Terrestrial Palaeoecology and Global Change

296 Valentin A. Krassilov. Terrestrial Palaeoecology
Extrapolation of (z) is a weak point of the estimates because a palaeobiome might
differ substantially from its extant ecological analogue in the spatial heterogeneity of (a).
Conceivably, if species richness calculated on the basis of the total diversity and turnover
rates (above) is in excess of that derived from the species-area equation, then spatial
heterogeneity might have been greater than in the present-day analogue, implying the
higher values of intercommunal diversity β, interregional diversity γ or both.
The heterogeneity constant (z) is maximal in the case of disjunctive floristic regions.
Z=0.27 indicates complete geographic isolation of floristic regions when derived from
the canonical equation (Preston, 1962):
(A/C) 1/z – (B/C) 1/z = 1
(A, B = species numbers of two floristic regions, C = the number of their shared
species).
Palaeogeographic implications of the threshold z value may pertain to “exotic terrains”
supposedly transported from remote locations (examples in V.6.3). Z greater
than 0.27 would confirm a former separation of continental blocks while a lesser z
would suggest land continuity. At least for the northern and southern Cathaysian floras
(species lists in Li & Wu, 1996; Li, 1997), z ~10 is far beneath the threshold value,
indicating continuity of respective “terrains” rather than their wide separation over the
eastern Tethys.
VIII.3.2. Species richness and disparity
Diversity as a number of different entities in a sample does not tell us how distinct
these entities are. Two assemblages of equal species richness may differ in distinctness
of their constituent species. Hence, there are at least two components of species-level
diversity: the richness (diversity in a common but inexact usage of the term) and disparity
in terms of morphological, etological or biochemical distinctness. Richness might grow
by addition of sibling species without an appreciable increase in morphological disparity.
On the other hand, an introduction of a single extremely deviant form would increase the
disparity with a negligible effect on the richness.
Disparity is measured as a mean pairwise difference (distance) between taxa
estimated for a selected set of characters (Briggs et al., 1992; Foote, 1995). As
such, disparity relates to the initial adaptive radiation, subsequent divergence, and,
inversely, to survival of intermediate types (Jernvall et al., 1996). A high initial disparity,
as in proangiosperms (Krassilov, 1997a), means either saltational or polyphyletic
origins, with convergence at a later stage. In contrast, a conventional mode of
gradual divergence implies a low initial disparity that increases with introduction of
unique characters over time.
In the early land plants, a high disparity/low diversity of growth habits, in particular of
the dichotomous, monopodial or H-type branching patterns, stands in sharp contrast with