Impact of atmospheric pollutants on the current and future status of protected habitats.

The Environment Agency has a statutory duty to ensure that operations carried out by industrial operations do not have an unacceptable impact on habitats and species protected under the EU Habitats Directive 1992 and the Conservation of Habitats & Species Regulations 2010. Intensive livestock units are a particular type of industrial process that are required to apply to the Environment Agency for a permit under the Environmental Permitting Regulations (England and Wales, 2010). Natural England, Natural Resources Wales, and SEPA are consultees in this process.

Much research has linked ecosystem damage to nitrogen deposition from agriculture and fossil fuels with demonstrated impacts on biodiversity, ecosystem services, N saturation and the onset of leaching to freshwaters, in experiments and spatial surveys (Phoenix et al 2012; Stevens et al 2010). Currently, around 60% of UK protected sites are expected to exceed their critical load (RoTAP 2012), yet at many there are limited obvious signs of direct impact from N.
This contradiction between the science and site based evidence creates a legislative problem for regulatory bodies with confusion in the dose-response relationships between vegetation and pollution. Modelling based around contemporary pollution suggests a curvi-linear (Field et al 2014), linear (Maskill et al 2010), and threshold responses (Tipping et al 2013). However, the effects of N deposition are thought to be cumulative over time (Phoenix et al 2012) and when species richness values from spatial surveys are modelled using cumulative N, the relationship becomes more linear. It is not clear if this change in relationship type is ecological or mathematical and the legitimacy of either needs further investigation.

The use of cumulative N as an alternative to contemporary pollution deposition also raises questions due to the assumption of a blanket, cross-habitat response to cumulative N. Different habitats and the plant functional groups within those habitats respond to long-term pollution over different timescales ranging from a few years to centuries (Evans et al 2006; Posch et al 2011). There are also issues in accurately calculating cumulative deposition, with estimates of long-term pollution to areas which have experienced increases in N deposition over recent decades likely to be too high, whilst estimates for areas with historically high pollution likely to be too low (Fowler et al 2005). The NERC funded Long term Large Scale Project is attempting to more accurately model 200 year cumulative deposition.

Dynamic modelling using models incl. MAGIC, GBMOVE and For-SAFE VEG may also offer some solutions to this problem by enabling specific habitat "N-limits" to be calculated, thereby, customising the N response through the addition of a temporal element (Evans et al 2006). Recent testing of models highlights their potential to predict plants species occurrence (Rowe et al 2011) and cover (De Vries et al 2010). An interesting application of dynamic modelling is the reverse calculation of critical loads from modelled vegetation data, with the advantage that future scenarios can be considered (e.g. pollutant reduction). Disadvantages of dynamic modelling include poor forecasting of lower plants that respond directly to N (Smart et al 2005), and a lack of consideration of the different forms of N, particularly the high concentrations of ammonia around intensive farming units (Sheppard et al 2011).

Recent research has also suggested that an absolute ecosystem critical load is unlikely, proposing instead ranges of deposition that drive soil properties into optimum zones for plant species (Posch et al 2011). There is some evidence that this has occurred in surveys, with sites resting on a pH threshold, vulnerable to a step-changes (Field et al 2014), and many species occurring over a deposition range though appearing to decline above a change point (Payne et al 2013).