Switchgrass over corn? The argument for maintaining ecological diversity

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Corn, corn, corn…the legacy of this crop in the US has placed it as the default crop choice for biofuel production.  But what impact does corn have on wildlife systems, and are there better choices that could provide biomass for energy as well as maintain ecological health where it is grown?  A new study by Werling et al in PNAS  says there are likely better choices if we consider priorities beyond biofuel production alone, using a multidisciplinary approach to study the broad effects of biofuel production on ecosystem diversity and richness.

Biofuels are one of the primary renewable energy sources being considered for the future, along with solar and wind power, among others.  Using biofuels is a carbon neutral process because it involves using plants that consume CO2 to grow.  So even though burning biofuels still emits CO2 to the atmosphere, just like fossil fuels, the entire process of burning and growth results in zero net carbon emissions.

Figure courtesy of: http://www.energyfuturecoalition.org/biofuels/benefits_env_public_health.htm#1

One of the major conflicts regarding biofuels is the competition of land use for food or energy production.  To avoid a situation in which biofuel directly competes with the primary need for food, recent research has focused on using marginal lands only for biofuel production.  Marginal lands are regions that would provide sub-optimal food production, due to poor soil nutrition or other factors, but provide enough small pockets of land suitable for agriculture for biofuel production.

The question then remains: what type of crop is best suited for these marginal lands?  Maize (corn), switchgrass, and prairie grass are three major types that could be used for this use.  Maize has long been at the center of US discussions of biofuel production, but this is mainly due to its legacy as the central crop for US farmers and a whole host of lobby groups pushing for grain ethanol to be the main form of biofuel.  However, it’s important to understand how maize affects the ecology of these marginal land systems compared to other possible choices.  Corn should not be chosen because of its past in this country; if there are better options, we need to look at them.

Werling et al investigate the effects of each of these on various economical and ecological factors, such as biofuel production, methane consumption (limiting greenhouse gas emission), and various wildlife populations (arthropods, bees, birds).  They gathered samples from 115 planting sites of maize, switchgrass, and prairie in agricultural areas in Michigan and Wisconsin.  The study found major differences between maize and switchgrass or prairie grass across all these factors.

Figure courtesty of: Werling et al (2013), PNAS

Figure courtesty of: Werling et al (2013), PNAS

In each graph above, the ‘taxonomic richness‘ is reported for regions of maize (left bar), switchgrass (middle), and prairie grass (right), measured in this study by taking many samples from each of the 115 sites and counting constituents of each category.  Across all species types except methanotrophs (bacteria that consume methane), switchgrass and prairie provide a statistically significantly greater amount of taxonomic richness, in terms of plant species, arthropods, bees, and birds (many of which are at critically low numbers).

Figure courtesty of: Werling et al (2013), PNAS

Figure courtesty of: Werling et al (2013), PNAS

Regarding ecological processes, maize does show significantly higher biofuel yield (upper-left graph A), the most straightforward economic quantity.  This is usually the argument provided by ethanol lobbying groups to choose maize over any other biofuel choice.  But these other ecological processes shown above are discussed less and could easily be equally important for long-term yield and global warming impacts.  For example, switchgrass and prairie grass both provide much grater methanol consumption, which will limit greenhouse grass accumulation in the atmosphere (B).  In addition, the increased taxonomic richness seen in the first figure leads to greater predation on pest eggs (C) and a decreased pest population (F) in switchgrass and prairie.   Grassland birds, which are close to endangerment, were also observed more often in switchgrass and prairie grass land use.  So switchgrass and prairie may yield less on an annual basis, however they will lead to healthier ecosystems that will likely lead to more consistent yields less susceptible to disastrous spreads of pests, as well as provide more natural habitats for wildlife like grassland birds that would more and more marginalized by maize planting.  The authors also found that these effects spread to lands at least 1 km away from the marginal land boundaries, indicating a more widespread effect of ecological diversity in the neighboring regions.

This type of study is extremely important because it places emphasis on results that capitalistic economics does not – long-term benefits of pest control, species richness, and greenhouse gas control.  Our current economics would only see one thing – the biofuel yield – and label maize as the clear winner.  But we need more studies like this one to understand the multitude of effects that these crops have on the environment.  Even more importantly, we need to start basing our policy decisions off studies like this that equally emphasize production, diversity, and long-term health and maintenance.

ResearchBlogging.org

BP Werling, TL Dickson, R Isaacs, H Gainesv, C Gratton, KL Gross, H Liere, CM Malmstrom, TD Meehan, L Ruan, BA Robertson, GP Robertson, TM Schmidt, AC Schrotenboer, TK Teal, JK Wilson, DA Landis (2014). Perennial grasslands enhance biodiversity and multiple ecosystem services in bioenergy landscapes PNAS DOI: 10.1073/pnas.1309492111

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