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.