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This month in ecology: Oysters, big rivers, biofuels

Ecological dimensions of biofuels: state of the science

Are biofuels a renewable, environmentally friendly energy source? The Ecological Society of America reviews bioethanol and biodiesel in conventional production as well as feedstocks still in development. Biofuels in commercial scale production are made from the sugars and oils of food crops, and share the ecological impacts of high intensity agriculture. Corn, the primary biofuel source in the United States, demands a lot of fuel to produce fuel. It needs nitrogen fertilizer, fixed using energy-intensive industrial processes. Much of that nitrogen ends up in waterways, where it causes problems for fish and fisheries. In some of the drier western states, farmers are drawing down groundwater resources to irrigate corn for biofuel. Cornfields are usually tilled, which releases greenhouse gasses stored in soil, loses topsoil to erosion, and loses water to evaporation. Much hope has been placed in the "cellulosic" biofuels for their superior environmental benefits. Made from grasses, woody crops like poplar, crop silage and other plant wastes, cellulosic ethanol does not compete with the food supply for feedstocks, which consume fewer resources, and are potentially more compatible with wildlife. Mixes of perennial native grasses, for example, offer better habitat than monocultures and don't need intensive fertilizer, pesticide, and water inputs. But cellulosic ethanol contributes only 0.5% of current biofuel production, and still faces major implementation challenges to become commercially viable. Algal biofuels remain in development. The authors conclude the report with recommendations to get the most out of biofuels going forward, improving ecosystem services, reducing greenhouse gases, and providing new income for rural communities.

  • Net greenhouse gas emissions: vary greatly by feedstock. Conversion of fallow, range, or wild lands to biofuel production releases greenhouse gases stored in soil. Tilling existing croplands also contributes greenhouse gasses. High intensity agriculture uses fuel for irrigating, fertilizing, sowing, harvesting, and transporting biofuel crops.
  • Water: biofuel processing plants do not use much water, but some of the biofuel crops do. Perennial crops such as switchgrass and mixed prairie grasses do not demand the irrigation, nitrogen fertilizer and yearly soil tilling typical of high intensity corn production.
  • Land use and wildlife diversity: biofuel crops compete with food crops. Demand for biofuels drives conversion of prime agricultural land, expansion into marginal agricultural lands and reopening of reserves. To meet current US energy demand through biofuels alone would require conversion of 41% of US land to corn, 56% to switchgrass, or 66% to rapeseed, but potentially only 3-13% to algae. Drought tolerance, fast growth and pest resistance, traits that make plants fine candidates for biofuel feedstocks, also make them fine candidates for becoming invasive. Some, such as the old world grass miscanthus, are already invasive in North America.

Ecological Dimensions of Biofuels. Cifford S. Duke, Richard Pouyat, Philip Robertson, and William J. Parton. Issues in Ecology No. 17, Spring 2013.

The review is freely available and follows a series of reports on biofuels and sustainability from 2010.

Looking to large tributaries for conservation gains

On big rivers like the Mississippi, the infrastructure of modern civilization dams, locks, dikes, power plants, cities has made life easier for people, but harder for fish and other denizens of the river. Restoration is a tricky problem. Economic reliance on these big rivers makes fundamental reversals like dam removals unlikely. Conservation laws and projects tend to be local, on the city or state level, and the river crosses many borders, complicating the restoration picture. Brenda Pracheil and colleagues at the University of Wisconsin, Madison, say tributaries have under-appreciated potential to compensate for habitat loss on the major concourses of the Mississippi Basin. The Platte, for example, has 577 kilometers of free-flowing, relatively intact habitat. It feeds into the heavily altered Missouri, a large mainstem river in the Mississippi Basin, and harbors many of the same fishes.

Pracheil found a correspondence between the volume rate of water flow and the presence of 68 large-river fishes, including paddlefish, blue catfish, and silver chub, most of which are threatened. A steep threshold separates tributaries with large-river fish from those without; 166 cubic meters per second is big enough for roughly 80% of large river specialist species. Below the threshold, almost none of these species are around. Pracheil says this threshold could be used to target tributaries for conservation attention. Existing regulatory structures don't allow improvements on tributaries to count toward mainstem restoration mandates. The UW scientists argue that more flexibility could, in some cases, provide a better return on investment of conservation dollars, complementing efforts on the larger rivers downstream.

Enhancing conservation of large-river biodiversity by accounting for tributaries (2013) Brenda M Pracheil, Peter B McIntyre, and John D Lyons. Frontiers in Ecology and the Environment 11(3): 124-128

Learn more on the University of Wisconsin Center for Limnology blog.

Oyster reefs buffer acidic inputs to Chesapeake Bay

When European settlers arrived on Chesapeake Bay, it was encrusted with a treasure trove of oysters and other bivalves. The living oyster reef and its stockpile of empty shells was voluminous enough to influence the water chemistry of the bay, says marine ecologist George Waldbusser and colleagues. Based on harvest records from the 17th century, he estimates that the oyster-impoverished bay of 2013 is running "at least 100 million bushels behind where it was before we started harvesting, in terms of shell budget."

Oysters eat microscopic phytoplankton, including algae, which the bay generally has overabundance of thanks to excess fertilizer runoff. Oysters are not just a tasty economic resource they make Chesapeake Bay cleaner. The missing shells are a direct loss to oyster restoration, because oyster larvae are choosy about where they glue themselves down and start building their shells. They prefer other oyster shells as anchorage.

But in addition to providing habitat for future generations, oyster reefs appear to alter their local water chemistry. Like slow dissolving Tums in the belly of the estuary, disintegrating oyster shells are slow release capsules of calcium carbonate, an alkaline salt and a buffer against acidity. Seawater mixing in on the tide has a relatively high capacity to absorb acid inputs without a large change in pH. Fresh water flowing out to sea generally has a low buffering capacity, and is sensitive to acid sources, whether from human made point sources like coal plants or natural processes like the oysters' own respiration. Coastal estuaries, where the waters meet, are also where oysters tend to cluster.

Ecosystem effects of shell aggregations and cycling in coastal waters: An example of Chesapeake Bay oyster reefs. (2013) George G. Waldbusser, Eric N. Powell, and Roger Mann. Ecology 94(4): 895-903 (Currently in authors' preprint; contact Liza Lester for a type-set copy).


Contact: Liza Lester
202-833-8773 x211
Ecological Society of America

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