Are humans a natural disturbance?


I think contemporary human intervention in our landscapes is unnatural compared to the effects of other native and wild species. Some will argue instead that our intervention over history has enhanced the value of landscapes and, anyway, there are too many of us now for it to be any other way in the UK – this is the natural effect of modern-day humans, the “proper place of human activity in ecology” (1). Can this really be true?

As a Permaculturist, observation of natural systems, recognising their patterns and relations, is important to me in applying the many lessons from natural ecology to the cultivated ecology I seek to achieve. It’s why I study how plants and animals grow and interact together in communities (guilds), discovering what function they fulfil and where I may contribute and benefit. It’s about the opportunities and yields found in edges and cycles, and in the multi-dimensional richness of complex successional assemblies of vegetation that support a mass of food webs (the trophic cascade) in landscapes that pulse with vitality and potential (2).

Ecological succession as a concept has had a hard time since the first studies just over a century ago identified that vegetation is constantly undergoing a process of change to a “higher type of community” in the development of a climax state at equilibrium (1). The stability of the climax is created by the dominance exerted by these higher plants over the site conditions, so that they exclude the invasion of other potential dominants. If you are thinking forest, then that’s an example.

The concept of succession was an early attempt at understanding the dynamic functioning of ecosystems, and other concepts arose from it. One was retrogressive succession, a “degradation of vegetation to a lower type” whereby factors - such as grazing, gradual leaching of minerals (think of moorlands), drying up, or waterlogging – could reverse the progress towards domination and instead allow the invasion and growth of different, replacement species. Countless arguments followed about the causal factors for these modifiers of vegetational change. Tansley, one of the early authors on succession, believed that it didn’t matter what was the initiator of the change – thus grazing by livestock introduced by man was equivalent to the natural grazing by buffalo of the North American prairie, or the antelope on the South African veld - because the response to the change would be the same, a similar equilibrium would be reached (1).

You may be interested to know of the contemporary management dilemma faced by prairie restorationists. The natural choice would be expected to be buffalo, but domestic cattle provide a more reliable management solution because they graze more uniformly throughout the prairie. Those troublesome buffalo tend to overgraze patches as large as a couple of acres, ignoring the surrounding prairie, and creating bare earth which, along with their earth wallows, are prime areas for the invasion of woody species (3). Thus is the grazing by livestock the natural equivalent of buffalo?

Natural disturbance - gaps and patches

It is fashionable now to denigrate the concepts of succession and climax, but no one has come up with an alternative term for peak development.  Climax as a concept was overtaken when later studies led to the idea of cycles of change brought about by natural disturbances that disrupted the formation of dominating equilibrium communities - natural disturbance preventing communities from reaching a stable state. The terms community succession and community assembly that are now used were also an important advance, as they gave a focus to species composition in the course of community development. The proximity of seed sources, differential timing of maturity and age of reproduction, annual seed production, and timing of disturbance have all been found to influence the spatial and temporal aspects of community development (4). Thus Egler, in his Initial Floristics Model, proposed that variation in initial composition, unrelated to abiotic site differences, resulted in the establishment of different community types (5). Among late-successional species, those already present in the seed bank or arriving shortly after a disturbance event were able to establish in sufficient numbers such that later arrivals were not able to change the course of community development, a  priority effect that is essentially about getting there first.

The certainty of endpoint and stability of the concepts of succession and climax were replaced by the gap and patch dynamics of modern ecology in which disturbance is a natural phenomenon that varies along a gradient of intensity (6). In minor scale, it creates local instability in natural systems that then react, sometimes returning it to what it was, but if the disturbance is greater – or even catastrophic - then the trajectory of the landscape may be altered. In addition, opportunistic species may move in after large scale or continual change, the overall pattern of the landscape altering to give rise to a mosaic of habitats rather one dominating habitat. Thus cyclical change or disturbance of sufficient scale can promote persistence in the varied patterns (patches) of the plant community.

