Terrestrial Palaeoecology and Global Change

294 Valentin A. Krassilov. Terrestrial Palaeoecology
R = O – E
While Sp is an ecologically meaningful variable, St is not. For each time plane, tp, the
standing crop diversity, Sp’, is a function of initial diversity (Ss), time passed since the
beginning of the period (tp – ts), and of mean turnover rate (Rm) for the period:
Sp’ = Ss + Rm/t (tp – ts)
A crude estimate of species richness for a geographic realm can be obtained from
the species-area relationships. A vast literature on the subject (Arrhenius, 1920; Preston,
1962; MacArthur & Wilson, 1967; Malyshev, 1975; Williamson, 1988; Harte &
Kinzig, 1997; Gaston, 2000; Williamson et al., 2001) holds the relationships as conveyed
by parabolic equation:
y = ax z
(y = species number per area x, a = species number per unit area, z = an empirical
constant of spatial heterogeneity).
In the following example, the variables and constants of the species-area equation
are provisionally calculated on the basis of palaeogeography and the present-day analogues.
Area (x). Palaeovegetation zones can be considered as areas of uniform z values
characteristic of the respective biomes. Calculations are made for the area of the northern
temperate forested (nemoral) zone north of the evergreen/deciduous boundary that
is most distinct over times due to the distinctions in the life forms and taphonomy (IV.3,
VII.4, VIII.2.3). The ranges of nemoral biome vary with climate and sea-level, spreading
to about 38°-35°N in the mid-Permian and mid-Jurassic, to 43°N in the Early Palaeocene,
and constrained at 50°N (60°N in Atlantic sector) in the Late Cretaceous and
Eocene, when their extent was additionally reduced by boreal transgressions.
Spatial heterogeneity constant (z). A compilation of data for vegetation zones
indicates typical z values of 0.10 for boreal conifer forests (taiga), 0.14–0.18 for temperate
broadleaved (nemoral) zone, about 0.17 for Mediterranean-type vegetation,
and 0.20–0.22 for tropical forests (Malyshev, 1975). Since z depends on ecological
convergence rather than taxonomy, these values should be valid for the respective
palaeobiomes as well. The upper present-day value of z = 0.18 is provisionally accepted
here for the Permian to Palaeocene nemoral zones on assumption of their relatively
high heterogeneity.
Diversity per unit area (a). In the present-day plant diversity estimates, the unit
area varies from 10 2 to 10 4 sq. km. For palaeobiomes, the natural sampling unit is sedimentary
basin supplying material for the basinal taphoflora. A modal area of hydrological
basins is about 10 3 sq. km, within the range of the present day sampling areas. Hence
species numbers for well-known taphofloras represent the contemporaneous (a) values.
In the present analysis these values are derived from the following taphofloras:
– The mid-Permian of Cisuralia (Naugolnykh, 1998),
– The Early Triassic of Tunguska Basin and Taymyr (Mogucheva 1973;
Sadovnikov, 1997),