The mean and range of regional salinities and temperatures, as
the main determinants of environmental heterogeneity, are given
in Table 1. Another potential predictor of heterogeneity and diversity,
the regional ecosystem surface area, is hard to define due to
lack of clear borders, except for landlocked embayment. Therefore
for consistency of cross-regional comparison we used the length of
the minimum spanning tree (MST) connecting the sampling stations.
MST (km) is a combination of vertices connecting all the
sampling stations in the geographic space, constrained to have the
shortest total length.