Introduction
Ecologist traditionally relied on the deterministic process, such as
environmental niche, and the coexistence of species was explained by a
stationary state of ecologically different species (Gause, 1934;
Hutchinson, 1957). Recently ecologist recognized also the role of
stochastic process. The neutral dynamics were derived from the primary
assumption of per capita equivalence among individuals and community
saturation, such as ecological drift, random speciation and migration,
and the coexistence of species was explained by a dynamical equilibrium
of ecologically equivalent species in a set of local communities
(Durrett & Levin, 1996; Hubbell, 2001; Rosindell, Hubbell, & Etienne
2011). In contemporary ecology, the reconciling of these divergent
ecological-evolutionary mechanisms is a promising perspective to explain
the origin and maintenance of biodiversity (Hubbell, 2001; Leibold &
Mcpeek, 2006; Munoz & Huneman, 2016; Rosindell et al., 2011).
Neutral dynamics were based on individual-based model. The models were
implemented mostly in the spatially-implicit and hierarchical
conjecture, in that the extinction was balanced by the speciation under
panmixis; while, the local extinction was balanced by the immigrats from
a metacommunity. The regional dynamics were predicted by population size
and speciation rate of metacommunity, and the local dynamics were
predicted by further two parameters of population size and dispersal
rate of local community (Hubbell, 2001; Etienne, 2005; Etienne, 2007;
Etienne & Oiff, 2004; Hankin, 2007; Munoz, Couteron, Ramesh, & Etienne
2007). However, the spatially-implicit conjecture seems inconsistent in
that the neutral dynamics assume panmixis in a metacommunity; whereas,
dispersal-limitation to each local community embedded in it (Etienne,
2007). The spatially-explicit conjecture was one step approach allowing
the speciation and dispersal-limitation across spatio-temporal scales. A
metacommunity was usually a portion of an infinite system, and the
models were implemented in a grid of local communities specifying
neighbourhood or more general dispersal kernel (Durrett & Levin, 1996;
Hubbell, 2001; Chave & Leigh, 2002; Chave, Muller-Landau, & Levin
2002; Pigolotti & Cencini, 2009).
Reconciling niche-based process and neutral dynamics in
spatially-implicit conjecture, ecologist tried to explain the community
assembly in successional patchy habitats, in that the vacant local
community created by perturbation was initially colonized by the
immigrants from a metacommunity under the ecological equivalence among
species; then, the competitive exclutions were followed in each local
community (Mouquet, Munguia, Kneitel, & Miller 2003). In stable
environment, the immigrants from a metacommunity was filtered along
environmental gradients, and the coexistence of species in each local
community was achieved under the per capita equivalence among
individuals in homogeneous environment (Jabot, Etienne, & Chave 2007;
Janzen, Haegeman, & Etienne 2015; Munoz, Ramesh, & Couteron 2014;
Munoz et al., 2018). In spatially-implicit conjecture; however, the
regional dynamics were not predicted but usually specified by a fixed
metacommunity. Strikingly, if the per capita ecological difference among
species contributed to the competitions in each local community or the
dispersals from a metacommunity; in general, the spatio-temporal scales
on which the per capita ecological difference and equivalence among
species contributed to metacommunity dynamics were credible assumpsions.
Reconciling niche-based process and neutral dynamics in
spatially-explicit conjecture, a metacommunity may be a portion of an
infinite system. The regional species pool may be already not free
parameter, and the divergent ecological-evolutionary mechanisms may
operate consistently at different spatio-temporal-environmental scales.
For instance, exploring the neutral model with nearest neighboring
communities, an approximate scale on which individuals were expected to
diffuse before speciation was predicted by a pair of speciation and
dispersal rates (Cencini, Pigolotti, & Muñoz 2012). In other words, the
area of model communities must exceed an approximate scale predicted by
these parameters to encompass at least one independent biogeographic
unit. In the synthetic model with periodic boundary condition, where the
environmental gradient repeats continuously across opposite sides of
system, the relative role of neutral dynamics may increase in an area
encompassing independent biogeographic units because the species
richness but the number of functional groups (i.e. guilds) may increase
through speciation.
In the present study, the individual-based model was implemented in the
two-dimensional grid with periodic boundary condition. In simulations,
the model parameters and the species properties in simulation outcomes
were known by researcher, and the focus was to detecting the
spatio-temporal-environmental scales on which the per capita ecological
difference and equivalence among species were emerged through divergent
ecological-evolutionary mechanisms. Exploring the model across a range
of parameters, I proved specifically three hypotheses. First, the
competitive exclusion may be advanced by the per capita ecological
difference among species so the number of guilds may achieve first to a
stationary state; while, the competitive exclusion may be retarded by
the per capita ecological equivalence among species so the species
richness may converge eventually to a dynamical equilibrium through
extinction-speciation balance (Leibold & Mcpeek, 2006). Second, the
relative role of neutral dynamics may increase in an area encompassing
independent biogeographic units. Third, the neutral dynamics may operate
among the species of each guild in patchy habitats (Leibold & Mcpeek,
2006; Economo & Keitt, 2008). The trajectory and contrast between
alternative processes establish the false premises that the niche-based
process is only important in each local community; while, the neutral
dynamics are only important in a set of local communities. I discuss
finally the implication of false premises and consistent patterns
observed in the present study for the performance of heuristic methods,
the manner of their applications, and the interpretation of their
results in field observations.