The rate of continental growth is of major importance to life and its ecosystems. The majority of Earth's biodiversity is today found on continents, and there is no reason to believe that this relationship has changed over the last 300 million years. As continents have grown through time, they have affected global climate, including the planet's overall albedo (its reflectivity to sunlight), the occurrence of glaciation events, oceanic circulation patterns, and the amount of nutrients reaching the sea. All of these factors have biological consequences and affect global biodiversity.
In the last chapter we proposed that diversity (roughly the number and relative abundance of species on the planet at any given time) is a major hedge, or defense, against planetary extinction or sterilization of life: High levels of diversity can counter the loss of body plans during mass extinctions. Plate tectonics can augment diversity by increasing the number and degree of separation of habitats (which promotes speciation). For example, as continents break apart, the seaways forming between them create barriers to dispersal. This in turn reduces gene flow and enhances the formation of new species through geographic isolation. Plate tectonics also increases the nutrients available to the biosphere, which may (or may not) also promote increased biotic diversity.
Plate tectonics promotes environmental complexity—and thus increased biotic diversity—on a global scale. A world with mountainous continents, oceans, and myriad islands such as those produced by plate tectonic forces is far more complex, and offers more evolutionary challenges, than would either totally land- or ocean-dominated planets without plate tectonics. James Valentine and Eldredge Moores first pointed out this relationship in a series of classic papers in the 1970s. They showed that changes in the position and configuration of the continents and oceans would have far-reaching effects on organisms, causing both increased diversification and extinction. Changes in continental position would affect ocean currents, temperature, seasonal rainfall patterns and fluctuations, the distribution of nutrients, and patterns of biological productivity. Such varying conditions would cause organisms to migrate out of the new environments—and would thus promote speciation. The deep sea would be least affected by such changes, but the deep sea is the area on Earth today with the fewest species. Over two-thirds of all animal species live on land, and the majority of marine species live in the shallow-water regions that would be most affected by plate tectonic movements.
The most diverse marine faunas on Earth today are found in the tropics, where communities are packed with vast numbers of highly specialized species. In higher latitudes the number of species lessens, and in Arctic regions there may be only a tenth as many species as in equivalent water depths or habitats found in the tropics. Not only are there fewer species in the higher latitudes, but the composition of species is also different there. Physiological adaptation constrains most species to fairly narrow temperature limits: Animal species adapted to warm, tropical conditions cannot survive in the cold, nor can the cold-adapted species tolerate the warmth of the tropics. Given that temperature conditions change rapidly with latitude, it's not surprising that north-south coastlines of continents show a continuously changing mix of species. North-south coastlines promote diversity because of the latitudinal temperature gradients. East-west coastlines, on the other hand, often show similar species.
As continental positions change through time, the relative abundance of north-south and east-west coastlines can change. Also, the larger the continents, the lower the environmental heterogeneity. If many or all of the continents were welded together into "supercontinents," biodiversity would be expected to be lower than if there were many smaller and separated continental masses. On one large continent, groups of land animals have fewer barriers to dispersal—and thus less opportunity to form new species. Clearly, continental size and position should affect biodiversity, and this appears to have been the case in Earth history.
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