Cumulative changes—new challenges for water management in agriculture
New challenges are emerging for water managers in agriculture as a consequence of the cumulative and sometimes synergistic effects of multiple drivers, including climate change and invasive spe- cies.
Global climate change is expected to directly and indirectly alter and degrade many ecosystems (Gitay and others 2002). For example, it will exacerbate problems associated with already expanding demand for water where it leads to decreased precipitation, while in limited cases, where precipita- tion increases, it could lessen pressure on available water. There are also major expected conse- quences for wetland ecosystems and species, although the extent of change is not well established (Gitay and others 2002; van Dam and others 2002; Finlayson and others forthcoming).
There is growing recognition of the important role that invasive species can play in degradation of ecosystems and ecosystem services (MEA 2005c). Invasive species, spread through water regula- tion for transport and water transfer and through trade, have altered the character of many aquatic ecosystems (see photo). Once established, invasive plants can block channels and irrigation canals and decrease connectivity within and between rivers and wetlands, replace valuable species, and
damage infrastructure (Finlayson and D’Cruz 2005).
Invasive species from forest plantations are also threatening water supply for downstream users, as shown in South Africa where cities such as Cape Town and Port Elisabeth depend on runoff from the natural low biomass vegetation in the catchment (Le Maitre and others 1996). Invasive species in ri- parian areas are a problem for water resources in several other parts of the world. The annual losses due to the invasive woody species tamarisk in the semiarid western United States reach $280–$450 a hectare, with restoration costs of approximately $7,400 a hectare (Zavaleta 2000).
Water hyacinth, a rapidly growing, free-oating invasive plant, has degraded many ecosystems
Photo by C. Max Finlayson
Aquatic ecosystems Water-related agricultural modifications have had major ecological, economic, and social consequences, including effects on human health, through changes in the key ecological components and processes of rivers, lakes, floodplains, and groundwater-fed wetlands [well established]. ese changes include alterations to the quantity, timing, and natural vari- ability of flow regimes; alterations to the waterscape through the drainage of wetlands and the construction of irrigation storages; and increased concentrations of nutrients, trace elements, sediments, and agrochemicals.
Aquatic ecosystems provide a wide array of ecosystem services [well established].
eir nature and value are not consistent, however, and our understanding of how eco-
system processes support many of these services is inadequate (Finlayson and D’Cruz 2005; Baron and others 2002; Postel and Carpenter 1997). In several areas around the world changes have contributed to a loss of provisioning services such as fisheries, regu- lating services such as storm protection and nutrient retention, and cultural services such as recreational and aesthetic uses. In some cases ecosystems have passed thresholds