A few examples come to mind of habitats in which natural cyclical disturbances are easily seen, and which prevent the formation of a dominating, stable climax. Coastal sand dunes are where both erosion and accumulation are natural elements of dynamic dune evolution. Coastal saltmarsh is eroded by exposure at its seaward edge but, given the freedom, will naturally reform although not necessarily in the same place. Similar arguments may be made about reed beds that silt up, but new ones can form elsewhere. Sea caves are fascinating habitats that experience strong wave surge, which sweeps up coarse sediment, cobbles and boulders that scour the cave walls. Physical conditions, such as slope, wave surge, scour and shade, change rapidly from the entrance and on into the cave, resulting in zonation in the communities present such as encrusting animal species, cup corals and sponges. Volcanic activity of the past would have resulted in a cyclical pattern of habitat destruction and fragmentation by lava, followed by habitat regeneration on newly formed soils.

Even in woodland, the early model for succession, the composition and dynamics are affected by wind, drought, fire, snow, disease, pests, and flood, with aspect and elevation overlaying these disturbances (7). Natural disturbance in woodland can thus be a source of diversity and renewal, but again there is a distinction between disturbance in small areas and at short timescales that generate localised instability, compared to larger scales – upto catastrophic disturbance – where even the largest and most spectacular impose a degree of dynamic stability.

At the lower end, windthrow of older, weaker trees is common in natural woodland, hurrying up their demise. The tendency is for this to create small, dispersed gaps throughout the woodland that probably only add up to 10% of the woodland area and, depending on species, the canopy will close over the gap within 15-20 years. Natural counterparts of traditional forms of human woodland management are rare. Coppice probably only develops in a fragmentary, irregular and temporary form from crushed saplings and shrubs after blowdown (and rarely also from browsing since it provokes instead a defensive style of shrubby, spiny-like growth). In larger scale, there have been five notable wind events in the last 50 years. However, only that of 1987 had the force of a catastrophic disturbance, and you have to go back to 1703 to find one comparable to that. This suggests that away from coasts and higher hills, devastating winds recur in the order of 200-300 years, and thus in the order of the natural ageing and turnover of woodland.

A matrix of artificial disturbance

People can of course devastate woodland in a much shorter timescale, and livestock will prevent it from returning. Absurdly, nature conservation draws support for its interventionist management approach on the natural and cyclical disturbances implicated in gap and patch dynamics, and from the belief that human intervention is only nature acting by other means. Chopping down and poisoning the naturally regenerated trees of secondary woodland, and preventing them from returning by grazing with livestock, is a common human disturbance in landscapes on conservation grounds, such as for chalk grassland or heathland. It's as if humans have determined when and where a catastrophic natural disturbance is going to take place, along with the influx of “wild” herbivores in response to the new growth. What I find perverse is that one of the early studies reckoned to have advanced our understanding of the effect of natural disturbance was based on a heathland landscape. Watt’s research (8) was located on the rabbit infested, sandy landscape of Lakenheath Warren. Watt wrote to his son in the 1960's about his research in the Brecklands (9):
This was a most interesting area with [sand] blow-outs in all stages of formation, development, and stabilisation, in which processes vegetation plays a very important part…… These studies forced upon me the idea of a cycle of change in which vegetation develops to a point where it becomes ‘old’ and more susceptible to hostile factors acting periodically or sporadically.”

Watt, who earlier had been a research student of Tansley's, was in fact observing the vegetational processes that were happening within a matrix of artificial disturbance - the natural effects of wind and drought being made worse by this. Thus the nature we get to see in the UK is often that which happens between and around human interventions. This is particularly so for Breckland (10):
“Although geology and climate are clearly important, much of what is interesting about Breckland is a consequence of human activity. Early stone-age hunter-gatherers occupied the region and may have burnt areas of woodland to attract deer to the subsequent regrowth. When Neolithic agriculturalists came to Britain more than 5000 years ago, Breckland was one of the first areas they colonised. The Neolithic people also brought grazing animals across the North Sea to Breckland. The grazing of these sheep, goats and cattle prevented the cleared woods from re-establishing - so the heathlands and bleak open landscape were created by the Neolithic people…….For the past 900 years much of Breckland has been heavily grazed by rabbits …….introduced by the Normans …
for the production of meat and fur….. From the 12th century the overgrazing by these huge populations of rabbits, combined with their scraping and burrowing, often led to the development of sand blow-outs.”

