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.