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.