Ecosystem services and invasive alien plants
Humankind benefits from a multitude of resources and processes that are supplied by natural ecosystems. These benefits are known as ecosystem services and include products such as clean drinking water and processes such as the decomposition of waste. The definition of these services was formalized by the United Nations Millennium Ecosystem Assessment. The economic evaluation of ecosystem services contains uncertainties, but some figures are available. For instance, Costanza et al. (1997) estimated the economic value of ecosystem services in the world at 33 000 billion dollars (while the world PIB is estimated at 18 000 billion dollars). The Economics of Ecosystems and Biodiversity (TEEB) project estimated the economic value associated with the protection of hydrographic basins by coastal ecosystems (such as mangroves and other wetlands) at 845 USD/ha/year in Malaysia, and at 1022 USD/ha/year in Hawaii. Bee pollination is estimated at 361 USD/ha/year (Ricketts et al., 2004).
The four following categories of ecosystem services are briefly described, and case studies of impacts of invasive alien species on ecosystem services for each of those categories are provided by Charles ; Dukes (2007):
- Provisioning services: food and crops (including seafood and game), species, water, pharmaceuticals, energy (hydropower, biomass fuel). Crops are negatively affected by invasive species: livestock are affected indirectly by plants that decrease forage quality or quantity such as Euphorbia esula which is avoided by cattle in the west of the USA; water is a critical resource, and numerous invasive plants such as Melia azedarach, Acacia mearnsii or Prosopis spp. decrease available water through evapotranspiration.
- Regulating services: carbon sequestration and climate regulation, waste decomposition and detoxification, purification of water and air, crop pollination, pest and disease control. Invasives such as Bromus tectorum increase fire frequency and enhance emissions of carbon dioxide and monoxide as well as nitrogen oxides, decreasing the air quality; Pueraria lobata and Eucalyptus spp. emit a large amount of isoprene which is highly reactive in the atmosphere and enhances the production of air pollutants; Spartina alterniflora reduces light levels in salt marshes, potentially decreasing estuarine algal productivity; invasive plants also have negative effect on water quality, Tamarix spp. form thickets along riparian corridors enhancing sediment capture and channel narrowing; invasives may also threaten pollination services by luring pollinators from native species, as was shown with Impatiens glandulifera in Central Europe.
- Supporting services: nutrient dispersal and cycling, seed dispersal. Invasive species also directly impact supporting services. Aquatic plants such as Eichhornia crassipes can decrease macroinvertebrate abundance by blocking light transmission and decreasing photosynthesis by phytoplankton and other plants, leading to anoxic conditions. In many cases, invasive alien plants increase net primary productivity as it is the case with Arundo donax and Phragmites spp. in marshes. Soil formation may be indirectly affected by changes in decomposition rates, soil carbon mineralization and geomorphological disturbance processes, as well as succession. Berberis thunbergii and Microstegium vimineum which have invaded forests in the Eastern USA, can significantly alter microbial communities, leading to changes in nitrification and increased soil nutrient concentrations.
- Cultural services: cultural, intellectual and spiritual, recreational experiences (including ecotourism) and scientific discovery. Alteration of cultural services is far more difficult to assess, given the subjective nature of these services. In addition, aquatic macrophytes such as Hydrilla verticillata that form dense layers impeed boating, swimming and diving. Terrestrial invasive alien plants may also form dense stands crowding out native species and diminishing recreational activities and tourism by making natural areas less accessible and by potentially reducing wildlife and rare-plant viewing, as it is the case for Fallopia japonica and Opuntia stricta. In the Galapagos, endemic plants have disappeared because of Lantana camara invasions.
The objective of the work on ecosystem services is to ultimately provide the necessary tools to decision makers to integrate the real value of ecosystem services in their decisions.
Sources
Charles H ; Dukes J (2007) Impacts of invasive species on ecosystem services. Biological invasions. Ecological studies193, 217-237. http://dge.stanford.edu/DGE/Dukes/Charles_Dukes_inpress.pdf
Costanza R, D’Arge R, de Groot R, Farber S, Grasso M, Hannon B, Limburg K, Naeem S, O’Neill RV, Paruelo J, Raskin RG, Sutton P ; van den Belt M (1997) The value of the world’s ecosystem services and natural capital, Nature 387, 253-260.
Millennium Ecosystem Assessment Website http://www.maweb.org/en/index.aspx
Ricketts TH, Daily GC, Ehrlich PR ; Michener C (2004) Economic value of tropical forest to coffee production, Proceedings of the National Academy of Sciences of the United States of America 101(34), 12 579-12;582.
The Economics of Ecosystems and Biodiversity (TEEB), accessed on October 2010. http://www.teebweb.org/
Wikipedia, Ecosystem services, accessed on October 2010. http://en.wikipedia.org/wiki/Ecosystem_services