Page 120 - Introduction to Paleobiology and The Fossil Record

PALEOECOLOGY AND PALEOCLIMATES 107

Box 4.7 Ecology of extinction events

We now have a massive amount of data across all the big fi ve Phanerozoic extinction events,
but are taxon counts a good guide to the severity of each extinction? Probably not! There is a
strong ecological dimension to each event. George McGhee and his colleagues (2004) have
ranked the ecological severity of each event and the order of severity is in fact different from that

established from taxon counts. First, the ecological impacts of the five Phanerozoic biodiversity crises

were not all similar (Table 4.2). Second, ranking the five Phanerozoic biodiversity crises by ecological
severity shows that the taxonomic and ecological severities of the events are decoupled. Most marked
is the end-Cretaceous biodiversity crisis, the least severe in terms of taxonomic diversity loss
but ecologically the second most severe. The end-Ordovician biodiversity crisis was associated
with major global cooling produced by the end-Ordovician glaciations; it prompted a major loss of
marine life, yet the extinction failed to eliminate any key taxa or evolutionary traits, and thus was
of minimal ecological impact. The decoupled severities clearly emphasize that the ecological impor-
tance of species in an ecosystem is at least as important as species diversity in maintaining an eco-
system. Selective elimination of dominant and/or keystone taxa is a feature of the ecologically most
devastating biodiversity crises. A strategy emphasizing the preservation of taxa with high ecological
values is the key to minimizing the ecological effects of the current ongoing loss of global
biodiversity.

Table 4.2 Classification of the ecological impacts of a diversity crisis.
Impact category Ecological effects
Category I Existent ecosystems collapse, replaced by new ecosystems post-extinction
Category II Existent ecosystems disrupted, but recover and are not replaced post-extinction
Subcategory IIa Disruption produces permanent loss of major ecosystem components
Subcategory IIb Disruption temporary, pre-extinction ecosystem organization re-established post-
extinction in new clades

used to develop models for both short- and now be mapped through time with some
long-term climate change? A range of geologi- degree of confi dence.
cal and paleontological criteria has helped
identify climatic zones through time (Fig.
4.24). Specific sedimentary rocks such as Climatic fl uctuations through time

calcretes (soils rich in calcium carbonate) Short-term trends
and evaporites (evaporated salts) can help
identify dry, arid climates whereas dropstones Many climatic events are short term, occur-
(stones that plummet from the bottoms of ring within a time span of 100 kyr. Many
melting icebergs into seabed sediments) and surface processes respond rapidly to climate
tillites (rocks and sand left behind by an change, for example the atmosphere and
advancing glacier) indicate polar conditions. ocean surface waters can change within days
These criteria have formed the basis for Chris- to a few years whereas the deep water of the
topher Scotese and colleagues’ reconstruction ocean basins and terrestrial vegetation may
of climates and paleogeogeography through take centuries to alter; the buildup of ice
time (http://www.blackwellpublishing.com/ sheets and associated sea-level changes,
paleobiology/). Global climate change can however, occur over millennia. Changes in