Biodiversity Patterns in Time
At first glance diversity seems to be far higher in summer than in winter, but in terms of how many species are present the two seasons aren't actually very different. Plants that die in winter have left seeds that wait for spring. Insects have also laid eggs that are waiting. Many mammals are hibernating. Many birds migrate to warmer areas in winter and some mammals and insects also migrate, but otherwise the species are still present, even if they may not be obviously visible. The major difference between the two seasons is in the number of individuals and the total weight of biological matter.
Over time, succession leads to greater diversity (see the Theory section for details) in an area. Diversity increases as succession continues, only leveling off when succession has reached its final stage. Disturbances, such as fires and severe ice storms, destroy this final stage and set the affected area to an earlier stage of succession. This disturbed area now has a lower diversity, but the area will then progress along the same path as it originally did, towards the final stage. Regions that have small disturbances within them will have higher total diversity than the a region entirely composed of the final stage; the different areas in different stages of succession have more species than just the climax stage. This is somewhat similar to the effect of habitat diversity, above, as the different stages of succession are similar to different habitats.
There have been numerous periods of glaciation known as ice ages, the height of the last one happening less than 20 000 years ago. During the last ice age, glaciers covered almost all of Canada, and its diversity obviously dropped dramatically. Ice ages are large-scale disturbances, and the system can recover afterwards, as in succession, above. The scale of destruction means that recovery can be extremely slow, as there are no healthy individuals nearby that can colonize the area; for instance, amphibians in Canada have yet to move back into the regions they lived in before the last glaciation.
Diversity has been increasing throughout the history of life on the planet, but the increase hasn't been smooth. There have been several mass extinctions, the one that claimed the dinosaurs at the end of the Cretaceous 65 million years ago being only the best known. New species arise fairly commonly, but groups at higher classification levels only rarely arise; no new class (examples of two classes are mammals and starfish) has come about in the past 65 million years, and no new phylum (examples of two phyla are flowering plants and molluscs) has arisen in the last 225 million years.