THE YEAR IN ECOLOGY AND CONSERVATION BIOLOGY, 2009 Effects of Air Pollution on Ecosystems and Biological Diversity in the Eastern United States Gary M. Lovett,a Timothy H. Tear,b David C. Evers,c Stuart E.G. Findlay,a B. Jack Cosby,d Judy K. Dunscomb,e Charles T. Driscoll,f and Kathleen C. Weathersa aCary Institute of Ecosystem Studies, Millbrook, New York, USA bThe Nature Conservancy, Albany, New York, USA cBiodiversity Research Institute, Gorham, Maine, USA dUniversity of Virginia, Charlottesville, Virginia, USA eThe Nature Conservancy, Charlottesville, Virginia, USA f Syracuse University, Syracuse, New York, USA Conservation organizations have most often focused on land-use change, climate change, and invasive species as prime threats to biodiversity conservation. Although air pollution is an acknowledged widespread problem, it is rarely considered in conser- vation planning or management. In this synthesis, the state of scientific knowledge on the effects of air pollution on plants and animals in the Northeastern and Mid-Atlantic regions of the United States is summarized. Four air pollutants (sulfur, nitrogen, ozone, and mercury) and eight ecosystem types ranging from estuaries to alpine tundra are considered. Effects of air pollution were identified, with varying levels of certainty, in all the ecosystem types examined. None of these ecosystem types is free of the impacts of air pollution, and most are affected by multiple pollutants. In aquatic ecosystems, ef- fects of acidity, nitrogen, and mercury on organisms and biogeochemical processes are well documented. Air pollution causes or contributes to acidification of lakes, eutroph- ication of estuaries and coastal waters, and mercury bioaccumulation in aquatic food webs. In terrestrial ecosystems, the effects of air pollution on biogeochemical cycling are also very well documented, but the effects on most organisms and the interaction of air pollution with other stressors are less well understood. Nevertheless, there is strong evidence for effects of nitrogen deposition on plants in grasslands, alpine areas, and bogs, and for nitrogen effects on forest mycorrhizae. Soil acidification is widespread in forest ecosystems across the eastern United States and is likely to affect the com- position and function of forests in acid-sensitive areas over the long term. Ozone is known to cause reductions in photosynthesis in many terrestrial plant species. For the most part, the effects of these pollutants are chronic, not acute, at the exposure levels common in the eastern United States. Mortality is often observed only at ex- perimentally elevated exposure levels or in combination with other stresses such as drought, freezing, or pathogens. The notable exceptions are the acid/aluminum effects on aquatic organisms, which can be lethal at levels of acidity observed in many surface waters in the region. Although the effects are often subtle, they are important to bio- logical conservation. Changes in species composition caused by terrestrial or aquatic acidification or eutrophication can propagate throughout the food webs to affect many organisms beyond those that are directly sensitive to the pollution. Likewise, sublethal Address for correspondence: Gary M. Lovett, Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545. [email protected] The Year in Ecology and Conservation Biology, 2009: Ann. N.Y. Acad. Sci. 1162: 99–135 (2009). doi: 10.1111/j.1749-6632.2009.04153.x C 2009 New York Academy of Sciences. 99 100 Annals of the New York Academy of Sciences doses of toxic pollutants may reduce the reproductive success of the affected organisms or make them more susceptible to potentially lethal pathogens. Many serious gaps in knowledge that warrant further research were identified. Among those gaps are the effects of acidification, ozone, and mercury on alpine systems, effects of nitrogen on species composition of forests, effects of mercury in terrestrial food webs, interactive effects of multiple pollutants, and interactions among air pollution and other environ- mental changes such as climate change and invasive species. These gaps in knowledge, coupled with the strong likelihood of impacts on ecosystems that have not been stud- ied in the region, suggests that current knowledge underestimates the actual impact of air pollutants on biodiversity. Nonetheless, because known or likely impacts of air pollution on the biodiversity and function of natural ecosystems are widespread in the Northeast and Mid-Atlantic regions, the effects of air pollution should be considered in any long-term conservation strategy. It is recommended that ecologically relevant standards, such as “critical loads,” be adopted for air pollutants and the importance of long-term monitoring of air