Discussion
The simulation results are consistence to the observations, which
demonstrates that the EGUS is a measurable driver of species richness.
Each environmental gradient is regarded as an optimal niche, a low EGUS
means a low number of optimal niches. Nevertheless, environmental
gradients exceeding the range in which optimal niches concentrate should
only provide optimal niches for few species that prefer stressful
environments and many environments are unavailable for species. Thus,
considerable increases in EGUS could not significantly increase the
numbers of species; further, such increases elevate abiotic filter and
dispersal limitation in the way that migrants move towards spaces
exhibiting their optimal niches, thereby reducing immigration success,
and such increases decrease the sizes of the space occupied by each
optimal niche. Population sizes are small within narrow spaces and both
extinction risks and dispersal limitation are high (Annette 2005; He
2012). The optimal niche number is relatively high in the areas with
middle EGUS except that such areas exhibit a high environmental stress
levels and those negative effects are relatively little. In this way,
both the simulations and the observations can show a unimodal algal
richness-EGUS relationship.
The consistency between the simulations and observations imply algal
species probably exhibit a difference of environmental preference and a
conditional equivalence. Species equivalence has been criticized because
a species often performs differently in different environments and
different species in same environments exhibit different performances
(Rohde 1992; Rahbek 1995; Nogues-Bravo et al . 2008; Mellardet al . 2012; Mandal et al . 2018). According to these
observations, species equivalence should be understood as a conditional
equivalence and the conditions should be same type species and average
performances of these species. The changes of performances of a species
along environmental gradients probably cause that the averages of
performances along a relatively wide environmental range are
approximately same among same type species such as among algae, amongK -strategy species and among r -strategy species. Thus,
again, when same type species randomly distributes among environments,
the averages of births, deaths, migrations, tolerances, dispersal
deaths, and environmental capacities of species are equivalent among
environmental levels. The results also imply that deterministic
competitions and environmental filters, and dispersal limitations and
stochastic abundances should influence algal richness, supporting our
predictions.
In addition to environmental range, fragmentation levels involving
habitat loss and habitat apart and the composition of factors like
nitrogen availability ratios and light spectra are often measurements of
the environmental property (Fahrig 2003; Schuler et al . 2017;
Mandal et al . 2018), but spatial scales are subjective. We can
employ the EGUS to connect fragmentation levels and compositions of
factors in each sampling position to sort the areas independently of
spatial scale and relative position of areas. Such classification is
objective and can indicate community boundary. Further, the EGUS can
profoundly influence immigrant success, population extinction and niche
availability. Therefore, the EGUS is an importantly measurable driver of
species richness, and the theory provides a potential understanding and
prediction of terrestrial plant richness and invasive plant spread when
the migrants in the study are regarded as propagules, especially, seeds
of annual plants.