Two theories can be tested:

• Competitive displacement

The later group out-competed the first, and drove it extinct. The grey squirrel (opposite) in Britain is doing just this to the native red squirrel.

• Independent replacement

The other says that the first group declined and went extinct for some reason - environmental change, perhaps - unrelated to the presence of the second group, and the second group only radiated after the first had been cleared away.

The pattern of change in diversity of the two groups provides the best evidence to test between the two theories. If the first group declines before the second expands, it suggests competition was not influential; whereas if the first group declines in proportion to the increase in the second, it suggests competition.