Fossils of insect-damaged leaves reveal clues about how plants may respond to future global change

Sep 28 2012 Published by under Uncategorized

In my previous posts, I've discussed some of the impacts that very large, "mega-" herbivores have on plants. There are many microherbivores, however. While they may be small, insects have packed a large whallop in the evolutionary arms race with plants. We know a surprising amount about this, considering that the players are smaller and more fragile than the other organisms that make up the fossil record. While insects are rarely preserved in the fossil sediment record, signs of their presence survive in the fossils of leaves that they consumed. Can these fossils tell us anything about modern plant-insect relationships?

Let's start with what we do know. Carbon dioxide concentrations have probably not exceeded 280 ppm in the last 650,000 years; current values (>380 ppm) are predicted to be greater than 550 ppm by 2050. Temperatures are predicted to increase by as much as 6 degres C by the end of the century, according to our best estimates (IPCC 2007). It's well-known that plants that grow in high CO2 environments tend to be nutrient poorer than those that grow in low CO2 conditions, because the ratio of carbon to nitrogen (C:N) decreases as CO2 is more available. As a result of this lower-quality forage, herbivores of all kinds need to eat more leaves of high-CO2 plants in order to get the same amount of nitrogen. Given what we know about how plants respond to temperature and CO2, and what's likely to happen in the future, understanding how plants--especially plants we eat-- will respond to these changing conditions has been an area of extensive research.

The last 56 million years of Earth's climate history. The PETM was an abrupt blip in an otherwise smooth transition to warmer temperatures during the mid-Eocene. Courtesy of Wikimedia Commons.

In order, then, to understand how plants and insect herbivores have responded to a combination of warmer temperatures and elevated CO2 levels in the past, you have to go back more than fifty million years to find the best analog to the conditions we're predicted to be in in the next century. In the early Cenozoic, some ten million years after the extinction of the dinosaurs, temperatures had been gradually warming for some time. Around 55.8 million years ago, this gradual transition was interrupted by a 100,000-year "spike" of especially rapid, warm, and high-CO2 conditions. The partial pressure of CO2 in the atmosphere (pCO2) is thought to have tripled or quadrupled, as global mean surface temperatures warmed by 5°C over around 10,000 years (geologically speaking, a short period of time) (McInerney & Wing 2011).

Much of what we know about how plants and animals responded during the "Paleocene-Eocene Thermal Maximum," or PTEM, comes from a series of amazing fossil deposits in the Bighorn Basin of Wyoming, some of which can be seen here. Some responses, such as the movement of subtropical plants into Wyoming, are fascinating but not particularly surprising. One of the things that caught researchers by surprise, however, is that the leaves from the PETM have a greatly-increased rate of insect damage from herbivory than the fossil leaves above and below the PETM layer. One study of more than 5000 leaves from the Bighorn Basin found that 57% of PETM leaves were damaged, as opposed to 15-38% before or 33% after (Currano et a al., 2008). Additionally, more kinds of insect damage were recorded in the PETM fossils than those that came before or after, presumably because warm-loving insects invaded from the tropics to enjoy a broader range of plants. The scientists concluded that this increase in both the rate and extent of damage was due to the fact that insects would have had to have eaten more leaf material to get the nitrogen they needed.

A selection of the diversity of fossil insect damage from the PETM. Photo from Currano et al., 2008.

One prediction that comes out of this research is that with increasing pCO2 and temperature concentrations in the Anthropocene, we should expect increased activity from insect pests. This has been tested empirically in some cases, such as the increased insect damage to soybean crops grown with experimentally high levels of CO2 (Zavala et al., 2008). Such a conclusion comes with a few caveats, not the least of which is the fact that the response of plants and insects to changes in temperature and pCO2 his highly variable among species of plants and insects; some plants may produce more defensive chemicals and reduce herbivory (Knepp et al., 2005), and some insects may become more common in a high-CO2 world, even though the rate of their herbivory may decline (Stiling et al., 2009).  Another issue is that phenology (the timing of ecological events) plays an important role in plant and insect life cycles, and so the ability of an insect to consume a plant is going to depend a lot on the timing working out for all parties involved (DeLucia et al., 2008). Ultimately, the indirect effects of increased CO2 and warmer temperatures (not to mention moisture!) on plants' abilities to tolerate herbivory may be more important than the number of insects or the rate of insect herbivory (Lau & Tiffen 2009).

The concept that we have to look back more than 50 million years to find the closest analog to the Anthropocene when it comes to climate and the atmosphere is sobering, especially when you consider that the climate change recorded in our closest analog took place over 100,000 years, not 100. It's amazing to me that the damage created by insects on leaves is captured in the fossil record as something we can look to to see how plant-herbivore interactions have changed through time, under different environmental conditions. There are widespread insect outbreaks today that are having a devastating impact on North American forests, from the hemlock woody adelgid in the eastern United States to the mountain pine beetle in the American West. The paleorecord may reveal clues about how ecosystems responded to similar outbreaks in the past. There are a lot of unknowns about how insect herbivory will affect plants in the future, and generalizations should be avoided. Still, by looking at the evidence left behind in meals eaten 50 million years ago, we might just learn something about the next 50 years have in store.

Currano, Ellen D., et al. 2008. Sharply increased insect herbivory during the Paleocene–Eocene Thermal Maximum. PNAS 105: 1960-1964

DeLucia et al., 20008. Insects take a bigger bite out of plants in a warmer, higher carbon dioxide world. PNAS 105: 1781-1782

Intergovernmental Panel on Climate Change, 2007. Fourth Assessment Report of the Intergovernmental Panel on Climate Change

Knepp, Rachel G., et al. 2005. Elevated CO2 reduces leaf damage by insect herbivores in a forest community. New Phytologist 167: 207-218.

Lau, & Tiffen. 2009. Elevated carbon dioxide concentrations indirectly affect plant fitness by altering plant tolerance to herbivory. Oecologia 161: 401-410.

McInerney & Wing. 2008. The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth & Planetary Science 39:489–516

Stiling, Peter, et al. 2009. Seeing the forest for the trees: long-term exposure to elevated CO2 increases some herbivore densities. Global Change Biology 15: 1895-1902.

Zavala, J. A., et al. 2008. Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects. PNAS 105: 5129-5133

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