Terrestrial Palaeoecology and Global Change

Chapter 9. Crises
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lobes, making its second entry in the latest Permian (Krassilov, 1999b; Fig. 128). A
Cretaceous example is Czekanowskia, a widespread pioneer of Mesozoic peatlands,
exceedingly rare over the Late Cretaceous in general, but prominent in the Maastrichtian
dinosaur beds of Amur Basin (Bugdaeva, 2001). Encephalartites, a semiextinct
Mesozoic cycadophyte, is likewise common in the contemporaneous deposits of Koryak
Highlands (Krassilov et al., 1988) while the relatively poor KTB assemblage of Augustovka,
western Sakhalin, contains two such cycadophytic genera, Pterophyllum and
Cycadites, giving it an archaic mid-Cretaceous aspect (Krassilov 1979; Fig. 146).
An association of macromutational newcomers with Lazarus forms is a distinctive
feature of crisis communities that combine a low diversity with a high morphological
disparity and morphological polymorphism of dominant species (IX.9.3).
IX.8. Recovery
In plant communities, recoveries after local crises tend to restore the pre-existing
structure. The models of recovery based on recent examples (Carr-Kitchell, 1980; Barry
et al., 1991; Rosenzweig & McCord, 1991; Frederiksen, 1994) emphasize the significance
of an initial diversity and of speciation rates that are fostered by vacant ecological
niches as well as by immigration.
The climax cut-off model (Krassilov, 1992a; IX.6) maintains that, in a stressful environment,
ecological succession is halted at an early stage never reaching the potential
climax. In effect, the recovery of climax species is hampered and, in the long sustained
disclimax situations, might never occur. Major ecological crises truncate ecological successions
over a wide range of terrestrial and marine habitats, thus driving the climax species
to extinction. This explains the great extinctions that are recorded as nearly simultaneous
disappearances of dominant forms over a wide range of habitats.
The post-crisis recoveries eventually bring forth new dominants that are recruited
from among pioneer species of pre-existing, or recently arising, taxa. Thus, dinosaurs
recovered, with new dominant forms, after their near-extinction over the mid-Cretaceous
crisis but were replaced by mammals over the KTB crisis (IX.5).
The recovery or non-recovery of a formerly dominant group basically depends on its
ecological status, with severity of disturbance and competition as additional factors. Recovery
usually involves pioneer species or, in a broader sense, taxonomic groups having
their representatives in the pioneer/early seral stages. It is problematic for the taxa represented
by climax forms only. In biotic communities, the pioneer and seral species may
belong to the same higher taxon (evolutionary grade) as the climax species. For instance,
in broadleaved forests, the climax (lime, oak) and seral (poplar, maple) dominants are of
the same evolutionary grade. The recovery of the lime–oak climax is impeded in heavily
polluted environments but the broadleaved forest may recover through its persistent
seral stages.