What threatens freshwater biodiversity? Climate change has some destructive allies...

The IPCC underlines that although climate change will affect biodiversity directly (through rising temperatures, changing rainfall patterns and habitat loss through sea level rise), many of it's negative effects will have come about in conjunction with other anthropogenic pressures to biodiversity (IPCC, 2002). It's important to remember that anthropogenic impact has a history far longer than recent concern about climate change, and that these pressures have already lead to biodiversity loss - I think climate change can only exacerbate damage already done. In this post I'll build on themes mentioned in the introduction to look at the important 'other' biodiversity threats specific to just freshwaters, and their likely interactions with climate change. 

Perhaps the largest threat suggested by the literature is habitat disturbance, fragmentation and loss. Groombridgeand Jenkins (1998) consider the habitat loss from draining wetland environments and building on floodplains, as well as the creation of poor habitat by water management strategies such as channel straightening and reservoir creation. Projects like these clearly remove habitat, but also make travel between remaining habitat difficult as a barrier of unsuitable conditions lies in between (for example an area that is too shallow to too exposed to terrestrial predators). Another water management strategy, increasingly common in countries experiencing seasonal water shortage, are dams - these physical barriers are well-known to adversely affect migratory species. It's these types of fragmentation that will interact most negatively for biodiversity with climate change, preventing vulnerable species from migrating away from areas where climatic conditions are unsuitable.

It's a peculiar situation in freshwaters that while strategies to manage water resources leave biodiversity vulnerable to climate change, as shown above, these strategies can be vital to prevent human vulnerability to climate change. As Vörösmarty et al. (2010) point out, most development projects to increase water security for people have negative impact on biodiversity. As adequate legal protection measures are not in place to protect freshwater biodiversity, and the future financial resources for water security will likely be far greater than for conservation (Vörösmarty et al., 2010), biodiversity needs are unlikely to be prioritised. Here, climate change will threaten biodiversity indirectly - as precipitation patterns change, creating water shortage in some parts of the world, more dams (and other water management projects) will go ahead, adding extra pressure to species already pressured by the direct physical and chemical impacts of climate change.

Pollution is another key threat, particularly in some parts of the world; 45% of China's major rivers were moderately to badly polluted in 2008 (Vörösmarty et al., 2010). Pollution can derive from diffuse sources, usually agricultural fertilizer and pesticide run-off, or point sources, such as waste dumps and effluent pipes. The study by Vörösmarty et al. (2010) found that downstream proximity to urban areas was a reliable predictor for pollution, although river habitat far upstream could be affected by urbanization at the river mouth. Pollution problems in freshwater systems make the community more vulnerable to climate change, as it may already be in an unstable state, with threatened species. Climate change is also likely to increase loading of pollutants like nitrates and phosphates (responsible for eutrophication) as warmer temperatures release organic material from permafrost and increase soil and sediment erosion.

Invasive species can cause huge problems in freshwater systems by competing for native species for resources, spreading disease or introducing parasites. Much literature has focused on this, for example, Dextrase and Mandrak (2006) found alien species to be the second largest threat to Canadian freshwater fishes after habitat degradation. Gurevitch and Padilla (2004) suggest that exotic species do cause dramatic ecosystem alterations, although little evidence shows they are solely responsible for the extinction of native species. Just like pollution, however, this pressure could combine with climate change to reduce biodiversity, and will also increase with warmer temperatures as species shift towards higher, cooler latitudes.

Ultimately, the take home message is that the future impact of climate change upon freshwater biodiversity will be complex, and that it won't be enough to try and fix any one aspect of the problem, as all anthropogenic pressures are interlinked.

References:

Dextrase, A.J and N.E. Mandrak (2006) 'Impacts of alien invasive species on freshwater fauna at risk in Canada', Biological Invasions, 8, 1, 13-24.

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

Gurevitch, J. and D.K. Padilla (2004) 'Are invasive species a major cause of extinctions?', TRENDS in Ecology and Evolution, 19, 9, 470-4.

IPCC (2002) Climate Change and Biodiversity, IPCC Technical Paper V.

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.