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Box 4.7 Ecology of extinction events

We now have a massive amount of data across all the big five 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 importance of species in an ecosystem is at least as important as species diversity in maintaining an ecosystem. 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 Category II Subcategory IIa Subcategory IIb

Existent ecosystems collapse, replaced by new ecosystems post-extinction Existent ecosystems disrupted, but recover and are not replaced post-extinction Disruption produces permanent loss of major ecosystem components Disruption temporary, pre-extinction ecosystem organization re-established postextinction in new clades

used to develop models for both short- and long-term climate change? A range of geological and paleontological criteria has helped identify climatic zones through time (Fig. 4.24). Specific sedimentary rocks such as calcretes (soils rich in calcium carbonate) and evaporites (evaporated salts) can help identify dry, arid climates whereas dropstones (stones that plummet from the bottoms of melting icebergs into seabed sediments) and tillites (rocks and sand left behind by an advancing glacier) indicate polar conditions. These criteria have formed the basis for Christopher Scotese and colleagues' reconstruction of climates and paleogeogeography through time (http://www.blackwellpublishing.com/ paleobiology/). Global climate change can now be mapped through time with some degree of confidence.

Climatic fluctuations through time_

Short-term trends

Many climatic events are short term, occurring within a time span of 100 kyr. Many surface processes respond rapidly to climate change, for example the atmosphere and ocean surface waters can change within days to a few years whereas the deep water of the ocean basins and terrestrial vegetation may take centuries to alter; the buildup of ice sheets and associated sea-level changes, however, occur over millennia. Changes in

Today Pleistocene

Tertiary

Today Pleistocene

Tertiary

Warm

Cool

Jurassic

Tropical Coal Bauxite • Laterite

Cool temperate Coal and Tillites f

Warm temperate Kaolinite(and coal and evaporite) Crocodiles «i^ Palms and mangroves f

Arid

Evaporite Calcrete

Cold Tillite

Dropstone

Glendonite

"Paratropical" = high-latitude bauxites

Figure 4.24 Some key indicators of climate and temperature. (Courtesy of Christopher Scotese.)

Carboniferous

Devonian

Ordovician

Cambrian

Precambrian

Ordovician

Cambrian

Precambrian

Climate Greenhouse Icehouse Devonian

Average global temperature (°C)

Figure 4.23 Climate change through time, showing alternations between icehouse and greenhouse worlds. (Courtesy of Christopher Scotese.)

Average global temperature (°C)

Figure 4.23 Climate change through time, showing alternations between icehouse and greenhouse worlds. (Courtesy of Christopher Scotese.)

precipitation and temperature in the recent past may have influenced the course of human events and almost certainly impacted on the direction of hominid evolution during the Late Pliocene and Pleistocene. Many short-

term climatic fluctuations have been related to Milankovitch cycles (see p. 36), patterns of change in climates and sedimentation patterns that are driven by changes in the eccentricity, obliquity and precession of the Earth's orbit and generally on scales from 20 to 400 kyr. These short-term trends are associated with evolutionary changes at the speciation level and more local regional changes in the composition and structure of ecosystems (Box 4.8).

Long-term trends

As noted above, the Earth has oscillated between greenhouse and icehouse conditions (Box 4.9) at least five times in the past 900 myr (Frakes et al. 1992). These megacycles have been compared with patterns of change in extinctions, sea level and volcanicity (Fig. 4.26). Moreover, there may be a correlation between these variables and the assembly and breakup of the supercontinents. In marine environments two extreme states occurred:

1 The icehouse state involved unstratified, unstable oceans, cool surface waters between 2 and 25°C and bottom waters ranging from 1 to 2°C together with rapidly moving bottom waters, rich in oxygen and with high productivity in areas of upwelling.

Box 4.8 Climate change and fossil size

There is strong evidence that climate and environmental changes have controlled extinctions and speciations, but do they have a direct influence on the size of organisms? Daniela Schmidt and her colleagues (2004) have investigated size changes in planktic foraminiferans during the last 70 myr from well-dated cores furnished by various ocean drilling programs. There was a sharp decrease in size at the Cretaceous-Paleogene boundary with the disappearance of many large taxa, and after this extinction event high-latitude taxa remained consistently small. Fluctuations in size, however, occurred in low-latitude assemblages (Fig. 4.25). A first phase (65-42 Ma) is characterized by dwarfs, a second (42-12 Ma) contains moderate size fluctuations, whereas the third (12 Ma to present) has the relatively large-sized taxa that typify Modern assemblages. Size increases are correlated with intervals of global cooling (Eocene and Neogene), when there were marked latitudinal and temperature gradients and high diversity. More minor size changes in the Paleocene and Oligocene may have been associated with changes in productivity. Cenozoic planktic foraminiferans thus provide strong support for a stationary model of evolutionary change, with size changes being strongly correlated with extrinsic factors such as fluctuations in latitudinal and surface-water temperature gradients.

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