On the 11th February Ellen D. Currano and others published an article, ” Sharply increased insect herbivory during the Paleocene–Eocene Thermal Maximum”. They argued that the Paleocene–Eocene Thermal Maximum (PETM, 55.8 Ma), an abrupt global warming event linked to a transient increase in pCO2, was comparable in rate and magnitude to modern anthropogenic climate change. They used plant fossils from the Bighorn Basin of Wyoming to document the combined effects of temperature and pCO2 on insect herbivory, examining 5,062 fossil leaves from five sites positioned before, during, and after the PETM (59–55.2 Ma).
The amount and diversity of insect damage on angiosperm leaves, as well as the relative abundance of specialized damage, correlate with rising and falling temperature. All reach distinct maxima during the PETM, and every PETM plant species is extensively damaged and colonized by specialized herbivores. Their study suggests that increased insect herbivory is likely to be a net long-term effect of anthropogenic pCO2 increase and warming temperatures. Their study showed that the leaf damage recorded in the sample leaves jumped from between 15% and 38% prior to the PETM to 57% during the PTEM episode and then returned to about 33% after temperatures fell.
Rising CO2 levels made leaves less nutritious because they contained relatively smaller concentrations of nitrogen, a high CO2 environment made photosynthesis easier, so less protein was required in the leaf. Insects therefore needed to eat more leaves in order to acquire the same level of nutrients, and the article argues that this resulted in the observed higher levels of leaf damage.
The consequences of this for the future are not entirely clear, but if the PETM period is a guide to the future then insect attacks on plants could rise, and the research on the nutrient levels in plant leaves in high CO2 environments suggests that this could also have a negative impact on human nutrition, humans need plant proteins as much as insects, so we may find that our diet becomes less nutritious as CO2 levels rise.
See http://www.pnas.org/cgi/content/abstract/105/6/1960 for the full article.