Human disturbance of the Amazon rainforest is more extensive than previously thought say a team of scientists writing in the current edition of the journal Frontiers in Ecology. Reviewing recent research on the Amazon ecosystem, they note that human activities are affecting the health of the forest and impacting the ecological goods and services the Amazon provides mankind.
“Emerging research indicates that land use in the Amazon goes far beyond clearing large areas of forest; selective logging and other canopy damage is much more pervasive than once believed,” the authors write. “Deforestation causes collateral damage to the surrounding forests — through enhanced drying of the forest floor, increased frequency of fires, and lowered productivity. The loss of healthy forests can degrade key ecosystem services, such as carbon storage in biomass and soils, the regulation of water balance and river flow, the modulation of regional climate patterns, and the amelioration of infectious diseases.”
Gross versus net deforestation. Deforestation in the Amazon is not just the simple removal of trees. A more complete description must consider the different rates of gross deforestation (clearing of rainforest for pastures or croplands), the management regimens of croplands and pastures, the abandonment of fields leading to the regrowth of secondary forests, and the resulting net changes in forested area (gross clearing minus regrowth). Here we have adapted a Markov land-use transition model developed by Fearnside (1993) to estimate that roughly a third of the total deforested land in the Amazon between 1961 and 1997 is currently regrowing (see also Cardille and Foley ). (M = megatons). Image and caption text courtesy of Frontiers in Ecology
In recent years the Amazon has experienced high levels of deforestation, including the largest loss of forest on record between 2002 and 2004. Growing interest in the region’s resources and agricultural potential will likely continue to fuel forest clearing and degradation. Further, roads and other infrastructure are opening up once inaccessible regions to development, accelerating human influence in the region.
Until recently the nature of land-use change in the Amazon was poorly understood, but new methods of remote monitoring are providing insight into how humans are altering the Amazon ecosystem. For example, it has long been believed that agricultural clearing and cattle pasture have the greatest impact on Amazonian forests, but studies now indicate that logging is widespread and pervasive outside cleared areas. Research by Daniel Nepstad and colleagues (1999) and Greg Asner and colleagues (2005) found that the area logged was roughly equal to the amount of area deforested each year — a critical finding since logged forest retains lower levels of biological diversity due to changes in forest structure. Further, Asner’s research shed light into the link between deforestation and logging in the Amazon:
“Even more unexpected was the finding that only 16% of the logged area turned into deforested (clear-cut) land the following year, and only 32% of the logged forests were consumed by clear-cut deforestation within 4 years (Asner et al. 2006),” write the researchers. “These results completely change our view of logging as a form of land use in the Amazon: first, selective logging often matches, and can even exceed, deforestation each year; second, for the most part, logging does not immediately precede deforestation — it is a distinct form of forest disturbance in and of itself. In short, the footprint of human activity on the Amazon landscape is roughly double that of previous estimates of deforestation alone (Asner et al. 2005).”
The authors go on to say that other studies indicate that the ecological impact of logging is highly variable depending on its intensity, but that it can affect fire regimes, making forest areas more susceptible to burning, especially in dry years.
Impact on Ecological Goods and Services
The authors say that while discussions on deforestation often focus on the “negative consequences for biodiversity and ecosystem functioning”, forest conversion does “offer many benefits” to local populations including the provision of “essential resources” and employment opportunities.
“Tropical deforestation therefore represents an inherent societal tradeoff (DeFries et al. 2004),” they state. “In general, land-use practices allow some ecosystem goods to be more readily appropriated by human societies, often yielding key economic and social benefits, at least in the short term. However, land use may degrade other ecosystem services — especially those tied to the long-term functioning of the ecosystem.”
It’s these long-term changes that are of particular concern to the authors, who review recent scientific literature to produce four examples of ecosystem services that are negatively affected by deforestation and degradation: carbon storage, water flow, regional climate influence, and disease vectors.
