We all know that too much of a good thing can be bad for us, but how many of us realise the natural environment can also be harmed by having too much ‘nutrition’? Ecologist Ben Kite explores the unwelcome phenomenon known as ‘Eutrophication’.

When we walk or drive through the British countryside and absorb the bucolic agrarian scenes that it offers, we might be forgiven for thinking that all is well. After all, isn’t the countryside supposed to be ‘green’? Aldo Leopold, a ground-breaking American ecologist of the early twentieth century, suggests that a closer look may be required and states[1]: “One of the penalties of an ecological education is that one lives alone in a world of wounds. Much of the damage that is inflicted on the land is quite invisible to laymen.” I am reminded of this startling and profound statement whenever I am asked to contemplate the issue of nutrient pollution, as the damage that it causes is sometimes so insidious and cryptic that even experienced ecologists can fail to recognise it.

‘Eutrophication’ is the term given to the effect of nutrient pollution on living systems. When released into the environment, chemicals that act as plant nutrients (principally compounds of Nitrogen and Phosphorous) stimulate the growth of plants. In small amounts, an ecosystem can ‘metabolise’ these nutrients and no harm occurs. In larger quantities however, those species of plants best able to exploit the surplus nutrients will grow vigorously, out-competing their more delicate neighbours and causing localised extinctions; typically, this means more delicate or specialised native species.  Eutrophication leads to a range of effects on different ecosystems, but an almost universal outcome is the rapid decline in biodiversity that it precipitates.

An example of damage caused by eutrophication can be seen in my local ancient woodland, where the farmer of adjacent arable field recently decided to plough up the field margins and apply fertiliser when wind conditions must have been somewhat less than settled. The result, in the edge of the woods adjacent to the arable, has been the rapid replacement of a rich ground flora comprising such species as Bluebell Hyacinthoides non-scripta, Wood Anemone Anemone nemoralis, Dog’s Mercury Mercurialis perennis, Solomon’s-seal Polygonatum multiflorum, Sweet Woodruff Galium odoratum and Lesser Celandine Ficaria verna (see Image 1 below), with a dense matted tangle of Cleavers Galium aparine and Common Nettle Urtica dioica (see Image 2 below).

Fig 1Above: Image 1 showing native woodland flora without the influence of agricultural ‘eutrophication’ with Bluebells and Lesser Celandine in evidence. Photo: ©Ben Kite

Fig 2Above: Image 2 showing the effects of ‘eutrophication’ on the same woodland as shown in Image 1, with Common Nettle and Cleavers in the ascendant. Photo: ©Ben Kite.

Eutrophication can have long-lasting consequences, particularly where clay is present in the soil, as it will bind nutrients – slowing the rate at which they naturally leach from the soil. At Black Wood in North Hampshire, the Forestry Commission-owned Beech Plantation comprises compartments with varied histories. The Beech trees on both sides of the track, seen in the image below, are the same age, having all been planted together at some point either during or shortly after the Second World War (in the 1940s). The photograph was taken in April, and you can see that the trees on the left-hand side of the track are already breaking into leaf, whereas those on the right-hand side are bare. This strange difference is potentially explained by the fact that the area on the left was, according to the land utilisation survey map of the 1930s, in arable use prior to becoming a plantation, whereas the area on the right-hand side was previously part of an ancient medieval coppice known as ‘Stratton Wood’ (which was grubbed out and re-planted with Beech). There are alternative possible explanations for the varying performance of the Beech between these two adjacent compartments, but among them is the possibility that, despite the passage of more than 80 years, elevated nutrient levels still prevail in the former arable area. The soils here comprise clayey loams with flints over chalk bedrock.

Fig 3Above: Compare and contrast  – trees on either side of the track at Stratton Wood. Photo: © Ben Kite.

