Species interactions – species richness and endemism effects upon ploidy state
The set and number of macrophyte species present within the area of a given gridcell are measures of the assemblage which integrate all the various environmental stress and disturbance factors (Grime, 1979) that individually appear to affect ploidy state occurrence. Diversity is one such measure, and our findings suggest that macrophyte species richness (both for total species and endemics) has some influence on, or at least an association with ploidy state in macrophytes. Low interspecific competition (suggested by low number of species present per unit area, though competition is also strongly influenced by other characteristics of species assemblage in plants, especially production: Grime, 1979) probably acts to increase the incidence of polyploidy (Rice et al., 2019). The number of macrophyte species present per unit area of the planet’s surface decreases with increasing distance from the Equator, and particularly so at high latitudes (Murphy et al., 2019, 2020), and it is notable that the incidence of polyploidy in macrophyte assemblages increases within this reducing alpha-diversity of macrophytes at high latitudes. However, if we look at the percentage of polyploid species vs. latitude there is no strong change until higher than 40 or even 50°absolute latitudes, i.e. once cool-temperate conditions are reached, and temperature variation at a given latitude is hence also likely to be driving polyploid occurrence. This fits with our findings for latitude-associated climatic variables like temperature and evapotranspiration which are good predictors of ploidy state in our dataset.
As expected, there are differences in the mean ploidy state of gridcells occurring in the six target ecozones, primarily relating to differences in climate between the temperate and more tropical ecozones. These observations on geographical variations in ploidy state occurrence for ecozone-endemic macrophyte species lend support to the evidence that we found regarding the value of endemic species richness in predicting mixed ploidy state using BRT analyses.
The predictive power of the number of endemic taxa in a grid cell on the percentage of mixed ploidy species was remarkable, explaining 49% of the total variation in the percentage D&P. However, it is hard to determine whether endemic species richness has a direct effect on ploidy state or whether endemic species richness is perhaps more likely just a proxy summary of multiple factors that actually drive this pattern (see schematic summary diagram, Fig. 1b): notably climatic (annual temperature range, evapotranspiration) and landscape factors (area of water body and anthropic disturbance).
Lastly, under this heading, we need to emphasize the possible effects of information bias, since endemics, especially those of tropical regions, are under-represented in the macrophyte karyological literature (Ramsey & Ramsey, 2014), and we do not know the possible impact of this upon our results relating to macrophyte endemism. Future improvement in knowledge of ploidy state in such plants may help rectify this situation.