This historical account lays bare the crucial truth about landscapes primarily influenced by human disturbance, and now dependent for their continued existence on management techniques that reproduce old farming practices (10):
“The history of Breckland is an interesting part of our culture but it also indicates the management necessary to preserve the heaths. The consistent feature of Breckland is that it has always been disturbed by flint workers, farmers, rabbit warreners, the military and rabbits. This disturbance has often been accompanied by nutrient depletion and grazing. If we wish to maintain these unique habitats it is likely that we will have to mimic these unique aspects of their history. Disturbance is as much a traditional part of the management of Breckland as coppicing is for many ancient woods or haymaking is for hay meadows.”

Is this agricultural style of landscape management a natural disturbance? Tansley wrote (1):
“Forest may be converted into grassland by grazing animals…….It is true of course that when man introduces sheep and cattle he protects them by destroying carnivores and thus artificially maintains the ecosystem whose essential feature is the equilibrium between the grassland and the grazing animals. ……In such ways anthropogenic ecosystems differ from those developed independently of man. But the essential formative processes of the vegetation are the same, however the factors initiating them are directed.”

The effect of carnivores in natural systems isn’t just a numbers game, where predation of herbivores creates what is called a density-mediated trophic cascade. Herbivorous wild animals are redistributed around the landscape by predators so that the effects of their browsing or grazing are not spatially uniform, and thus it is also a behaviourally-mediated trophic cascade. A series of papers in the last few years document the changes in vegetation in the Yellowstone Ecosystem as a result of the reintroduction of grey wolves (11, 12). Willow species and aspen in riparian plant communities have shown significant regeneration along with the returning wolf population, these studies showing that the herbivorous actions of elk are effected by the increased risk of predation in differing locations, such as along river banks. Another paper links a decline in cougar numbers in some areas of the Zion National Park, Utah, with an increase in human visitor numbers, and a catastrophic decline in the whole trophic cascade due to the loss of recruitment of riparian cottonwood trees from the herbivorous action of mule deer (13).

The long-term effect of habitat alteration in the UK and removal of predators has been to place grazing and browsing levels throughout our landscapes at the mercy of human values rather than natural process. Tansley accepted that our destructive activities upset pre-existing ecosystems and eventually destroyed them, but he saw that new ones were forming of a very different nature (1). He stuck his neck out when he said that it would be difficult, not to say impossible, to draw a natural line between the activities of the human tribes which presumably fitted into and formed parts of "biotic communities" and the destructive human activities of the modern world”. However, in the modern world, we tend to forget that there are still locations where human behaviour is as much mediated by risk of predation as is the behaviour of other mammals. Tansley, an Oxford academic, could only say what he did because he could feel safe in a country that had eliminated all the threats to humans - and had completed the domination of its landscapes.

Going back 8000 years

We can gain some insight into the reality of this from an archaeological pollen study in a small woodland hollow in Migdale Wood, an ancient broadleaf woodland on the north shore of Dornoch Firth in East Sutherland, Scotland (14). The fossil pollen data revealed a number of phases of disturbance and stable cycling going back 8000 years to a point where the climate was warming after the retreat of the last ice age and where the trees were returning. Pine is initially the dominant tree back then, but over the next 1000 years there is a period of continuing transition in which birch, alder and some oak become more or less dominant as a result of disturbances such as gap dynamics, and probably natural fire. Then comes a phase of stable cycling that lasts from 6900 to 4200 years ago. Pine is no longer dominant in this phase. The cycling of alder, birch and oak indicates species groupings that undergo small scale disturbances, but return to the same or similar groupings. It is likely that Mesolithic people were making locally small clearings in the woodland at periodic intervals and that these would represent local disturbances around the site. The disturbances would not be sufficient to alter the overall stable cycling during this phase. Therefore human induced disturbances were probably mimicking the course of natural disturbance such as gap dynamics and natural fire regimes. This type of human land activity has resonance in the writings of Permaculturist David Holmgren, who advocates making use of small-scale, short-lived changes in a deliberate and co-operative way (see Principle 12 in (2)).

