Patterns of diversity with time fall along a scale axis that runs from one year to hundreds of millions of years. Nine orders of magnitude! The range makes trivial work of emphasizing how important it is to keep aware of scale. I shall treat scale patterns in order, from longest to shortest.

Evolutionary time

Phanerozoic time

For hundreds of millions of years, life has been leaving abundant testimony to its existence and history. The fossil record teaches us that the number of species has increased over that vast time scale (Sepkoski, 1984). Figure 3.1 shows the number of species of marine invertebrate fossil (per million years) for the ten geological eras preceding ours. (Diversities are divided by the length of the interval to avoid biasing the results. If diversities do not change, then longer intervals will accumulate more species because of turnover, i.e. speciations and extinctions.)

The increase did not proceed without reverses. Several monumental decreases in diversity punctuate the record. (More about these in the section on mass extinctions in Chapter 6.) But, on the whole, life expands.

Perhaps the record deludes us? After all, even rocks are not eternal. The proportion that survive must decline with time, taking their cargo of fossils with them as they erode or are carried down into the molten inner reaches of the earth.