Introduction

The complex geological history of Mexico’s land has given rise to a rugged topography which, combined with the diversity of climates, allows for a rich variety of water bodies including rivers, streams, lakes, lagoons and estuaries. Their waters can be freshwater or saline, and ephemeral, seasonal or permanent. The types of vegetation that thrive in these environments are varied: riparian forests, which extend along the banks of rivers and streams, popals, bulrushes, and certain types of submerged aquatic vegetation. In terms of biodiversity, although the total number of species inhabiting the planet's freshwater bodies is lower relative to marine environments, freshwater biodiversity is considerably higher than the marine biodiversity when their relative areas are factored in (Arriaga et al., 2000; Revenga et al., 2000; Groombridge and Jenkins, 2002).

Freshwater ecosystems provide a wide variety of goods to society, ranging from drinking water for human populations, agriculture and industry, to many types of food, fiber, medicinal plants, fuel and building materials, among others (Revenga et al., 2000; UNDP et al., 2000; Baron et al., 2003; Schuyt and Brander, 2004). These also provide important environmental services, including the maintenance of biodiversity, climate stabilization, mitigation of floods, assimilation and dilution of pollutants, nutrient recycling, restoration of soil fertility and recharge of aquifers (Arriaga et al., 2000; Revenga et al., 2000; UNDP et al., 2000; Harvey, 2001; Groombridge and Jenkins, 2002; Schuyt and Brander, 2004). As an example, most of the electrical power that supplies the world's population is generated at dams that make good use of inland freshwater flows (Groombridge and Jenkins, 2002).

The strong dependence of human societies on goods offered by freshwater bodies has deteriorated and threatens the permanence of many species, and hence the integrity and proper functioning of ecosystems. The condition and degree of threat of freshwater ecosystems is even greater than the one observed in forest and coastal ecosystems (Revenga et al., 2000; UNEP, 2007). To note, part of the pressure on freshwater communities derives from human activities at a watershed level. This pressure is exerted on two fronts: on the one hand, through a direct impact, i. e. by modifying or reducing the areas of these ecosystems and through the extraction and introduction of specimens; on the other, by modifying the amount and quality of water sustaining these ecosystems. The expansion of urban and tourist areas, the overexploitation of fisheries resources and the introduction of exotic species are some of the main activities that directly impact freshwater biodiversity. Also, alterations in courses of rivers by dams, water overuse and pollution by agricultural, municipal and industrial discharges are the most important sources contributing to the deterioration of habitats (Miller et al. 1989; EPA, 1992; Poff et al., 1997; Arriaga et al., 2000; Revenga et al., 2000; UNDP et al., 2000; Revenga and Kura, 2003; MEA, 2005).

References

Arriaga C., L., V. Aguilar S. y J. Alcocer D. Aguas continentales y diversidad biológica de México. Conabio. México. 2000.

Baron, J. S., N. L. Poff, P. L. Angermeier, C. N. Dahm, P. H. Gleick, N. G. Hairston Jr., R. B. Jackson, C. A. Johnston, B. D. Richter y A. D. Steinman. Sustaining Healthy Freshwater Ecosystems. Issues in Ecology 10. 2003.

EPA. The Quality of Our Nation´s Water. Office of Water. Washington D.C. 1992.
Groombridge, B. y M. D. Jenkins. World Atlas of Biodiversity. UNEPWCMC. University of California Press. USA. 2002.

MEA 2005. Ecosystems and human well-being, Current state and trends, vol. I Millennium ecosystems assessment. USA 2005.

Miller, R. R., J. D. Williams y J. E. Williams. Extinctions of North American fishes during the past century. Fisheries. 14:22-38. 1989.

Poff, N. L., J. D. Allan, M. B. Bain, J. R. Karr, K. L. Prestegaard, B. D. Richter, R. E. Sparks y J. C. Stromberg. The natural flow regime. Bioscience. 47(11): 769- 784. 1997.

Revenga, C., J. Brunner, N. Henninger, K. Kassem y R. Payne. Pilot Analysis of Global Ecosystems. Freshwater Systems. WRI. Washington, D. C. 2000.

Revenga, C., y Y. Kura. Status and trends of biodiversity of inland water ecosystems. CBD Technical Series 11. SCBD. Montreal. 2003.

Schuyt, K. y L. Brander. The Economic Values or the World´s Wetlands. WWF. Gland/Amsterdam. 2004.

UNDP, UNEP, WB y WRI. World Resources 2000-2001. WRI. 2000.

UNEP. GEO 4. Global Environmental Outlook. Environment for development. United Nations Environment Programme. Malta. 2007.