Emergent niche structuring leads to increased differences from neutrality in species abundance distributions

Abstract

Species abundance distributions must reflect the dynamic processes involved in community assembly, but whether and when specific processes lead to distinguishable signals is not well understood. Biodiversity and species abundances may be shaped by a variety of influences, but particular attention has been paid to competition, which can involve neutral dynamics, where competitor abundances are governed only by demographic stochasticity and immigration, and dynamics driven by trait differences that enable stable coexistence through the formation of niches. Key recent studies of the species abundance patterns of communities with niches employ simple models with pre-imposed niche structure. These studies suggest that species abundance distributions are insensitive to the relative contributions of niche and neutral processes, especially when diversity is much higher than the number of niches. Here we analyze results from a stochastic population model with competition driven by trait differences. With this model, niche structure emerges as clumps of species that persist along the trait axis, and leads to more substantial differences from neutral species abundance distributions than have been previously shown. We show that heterogeneity in "between-niche" interaction strength (i.e., in the strength of competition between species in different niches) plays the dominant role in shaping the species abundances along the trait axis, acting as a biotic filter favoring species at the centers of niches. Furthermore, we show that heterogeneity in "within-niche" interactions (i.e., in the competition between species in the same niche) counteracts the influence of heterogeneity in "between-niche" interactions on the SAD to some degree. Our results suggest that competitive interactions that produce niches can also influence the shapes of SADs.

On the Consequences of the Interdependence of Stabilizing and Equalizing Mechanisms

“…Indeed, the definitions of niche overlap r and the fitness ratio k 1 =k 2 in the two-species framework (Chesson 1990(Chesson , 2000(Chesson , 2012 are motivated by MacArthur's (1970) influential consumer-resource model. This model serves as the prototype for a large class of consumer-resource models (May and MacArthur 1972;Tilman 1982;Chase and Leibold 2003;Koffel et al 2016;Rohr et al 2016;Rael et al 2018). Here, for this reason, we translate the stabilizing and equalizing mech-anisms into their mechanistic meanings using MacArthur's model.…”

“…This idea takes various forms in the literature. One body of thought considers the relative importance of deterministic versus stochastic processes in generating largerscale community patterns (Gravel et al 2006;Haegeman and Loreau 2011;Rael et al 2018). Another (which will be our focus here) argues that coexistence is best viewed from the angle of where communities lie in figure 1 (Adler et al 2007;Mayfield and Levine 2010): communities with a fitness ratio close to 1 and small niche overlap are highly stable and driven by niche differentiation, communities with large fitness differences and large niche overlap are unstable and preclude coexistence, a fitness ratio of 1 and maximal niche overlap lead to neutral coexistence, and so on.…”

Emergent neutrality in consumer-resource dynamics

“…On the other hand, the theory has been criticized on two fronts: first, it is unclear whether neutral theory could successfully predict a broader range of community properties, particularly regarding community dynamics from generational to geological timescales [19][20][21][22][23][24][25][26][27]. Second, it has been argued that the patterns that neutral theory successfully fits do not uniquely reflect the underlying ecological properties of the species involved; in particular, it is possible that summarized indices of community structure and dynamics may look neutral even when species are not ecologically equivalent [2,[27][28][29][30][31].…”

Emergent neutrality in consumer-resource dynamics

“…On the other hand, the theory has been criticized on two fronts: first, it is unclear whether neutral theory could successfully predict a broader range of community properties, particularly regarding community dynamics from generational to geological timescales (Clark and McLachlan, 2003;Volkov et al, 2004;Pigolotti et al, 2005;Ostling, 2012;Wang et al, 2013;Kalyuzhny et al, 2014;O'Dwyer et al, 2015;Fung et al, 2016;D'Andrea and Ostling, 2017). Second, it has been argued that the patterns that neutral theory successfully fits do not uniquely reflect the underlying ecological properties of the species involved; in particular, it is possible that summarized indices of community structure and dynamics may look neutral even when species are not ecologically equivalent (Chave et al, 2002;McGill et al, 2007;Chisholm and Pacala, 2010;Al Hammal et al, 2015;D'Andrea and Ostling, 2017;Rael et al, 2018).…”