How is current freshwater biodiversity faring?

Define biodiversity?

The term is used in many different contexts - perhaps because it comes from the conservationist movement rather than biology - but is usually taken to mean the variety of all life on Earth. Broadly, the classic definition is: biological diversity is the genetic diversity within species, the diversity of different species and the variety of ecosystems on Earth (UN, 1992).

What's the current state of biodiversity for freshwater habitats?

Relative to the spatial extent of habitat, the species richness of freshwaters is very high in comparison to marine or terrestrial environments, and the bounded nature of lakes and rivers has lead to high levels of endemism (Groombridge and Jenkins, 1998). Unsurprising then, that Myers (2003) highlights the obligation to more thoroughly and extensively document freshwater species, suggesting that freshwater ecosystems could turn out to be crucial biodiversity 'hotspots' for conservation. Despite these gaps in the data the importance of freshwater biodiversity is well-known, and it's already a high priority for most conservation efforts. For example it's first on the agenda for the International Decade for Action from 2005-15 (Dudgeon et al., 2006).

Unfortunately this high conservation priority is not just down to the natural richness of freshwaters. In a recent study by Vörösmarty et al. (2010) which used threat indices to map global incident biodiversity threat, it's only a tiny percentage of the world's rivers that remain unaffected by people. Freshwater habitat is among the worst threatened by humans (Myers, 2003), and as a result it's quality is in decline; most monitored freshwater species populations are falling (Groombridge and Jenkins, 1998). Freshwater fish in particular are known to be highly threatened (Groombridge and Jenkins, 1998) and as these are often the only taxa monitored, many conservationists assume that fish species declines are indicative of increasingly poor freshwater ecosystem health in general.

References:

Dudgeon, D., A.H. Arthington, M.O. Gessner, Z.I. Kawabata, D.J. Knowler, C. Lévêque, R.J. Naiman, A.H. Prieur-Richard, D. Soto, M.L.J. Stiassny and C.A. Sullivan (2006) 'Freshwater biodiversity: importance, threats, status and conservation challenges', Biological Reviews, 81, 163-82.

Groombridge and Jenkins (1998) Freshwater Biodiversity: A Preliminary Global Assessment, Biodiversity Series No. 8, Cambridge: World Conservation Monitoring Centre. 

Myers (2003) 'Biodiversity Hotspots Revisited', BioScience, 53(10):916-917.

UN Conference on Environment and Development (1992) Convention on Biological Diversity, Rio de Janeiro: UN Conference on Environment and Development.

Vörösmarty, C. J., P. B. McIntyre, M. O. Gessner, D. Dudgeon, A. Prusevich, P. Green, S. Glidden, S. E. Bunn, C. A. Sullivan, C. Reidy Liermann and P. M. Davies (2010) 'Global threats to human water security and river biodiversity', Nature, 467, 555-60.