Introduction
An important role of ecologists is to explain and predict spatial
differences in species richness. To this end, niche and neutral theories
have been developed (Abrams 1983; Hubbell 2005; Schwilk & Ackerly 2005;
Gravel et al . 2006; Kadmon & Allouche 2007; Levine &
HilleRisLambers 2009; Hart et al . 2017). However, both two
theories do not predict the effects of space size that can influence
population size and thus population extinction risk on species richness,
and the potential prediction that unlimited species number of niche
theories and the prediction that absolute species equivalence of neutral
theories are contrary to numerous observations (Abrams 1983; Rohde 1992;
Rahbek 1995; Hubbell 2005; Tilman 2005 Nogues-Bravo et al . 2008;
Mellard et al . 2012; Mandal et al . 2018). Most studies
that have been conducted to reconcile the two theories are focused on
the variously contradictory predictions of the two theories, such as
differences and equivalences of species, competitive exclusions and
stochastic extinction, and environmental filters and dispersal
limitations (Schwilk & Ackerly 2005; Tilman 2005; Gravel et al .
2006; Kadmon & Allouche 2007; Peng et al . 2016; Mitchellet al . 2019). As a result, there are few comprehensive insights
into species richness, and such insights should link space size with
deterministic and stochastic processes and overcome the shortcomings
that unlimited species numbers and absolute equivalences.
Here, the environmental gradient per unit space (EGUS) metric, also
termed the environmental spatial change rate, is presented. The EGUS of
an area positively correlates with the environmental gradient in the
area. When each environmental gradient is regarded as an optimal niche,
the EGUS in an area consequently limits the number of species in an
area. The EGUS should positively
correlate with environmental filter and dispersal limitation and
negatively correlate with space size which is occupied by an optimal
niche and therefore is regarded as environmental capacity of a species.
Further, this metric is incorporated with stochastic abundance change of
each species, stochastic migration and equivalently average birth, death
and dispersal of species along environmental gradients. In this way, the
space size, differences and equivalences of species, and deterministic
and stochastic processes are integrated through the EGUS.
The EGUS model is developed to demonstrate how such integrations
influence species richness. An investigation of the relationship between
algal richness and water EGUS in a lake and a river is conducted. If the
simulation results are consistent with the observations, the EGUS should
be an importantly measurable driver of species richness.