Diversity Loss due to Nitrogen Deposition

In this post I’m going to look at another of the important environmental impacts of the anthropogenic alteration of the global nitrogen cycle: loss of diversity.  A key recent paper in this area has been written by Bobbink et al. (2010), who give a synthesis of the global changes in terrestrial plant diversity caused by nitrogen deposition. 

Bobbink et al. state that nitrogen accumulation is the main driver of changes to species composition across the whole range of global ecosystem types.  This is because N accumulation drives competitive interactions that lead to composition change and can also produce conditions that are unfavourable for some species. There are also other effects playing a supporting role, such as the toxic nature of N gasses, the long-term negative effects of increased ammonium and ammonium availability, and acidification, in addition to more localised secondary stresses.  The main impacts of increased nitrogen deposition on terrestrial ecosystems are summarised in the diagram below: (Bobbink et al., 2010)

 

(positive (+) and negative (-) feedbacks are shown in brackets, ↑ denotes productivity increase ↓ productivity decrease)  

Bobbink et al summarise the impacts of N deposition in every ecosystem type and the mechanisms by which they occur.  They have also created a new method to identify the ecosystems of high conservation value that are most threatened by the trend of increasing nitrogen deposition. This has been done by overlaying modelled nitrogen deposition with WWF G200 ecoregions, ensuring that both diversity hot spots and regions with their typical ecosystems are covered. Importantly, the ecoregions relate to ecosystem types whose response at different locations to N deposition can be compared and contrasted. The model show the areas of greatest deposition to be Europe, N. America, southern China and parts of southern and SE Asia, by 2030 Latin America and Africa are also predicted to be more significantly affected (see below).  

The total N deposition rates for (a) the year 2000 and (b) the year 2030 within the G200 ecoregions (deposition outside these areas is not shown.) (Bobbink et al., 2010)

Bobbink et al. state that it could be later than we think in the fight against biodiversity loss due to N deposition, as the problem in boreal forests, mediterranean systems, and some tropical savannas and montane forests could be more serious than thought previously.  They say that there are many questions about the impacts of N deposition on biodiversity still open, particularly in more remote regions.