The vectors of the eutrophication ‘ailment’ are diverse, for example including: excessive fertiliser use by farmers and land managers (causing both air pollution as spray drift and water pollution as runoff into surface and groundwater); ammonia/ammonium windblown as gas/dust respectively from animal faeces stored at intensive livestock installations (e.g. slurry lagoons); discharges of human sewage into watercourses (which have recently been blamed for damage to the marine environment in Hampshire[2]);  and the content of the exhaust gases from our cars (which contain ‘oxides of Nitrogen’ that act as fertilisers). We are all, directly and indirectly, contributors to the environment’s nutrient over-consumption problem.

In relation to air pollution alone, scientists evaluate whether air pollutants might do harm to ecosystems by establishing a threshold known as the ‘Critical Load’, beneath which no harm is caused ‘according to present knowledge’. On behalf of Defra, the Centre for Ecology (CEH) retrospectively models the extent of Critical Load exceedance across the UK every year from air pollution and produces updated ‘trends’ reports, looking back at past air monitoring data. The 2019 update[3] shows that in 2016 (the year analysed), although broadly speaking levels of air pollution were declining, 58.1% of UK habitats were still in exceedance of their Critical Load for nitrogen deposition (eutrophication); and between 78.6% and 98% of all designated Sites of Special Scientific Interest (SSSIs) exceeded the Critical Loads that have been assigned to them regarding one or more of the features for which they were designated.

Air pollution can be a particularly difficult driver of eutrophication to identify with any confidence, due to the huge range of other influences that affect vegetation composition (either resembling or masking the effects of eutrophication) combined with the limitations of environmental models. A team of colleagues and I recently undertook an extensive exercise of survey and statistics to explore what superficially appeared a clear-cut example of a well-documented outcome of air-pollution related damage to heathland. The damage symptom that was suspected manifests itself as the displacement of heather-dominated heathland by grassland, and broadly this phenomenon had been observed alongside the A31 road corridor in Hampshire’s New Forest (ostensibly caused by nitrogen deposition from vehicle exhausts). After onerous study over several months, we concluded[4] that the acid grasslands present were in fact the result of a combination of roadside heathland management (heather cutting, baling, burning and Bracken spraying) combined with the elevated grazing pressure and livestock ‘dunging’ that follows these interventions (due to the resultant fresh grass growth); and to a lesser degree the chemical influence of materials used in road construction and management. Further, the acid grassland areas in question were ecologically diverse, containing some species of considerable conservation importance – certainly not areas that should be discounted merely as ‘damage’.

A widely cited example of eutrophication is the plight of lowland species-rich neutral grasslands in the UK (referred to as ‘wildflower meadows’ in the press). The erstwhile ‘UK Biodiversity Action Plan’ for Lowland Meadow Priority Habitat[5] refers to a 1984 assessment which determined that the resource of ‘semi-natural’ (native species rich) grasslands in lowland England and Wales “…. declined by 97% over the previous 50 years…” and that “Losses have continued….. and have been recorded at 2-10% per annum…”. The Biodiversity Action Plan lays the blame for this squarely at the feet of what it terms ‘agricultural modification’, which includes fertiliser use as a main driver, alongside re-seeding and ploughing up of grasslands.

When I am out surveying, time and again I encounter ‘improved’ (in the agricultural sense) pastures, hallmarked by their ‘giveaway’ vibrant but uniform lime-green colour. These usually comprise an uninteresting near-monoculture of (usually) sown grasses; often Perennial Rye Grass Lolium perenne swards, typically with around 5-10 species of forb, including those ecologists consider ‘indicators’ of nutrient pollution when in abundance, such as Creeping Thistle Cirsium arvense , Cleavers, Common Nettle and Creeping Buttercup Ranunculus repens.

163168Above: Creeping Thistle, a classic indicator of nutrient pollution. Photo: ©Paul Sterry/Nature Photographers Ltd.

