Identify and discuss the evidence to support the view that nitrogen pollution is a cause for decline in biodiversity. Illustrate your essay with examples from habitats of your choice and include details of likely mechanisms by which nitrogen pollution may have its effect
Due to increasing industrialisation over the last century, Nitrogen (N) deposition has increased dramatically and, in the past twenty years, has increased exponentially as a result of agricultural innovation and in the amplified use of fertilisers and pesticides. There are numerous associated effects that are linked not only to biodiversity - the variety of organisms at all levels; from variants of the same species to the variety of ecosystems, but to spatial diversity, species richness and even indirectly affecting species of animal.
Nitrogen deposition can manifest itself in two main ways, 'non-point pollution' - meaning that it doesn't come directly from a source (i.e. it is leached into the soil through run-off, air pollution etc.), and 'point pollution' - meaning immediately from a source (such as a river being contaminated directly from a factory). This essay will look largely at non-point anthropogenic pollution - meaning pollution produced by humans, as it will be looking more at the effects on species of plant life and insects, rather than the broader spectrum of general 'point' pollution such as rivers (which involve processes of eutrophication and acidification).
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There have been numerous studies done on the subject of nitrogen deposition, many of which have covered a range of topics within the subject matter including responses of plants and insects to increased N, recovery from exposure to N and a number of studies relating to the various habitats in which N deposition has increased and therefore their own specific responses and reactions. The first of these habitats to look at is perhaps the most common in the UK: grassland.
Much of the countryside in Great Britain is grassland and therefore - if there is evidence that increased N deposits do cause a decline in biodiversity - then it will inevitably have an effect in wide-scale terms right across the country. Under normal circumstances, nutrients are obtained by plants through a number of processes; primarily through the weathering of - and therefore release of nutrients within - the bedrock; and nutrients from the atmosphere via wet fall, i.e. rain, snow and fog (Mackenzie, Ball, & Virdee, 2001). The first thing to note is that N is the limiting nutrient for plant growth, meaning that regardless of growing conditions, if there is very little N, then plant growth will be inhibited. From this, we can infer that an increase in N should theoretically increase cellular growth and therefore size, however this is not necessarily the case.
By measuring N deposition at different sites across the UK, and taking the area with the lowest measurement as 'pristine' i.e. the least affected, an equation can be utilised to work out what the average species richness is for that one less affected area. This can then be substituted for an area with much higher N deposition using the same equation and it has been found that there is an approximate 23% decline in species richness between the two (Stephens, 2004). This is for an area of average N deposition, so it can be assumed that regions with even higher concentrations will have even higher rates of decline. The same study has also shown that, perhaps as expected, nitrophilous species - those that grow well in soils with a high concentration of N - were shown to increase, and those species that respond negatively to N declined.
Further studies on grasslands have confirmed that high levels of N do indeed cause a decline in abundance - this time specifically in bryophytes - and that these can also helpfully be indicators as to the effects of and recovery from N deposition (Arroniz-Crespo, Leake, & Horton, 2008). Additionally, this particular study found that although there was a clear loss of abundance, up to 90% in some cases, there was virtually no recovery observed at all. This has led to further theories that not only do the non-vascular plants struggle to cope with increased N, but also that the higher plants that have a higher tolerance to it and therefore increase their growth create more shade, allowing less light to penetrate to the lower plants such as the bryophytes and lichens on the ground, further limiting their growth.
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This is however just a theory and very few studies have been done with regard to the effect of N on photosynthesis and plant competition. It should also be noted that this study included both N deposition and phosphorous fertilisation and so the results may be slightly skewed away from just the effects of nitrogen. However, because bryophytes lack good roots and most of their nutrient uptake is from the atmosphere, they are a very good gauge of atmospheric deposition of nutrients and therefore remain a useful form of indicating whether new legislations on cutting pollution are effective or not.
The next investigation looked exclusively at species of Sphagnum mosses and whether they modify their N uptake in reaction to changes in anthropogenic N levels. Two species - Sphagnum balticum & Sphagnum fuscum were collected from areas of low to medium N levels and then were subjected to varying increases in N. The study was done using species taken from peatland, a habitat that generally has limited amounts of N. It was found that higher concentrations led to a decline in biomass of the Sphagnum species and yet interestingly with the higher plants, it appeared that they reacted to a higher supply of N by actually using less; contrary to previous suggestions (Wiedermann, Gunnarsson, & Ericson, 2009). What must be kept in mind at all times is that all experiments are relatively short-term and N deposition is a long-term process. The conclusions drawn up in this research support the theory that increased N does indeed alter the composition and diversity of a number of species, both vascular and non-vascular.
Nitrogen deposition does not only have an effect on life-forms, as inherently once it is in the soil, a number of other biogeochemical processes can be influenced which in turn may themselves have impacts on the species living within the habitat. Particularly on moorland habitats, nitrogen is apparent in many forms including vegetation composition, eutrophication and interestingly, water acidification in lake water as a result of leaching through the soil (Curtis & Emmett, 2005). This again is a study of bryophytes and lichens in another habitat and similar results to support the theory have been found.
The lichens, mosses and bryophytes have been confirmed to not only be excellent indicators of excessive anthropogenic N, as found in other studies, but also for the first time, appear to be essential in reducing the effects of nitrate leaching and consequently acidification and eutrophication by showing 'far greater N recovery per unit biomass than grasses or ericaceous shrubs' (Curtis & Emmett, 2005). It is therefore imperative that any moorland management techniques - slash-and-burn, grazing etc. - do not destroy the bryophyte and lichen populations, as careful management of such species is significant in continuing to lower pollution as a whole. However, in the opinions of Curtis et. al. the only consistently effective way to reduce N leaching is a large-scale reduction in emissions.
It is interesting to observe the relationship between N deposition and plant composition, but to get a full understating of its effect upon biodiversity, one must also take into account the insects that live within the affected plant communities and the interactions between the two. A study was undertaken looking at the indirect effects of N deposition on an aphid population - Aphis nerii - downwind of an urban environment (where anthropogenic N is significantly higher) and it was found that not only is plant biomass increased, but - perhaps somewhat expectedly, as a result of this - aphid populations increase too. Moreover, it was shown that the aphids appeared to have a higher survival rate and most interestingly of all, that the development time of the insects decreased dramatically allowing for a much higher abundance than previously noted (Zehnder & Hunter, 2008).
Finally, to get a full understanding of the effects of nitrogen, one must look not only at how communities respond to increases, but also at how particular habitats respond to controlled decreases in order to calculate time-scales for recovery. In 1998 a collaborative study was undertaken by the Imperial College London and the University of York that lasted until 2005 that looked exclusively at heathland recovery to a decrease in N deposits and the findings were quite significant.
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After effectively completely ceasing N deposition on the test site, it was discovered that even after 8 years of virtually no anthropogenic N being released into the area, traces were still apparent on some species. This now suggests that prolonged deposition could result in effects on vegetation cover that last for decades. Because these results come from a study in which N deposition was completely halted, it is not necessarily realistic in real-world terms (it would be impossible to end pollution outright) and it has been suggested in the same study that this pollution may in fact continue to rise for at least the next 50 years (Power, 2005).
Supporting this, data from the Department for Environment, Food and Rural Affairs (DEFRA) shows that although UK emissions of oxidised nitrogen and similar chemicals have in fact decreased over the last two decades, the rate of deposition has not decreased to the same degree (DEFRA, 2010). As the results of these experiments show, there is undoubtedly an inextricable link between increased nitrogen pollution and decreased biodiversity across a range of habitats and populations and realistically, a huge and swift reduction in man-made pollution must become a reality.