Past climate change - Climate Change: Biological and Human Aspects - page 105

mechanism that enables it to photosynthesise more efficiently than C 3 plants at lower
carbon dioxide partial pressures (which is effectively similar to atmospheric con-
centration). Indeed, it is worth noting now, for we will only briefly return to this
point later, that the photosynthetically efficient C 4 grasses - being ideally suited to
a low-carbon dioxide atmosphere - will undoubtedly play a significant role in the
biosphere's continued evolution on the scale of hundreds of millions of years. Now
let's return to the Miocene.
Conventional wisdom has it that the general decline in atmospheric carbon dioxide
(as seen in Figure 3.1), hence long-term cooling of the planet, was enhanced from
the Miocene due to the photosynthetically efficient C 4 plants and this efficiency
enabled the C 4 photosynthetic grasses to expand considerably in the late Miocene
(approximately 8-4 mya). While C 4 grassland biomes had a significant presence on
the Miocene Earth, the grasses did evolve earlier and grass seeds have been found in
dinosaur coprolites (fossilised excreta). The idea is that not only did the C 4 grasses
help drive atmospheric carbon dioxide concentrations down, at lower carbon dioxide
levels they had more of an evolutionary advantage (such plants' other evolutionary
factors notwithstanding) over those plants with a lower photosynthetic efficiency. This
is the commonly held conventional wisdom, but as nearly always there are details and
it is prudent not to take too simplistic a view.
Other environmental factors facilitating the rise of grasses was that the late Miocene
was also the time of orogenesis (mountains building), which resulted in formation
of the Rocky Mountains, Andes and Himalayas and in turn in regional climatic
effects from rain shadows and rainy seasons. In addition, of course, the uplifted areas
themselves were cooler.
Whereas it is true that carbon dioxide levels were considerably lower in the Miocene
than compared with the previous couple of hundred million years, and that this gave
C 4 plants and grasses a competitive advantage, there is some evidence to suggest
that carbon dioxide levels had largely already stabilised (recent Quaternary glacials
notwithstanding) by around 9 mya. As we shall see in the next chapter, although
the evolution of C 4 plants was earlier, their actual global spread did not take place
until from about 8 mya (when there was a major expansion) and there was not a
decisive lowering of carbon dioxide at that time to act as a trigger for this grassland
expansion. So it is likely that C 4 expansion was driven by additional factors such as
tectonically related episodes, such as orogenesis or vulcanism (Pagani et al., 1999),
which caused cooling climate change that disrupted ecosystems, so enabling the
grasses to capitalise on their advantage in the already lower carbon dioxide levels.
This distinction may seem trivial and the point a subtle one, but it is nonetheless
worth noting. As we have seen with climate change itself, and as we shall see in
later chapters with regards to ecological change in response to climate change, the
resulting biological change is frequently a result of the necessary interplay of a
number of factors.
The importance of such evolutionary subtlety is also relevant to the broader change
in mix of global plant species. For example, it had long been recognised that ferns
are an old group of species and that one type, the leptosporanginates (which feature
the majority of contemporary or extant ferns), is more than 250 million years old
and today number over 10 000 extant species. However, it used to be thought that