Grasslands like these are now an extensive fabric in the tapestry of the countryside, but it wasn’t always this way – English poetry and literature from the eighteenth and early nineteenth centuries is strewn with accounts of quintessentially English but now illusory ‘wildflower meadows’. John Clare for example, in his poem ‘The Mores’ laments the disappearance (due to enclosure) of his local landscape “That never felt the rage of blundering plough, Though centurys wreathed spring’s blossoms on its brow”[6].

To appreciate what we’ve lost, it is necessary to contrast modern grasslands with what existed before. This is difficult, as relict species-rich grasslands are increasingly rare and falling out of the public’s living memory as a result – a phenomenon called ‘landscape amnesia’. Take however, as an example, Cricklade North Meadow National Nature Reserve (NNR) near Swindon. This incredible site, saved from nutrient application by virtue of being protected by an archaic ‘Court Leet’ since Saxon times, is famous for its spring display of Snake’s Head Fritillary Fritillaria meleagris (Figure 5), but the grassland here is also a wonderfully rich treasure trove of more than 250 other species of vascular plant (take a moment to compare that number with my previous description of modern grasslands). A brief stroll in spring or summer offers the opportunity to explore a complex and richly woven carpet of wildflowers, with species such as Greater Burnet Sanguisorba officinalis, Cowslip Primula veris and Marsh Marigold Caltha palustris in number.

Fig 5Above: Cricklade North Meadow National Nature Reserve, famous for its Snake’s Head Fritillary population. Photos: ©Ben Kite (main); Paul Sterry/Nature Photographers Ltd (inset).

The fumigation of diversity from our landscape is not in my view simply an academic curiosity or indulgent sentimentality – it matters. Impacts on plants at the bottom of the food chain have profound consequences higher up it, through a phenomenon known as a ‘trophic cascade’. Simplified, the fewer plant species we have, the fewer insects and other invertebrates there will be that would otherwise feed and live on them, and in turn, there will also be fewer larger animals such as birds and bats, that feed on them: “Big fleas have little fleas upon their backs to bite ‘em, and little fleas have lesser fleas, and so, ad infinitum…”[7]. Aside from the direct benefits that access to beautiful open spaces has for human health (mental and physical) therefore, the species at every level in this complex web provide what are known as ‘ecosystem services’ to humanity, such as waste-recycling or natural crop pest control. We ourselves are part of this global ecological machine and subject to its breakdowns. As Aldo Leopold wisely wrote: “To keep every cog and wheel is the first precaution in intelligent tinkering”.

161787Above: Arable farmland in north Hampshire: colourful, but about as intensively farmed as it gets. Photo: ©Rob Read/Nature Photographers Ltd

As the physicians of the natural world, ecologists, however skilled, cannot do much more to treat eutrophication than identify the condition and apply topical first aid to the damaged area. For acute cases, a prescribed nutrient diet may be warranted, but as with any form of collective over-indulgence, the prognosis for the long-term recovery of the countryside requires the holistic moderation of its nutrient intake. Whether we are farmers or car drivers, we each have a role to play in this.

Ben Kite is Managing Director and Principal Ecological Consultant at Ecological Planning & Research Ltd

 

References:

[1] Leopold A (1949) A Sand County Almanac. Oxford University Press

[2] https://www.bbc.co.uk/news/uk-england-hampshire-48634596

[3] Rowe E, Sawicka K, Mitchell Z, Smith R, Dore T, Banin L & Levy P (2019) Trends Report 2019: Trends in critical load and critical level exceedances in the UK.

[4] Kite B (2019) Will there be Adverse Effects on the Integrity of an SAC from Air Pollution? – The Confounded and the Confounding: A Case Study into Air Quality and the Heathlands of Hampshire’s New Forest. The Habitats Regulations Assessment Journal. Issue 12 July 2019. pp 20-24.

[5] Joint Nature Conservation Committee (JNCC) (2016) UK Biodiversity Action Plan Priority Habitat Descriptions: Lowland Meadows

[6] Clare J The Mores (1812-1831)

[7] De Morgan A (1806-1871) Siphonaptera