Citing figures that show the Amazon accounts for nearly 10% of the world terrestrial biomass, the authors write “the Amazon provides an important ecosystem service to the planet by storing organic carbon in biomass and soil, thereby keeping greenhouse gases (CO2 and CH4) from the atmosphere.” Deforestation, forest degradation, and fragmentation release some of the stored carbon into the atmosphere, contributing to global warming. The rate at which the carbon is released depends on how the forest is converted; burning releases carbon quickly, logging and subsequent use of wood and paper products releases carbon over longer period of time. Deforestation can indirectly affect carbon storage in neighboring forest areas through desiccation and changes in fire regimes.
Forests greatly affect the hydrological cycle of the Amazon basin. The authors produce studies showing that “runoff and stream discharge generally increase with increasing deforestation” even when precipitation levels stay the same. One model developed by Marcos Heil Costa and Jon Foley (1997) shows that “widespread deforestation” in the Amazon could increase runoff and river discharge by about 20%.
Regional and global climate regulation
Deforestation can reduce evapotranspiration of moisture into the atmosphere, weakening water recycling and causing local climate drying.
Deforestation in the Colombian Amazon – photo by Rhett A. Butler.
“Climate model simulations of large-scale deforestation in the Amazon Basin generally show a considerable reduction in evapotranspiration as tropical forest vegetation is replaced with grasses and shrubs,” the authors explain. “This has the effect of substantially warming the surface and inhibiting convection, regional precipitation, and cloud cover.”
The impact of deforestation can extend well beyond the Amazon basin. Simulations by Peter Snyder and colleagues suggest that deforestation could cause changes in the North Atlantic and European storm tracks, producing “substantial cooling in southern Europe and warming across parts of Asia in winter.” Separately, NASA studies indicate that Amazon deforestation can influence rainfall from Mexico to Texas and in the Gulf of Mexico.
Rainforests have also been linked to human health, specifically moderating the spread of infection disease by “regulating the populations of disease organisms (viruses, bacteria, and other parasites), their animal hosts, or the intermediary disease vectors (most often insects or rodents).” The authors cite research from Peru that found links between deforestation and malaria-carrying mosquitos. Anecdotal evidence from across the region (Colombia, Brazil, and Venezuela) suggest the link is widespread.
The authors say these are only some of the examples of the ecosystem services afforded mankind by the Amazon rainforest. Because some of these function at a larger scale, they are sometimes undervalued by local populations and policymakers. For this reason, the authors say there is an urgent need to develop better ways to assign monetary values to these services. They also argue that more cross-disciplinary scientific research is crucial to helping them more fully understand the links between forest health and human-caused disturbance. The authors highlight the Brazilian-led Large-Scale Atmosphere Biosphere Experiment in Amazonia (LBA), an initiative the aims to study the Amazon as an integrated entity, as an institution that has been particularly productive towards this end.
“I think the scientists, policy-makers, practitioners and everyday citizens should have an ongoing conversation about the value of ecosystems — both in terms of their commodities (like timber and agricultural products), but also in terms of their ecosystem services (like pollination, flood control, carbon storage, and disease control),” Jon Foley, an author and a professor at the Center for Sustainability and the Global Environment (SAGE) at the University of Wisconsin, told mongabay.com. “When we finally recognize the importance of these under-valued services — whether in economic, health or cultural terms — we will be able to make much better decisions about how people live on the land, and we can continue to do so far into the future.”
Citation: Foley, J.A. et al. (2007) Amazonia revealed: forest degradation and loss of ecosystem goods and services in the Amazon Basin. Frontiers in Ecology 2007; 5(1): 25—32. Authors include: Jonathan A Foley, Gregory P Asner, Marcos Heil Costa, Michael T Coe, Ruth DeFries, Holly K Gibbs, Erica A Howard, Sarah Olson, Jonathan Patz, Navin Ramankutty, and Peter Snyder.