pollution and its effects is emphasized. Key words: acid deposition; sulfur; nitrogen; mercury; ozone; eastern United States; forest; stream; lake Introduction a “big picture” makes it difficult for conserva- tion biologists and policymakers to determine Background the role of air pollution among the array of threats to biodiversity. The past four decades have seen much This report is a synthesis of scientific in- research on the effects of air pollution on formation on the impacts of air pollution on ecosystems. Much of this work has focused on elements of biological diversity (e.g., species, biogeochemical responses of lakes, forests, and natural communities, ecosystems) in the North- estuaries to elevated levels of sulfur and nitro- eastern and Mid-Atlantic regions of the United gen deposition. Many studies have shown that States. These elements are used by The Nature plant and animal species differ in their sensi- Conservancy (TNC), the largest conservation tivity to air pollution and its biogeochemical organization in the United States, to best rep- consequences, such as soil and water acidifica- resent biological diversity in planning efforts tion. For instance, lichens differ in sensitivity used to guide and focus conservation action. to sulfur dioxide (e.g., Thormann 2006), vas- By focusing on TNC targets, we hope to pro- cular plants differ in sensitivity to ozone (e.g., vide conservation organizations with sufficient King et al. 2005), fish and other aquatic biota information to gauge the scope and severity of differ in sensitivity to lake acidity (e.g., Baker the threat of air pollution in this region. Our et al. 1990), and grasses and forbs differ in sen- approach in this synthesis is to group species sitivity to added nitrogen (e.g., Wedin & Till- and communities in eight ecosystem types and man 1996). This differential sensitivity suggests to assess the evidence for the effects of four that air-pollution stress will have consequences key air pollutants in each ecosystem type. To for the biodiversity of an ecosystem, either in the extent possible, we attempt to quantify the shifting species composition or outright loss of levels of air pollution that produce adverse im- sensitive species. Because these impacts are pacts, and to assess the certainty of scientific species-specific and pollutant-specific, and knowledge on the subject. Our goal is to gain most ecosystems are exposed to multiple air a better appreciation for the combined impacts pollutants simultaneously, it can be difficult to of air pollution on multiple taxonomic groups assess the overall impact of air pollution on or- and ecosystem types in order to better inform ganisms in a particular ecosystem. This lack of biodiversity-conservation efforts about the Lovett et al.: Biodiversity & Air Pollution 101 relative significance of this threat. To our soils and the long-term changes in soil and knowledge, this type of collective synthesis of water chemistry caused by deposition of sul- impacts to biological diversity has not been pre- fate, nitrate, and ammonium. Nitrogen and S viously attempted for multiple pollutants across can be transported long distances in the atmo- multiple conservation target groups. sphere and can impact ecosystems hundreds of kilometers from the emission sources. Because Air Pollutants Considered sulfate and nitrate ions can be readily leached from soils to surface waters, deposition of these In this review we consider four air pollutants pollutants to terrestrial ecosystems may cause a of concern: sulfur (S), nitrogen (N), mercury cascade of effects that includes lakes, streams, (Hg), and ozone (O3). rivers, estuaries, and the coastal ocean. Sulfur and nitrogen are primarily released Ozone is a “secondary” pollutant, formed in from fossil-fuel combustion as S and N ox- the atmosphere from photochemical reactions ides, and these gases can be transformed in the involving N oxides and hydrocarbons. Ground- atmosphere to acidic particles and acid pre- level O3 is a widespread regional pollutant in cipitation (Driscoll et al. 2001). The gases and the eastern United States but tends to occur in particles may be deposited directly to vegeta- particularly high concentrations downwind of tion and soil surfaces in a process known as major urban areas (U.S. EPA 2006). Because dry deposition, or they may be incorporated O3 and its precursors can be transported long into cloud droplets, raindrops, or snowflakes distances, it is a threat to ecosystems far re- to increase the acidity of precipitation (Lovett moved from urban centers. Aside from its well- 1994). In mountaintop and coastal areas where known effects on the human respiratory system, cloud and fog are common, highly acidic cloud O3 is known to reduce photosynthesis in most