features evolve
alongside others, reducing trade-off costs in a way that enables species
to be, often, as adept at extracting resources from a given niche as
their competitors (Codron & Clauss 2010; Fraser & Rybczynski 2014;
Codron et al. 2019).
Equality in fitness is more likely among species within clades than
between them. Bursts of adaptive radiations occur within clades
(Schluter 2000), either due to invasion of new niches or due to
incumbent replacement (Rosenzweig & McCord 1991), and
reproduction-related life history fitness emerges as a major
characteristic of clades with little differentiation of species within
taxa (Clauss et al. 2020). Coexistence is thus biased towards
multiple species of a specific clade rather than species from multiple
clades. In our case, the dominating taxa are ruminants. Expanding
similar analyses to include additional taxa, such as rhinoceroses,
elephants, and rodents, could possibly help identify further
characteristics required to allow coexistence in the presence of niche
overlap.