7 Contemporary global biodiversity challenges

The natural world is not static and unchanging. In the past, there have been changes resulting from long-term processes such as continental drift and short-term events like volcanic eruptions or meteorite strikes. Some changes have caused extinctions, not just of individual species but of whole groups of organisms.

Figure 19 Impact of five major extinction events on the numbers of marine families over time. The geological periods are shown with their initial letter only but the full names can be seen below.

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The graph has a horizontal time axis with the Geological Eras and Periods marked. The left-hand vertical axis is the total extinction rate in number of families per million years. The plot has 5 large peaks, at 444, 359, 251, 200 and 65.5 million years ago. The right-hand vertical axis is the number of families extant. The plotted graph shows an increase followed by a levelling off up to 251 million years ago. There is a substantial drop at that point to less than half the number, followed by a gradual increase to four times the number by the end of the Quaternary period.

Figure 19 Impact of five major extinction events on the numbers of marine families over time. The geological periods are shown with their...

As you know from Activity 5, there have been five major extinction events in the past. These are marked on the time scale in Figure 19 by red arrows. The blue line shows the number of families of marine organisms and the decline in numbers at each extinction event. The red line shows the rate of extinction in families of marine organisms, with a peak value at the end of the Permian Period (P). In this particular mass extinction, 96% of species disappeared and all of the current species are descended from the surviving 4%.

The question that arises from a consideration of the past is, what does the future hold? Are we now in the period of a sixth mass extinction?