All is change in a brief period next of high disturbance that lasted 200 years, and which was marked by a large decline in pine and other tree species, except for birch. It is likely that this was due to an increase in the intensity and scale of human disturbance, coupled with what may have been a local change in climate that helped with clearing the trees. The response to the disturbance was a shift to a vegetation mosaic of birch and agricultural herbs, different to anything seen before, and which ushered in a new period of stable cycling that lasted between 4000 and 500 years ago. Agricultural activity probably dominated the landscape, and this was mostly wood pasture with scattered fields. It is not surprising that this phase has the highest floral diversity as the opening up of the landscape from woodland allowed a range of different open landscape species to flood in.

The last 500 years saw another disturbance-transition phase, where birch and pine are the most common trees and diversity drops off. The vegetation mosaic is not in a stable cycling phase, probably as a result of differing management objectives, especially into the 19th century from oak tanning and conifer plantation that became key activities. The pollen data of this period shows species groupings that have no match in the previous 7500 years.

Given the choice, where would you place yourself in this timeline? In the hunter-gatherer/minor cultivated ecology of the first stable cycling phase, or the agriculturist of the second? Clearly the last 500 years seem the least appealing at that location, but can we make such choices now? I think we can and I am supported in this by a recent thought provoking analysis of a range of scenarios for British agriculture produced by Simon Fairlie (15). He set himself the task of deciding whether Britain could feed itself, and his comparisons are of a conventional, an organic and a Permacultural approach to agriculture, and how converting to a vegan diet would affect each of them.

As would be expected, giving up meat eating in Britain would have a profound effect on the amount of land used in any approach to agriculture, raising the prospect that we could give substantial space back to wild nature. In his comparison for livestock-based agriculture, organic comes out as the most land hungry, while a conventional and a Permaculture approach are similar in overall land amount used, but the Permaculture approach is markedly different in its approach to land use - as it is also different to the organic approach. The Permaculture approach has a more dispersed human settlement pattern, makes much more use of land for woodland and has a range of additional yield, including wild meat. It’s an approach that surely starts to bring wild and natural processes back into being a force in our landscapes.

Mark Fisher 15 December 2007

(1) The Use and Abuse of Vegetational Concepts and Terms, Tansley, AG (1935) Ecology 16:284-307

(2) Permaculture: Principles and Pathways Beyond Sustainability, David Holmgren (2003) Holmgren Design Services ISBN 0646418440

(3) Gardening with prairie plants, Sally Wasowski (2002) U. Minnesota Press ISBN 0-8166-3087-9

(4) Young, T.P., Chase, J.M. and Huddelston, R.T. (2001) Community Succession and Assembly. Ecological Restoration 19:5-18

(5) Egler, EE. (1954) Vegetation science concepts. I. Initial floristic composition, a factor in old-field vegetation development. Vegetatio 4:412-417

(6) The Ecology of Natural Disturbance and Patch Dynamics, Pickett, ST & White, PS, eds (1986) Academic Press ISBN-13: 978-0125545211

(7) Natural Woodland - Ecology and Conservation in Northern Temperate Regions, George F. Peterken (1996) Cambridge Uni. Press ISBN 0-521-36792-1

(8) Watt, AS (1947) Pattern and process in the plant community.  Journal of Ecology 35:1-22

(9) From “This Week’s Citation Classic”, Block, M, Current Contents, No 30, 28 July 1986

(10) Historical clues to conservation: Management techniques that reproduce old farming practices may yet save one of the last remaining heathlands in Britain, Dolman, P & Sutherland, W, New Scientist, 12 January 1991

(11) Linking wolves to willows via risk-sensitive foraging by ungulates in the northern Yellowstone ecosystem, Ripple, WJ & Beschta, RL (2006) Forest Ecology and Management 230:96–106

(12) Restoring Yellowstone’s aspen with wolves, Ripple, WJ & Beschta, RL (2007) Biological Conservation 138:514-519

(13) Linking a cougar decline, trophic cascade, and catastrophic regime shift in Zion National Park, Ripple, WJ & Beschta, RL (2006) Biological Conservation 133:397-408

(14) “More questions than answers? High resolution pollen records as a tool for conservation management at Ledmore and Migdale woods, Sutherland”, Davies, A & Smith, M, Conference Paper presented at: ‘Working and Walking in the Footsteps of Ghosts’: The international conference held at Sheffield Hallam University, 2003

(15) Can Britain feed itself?, Simon Fairlie, in The Land, Issue 4 Winter 2007-8 ppg 18-26, The Land is Ours/Chapter 7