Landscape Ecology vol. 10 no. 4 pp 191-196 (1995) SPB Academic Publishing bv, Amsterdam

Human-mediated vegetation switches as processes in landscape ecology

J. Bastow Wilson and Warren McG. King Botany Department, University o f Otago, P.O. Box 56, Dunedin, New Zealand

Keywords: Switch, boundary, ecotone, landscape, mowing, trampling, human effects, positive feedback, garden design

Abstract

Switches are processes in which there is positive feedback between vegetation and environment. Landscape features can be created and modified by switches. The concept has previously been used with physical factors and non-human animals as the switch mediator, i.e. the factor which the vegetation modifies and which in turn affects the vegetation. Here, the switch concept is extended to include some types of human behaviour as possible switch mediators. With this extension, the switch concept can explain the impact on the landscape of some types of human behaviour. Examples are given of the behaviour of mower drivers, mowing up to a boundary which they create and/or maintain, and of walkers trampling tracks which they create and/or maintain. Other possibilities are discussed briefly. It is concluded that the concept of a human-mediated switch can unify the study of human behaviour, vegetation processes and landscape ecology.

Introduction

Human impact is now the main determinant of landscape pattern over much of the globe (Hansen et al. 1988; Fuentes 1990; Lepart and Debussche 1992). The processes of disturbance and manage- ment by humans are often considered qualitatively different from the 'natural' processes that are studied by most ecologists. In this paper, we suggest that some types of human impact are mediated by a basic process that is in common with natural com- munities - the vegetation switch.

Wilson and Agnew (1992) introduced the concept of the 'switch' in vegetation - a positive feedback process between species composition and the envi- ronment. A switch operates when a community modifies the environment in a direction that favours the maintenance of that community. The term 'switch' is used following analogous use by Odum (1971).

A switch can operate in time: vegetation/envi- ronment change can be accelerated or delayed, giving the possibility of a sharp temporal change. A switch can also operate in space: a small initial difference in biota or environment can switch be- tween alternative stable states of vegetation/envi- ronment across an abrupt boundary. A spatial switch can create a mosaic in a previously near- uniform area. It can also sharpen an existing gra- dient, creating an abrupt ecotone. We are con- cerned here mainly with the latter process, which clearly has the potential to modify the landscape (Armand 1992; Wilson and Agnew 1992; Agnew et aL 1993).

Wilson and Agnew (1992) gave examples of switches mediated by a range of factors, and able to create landscape features. Some mediators are abiotic, e.g. water supply, pH, light, fire and wind. A well-known example is the fire/light switch, which creates the savannah/closed-forest mosaic

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Fig. 1. A boundary between tall and mown grass, maintained by a mowing-mediated One-sided switch, producing a Sharpening situa- tion. Port Chalmers golf course, Dunedin, New Zealand.

landscapes in Africa and Australia. Other switch mediators are biotic, e.g. microbial population, ter- mite nests, grazers. An example is the occurrence of patches of trees/shrubs centred on termite mounds, creating the Termitensavanna landscape of the Kenya Highlands (Darlington 1982; Wilson and Agnew 1992). In this paper, we extend the switch concept of Wilson and Agnew to include human influence. By viewing some types of human be- haviour as the mediator of a switch we can explain some of the ways that human behaviour modifies the landscape. We give examples.

Mowing

In managed vegetation, there is often a sharp boundary between mown grassland and rough grassland (Fig. 1). This boundary is usually set deliberately, but it is maintained informally, as the mower operator mows up to the edge of the short grass and not beyond, using the previous discon- tinuity in sward height as a signal to the mowing

limit (Buchanan 1981). This maintains the sharp boundary, since the unmown area is tall, and there- fore is left unmown, and the mown area is short, and therefore continues to be mown. Mowing can

affect species composition (Bassett 1980; Parish et al. 1990; Gibson et aL 1993), and this could rein- force the process, the distribution of conspicuous species acting as a signal to the mower driver as to the position of the mown/unmown boundary.

The two elements of the mowing switch are there- fore: (i) The taller grass deters mowing, and its species

composition may signal that the area is to be left unmown.

(ii) Lack of mowing enables the taller grass to grow/remain tall, and allows the invasion of mowing-intolerant species, reinforcing element (i), and completing the positive-feedback loop that is intrinsic to a switch.

In terms of the categories of Wilson and Agnew (1992) this represents a One-sided (i.e. Type 1) switch, since the taller community deters the

mower-driver, but the shorter community does not attract him.

Where there is an underlying gradient, e.g. in slope, water supply, soil type etc., such behaviour could sharpen the gradient into a boundary. Differ- ences in slope can be reflected in species composi- tion (Dargie 1987), and such a vegetational gradient could be sharpened by a mowing switch. Alterna- tively, a switch mediated by the behaviour of a mower-driver could produce a boundary in an initially-uniform area of grassland, due to an initial decision by a mower operator. However, the initial position of the chosen boundary is likely itself to be caused by an implicit gradient. This implicit gra- dient might be a psychological one, such as distance from a land boundary (e.g. a fence). All these are examples of what Wilson and Agnew (1992) saw as a Sharpening (category B) outcome of a switch.

The same process is seen even more markedly at a boundary between mown grassland and shrub- land or woodland (Gysel 1951). Gysel suggests that the exact boundary is often the dripline of the tree, but the coincidence is not necessarily directly causal, for example the mowing line might be in- fluenced by the dampness of the grass.

A similar switch could operate when a farmer clears the scrub on an area of land, and maintains the shrubless state up to the boundary.

The boundary produced by a One-sided switch is intrinsically unstable, in that the boundary can move into the area in which the switch does not operate, in this case into the mown area. This would occur if the spatial extent of mowing is repeatedly slightly short of the tall grass, so that the tall-grass zone gradually extends into the mown area. This occurs on the University of the South Pacific com- pounds (A.J. Watkins, pers. comm.). A gradual change could also occur in the opposite direction, with extension of the mown area, if the switch is less than fully efficient.

Track trampling

Informal pathways or tracks can be established in semi-natural vegetation, or even across mown grass where there is frequent pedestrian traffic (Fig. 2).

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The first pedestrians start by walking along a cer- tain route. This route is not necessarily the shortest distance between two points, it can be influenced by environmental variation such the topography or the softness of the ground (Bates 1950). Once a path- way is established, other pedestrians follow it, and reinforce it (Leland et al. 1986). They do this partly by following an indication of the appropriate route without any other reason for preference. However, in semi-natural vegetation there may be a genuine advantage in following an established route because shrubs are trampled and the way made easier.

Once the path is established, vegetation change will occur (Williams and Lambert 1960; Page et al. 1985; Sun and Liddle 1991; Liddle 1991). Some- times this change affects only species abundances: Bates (1935), comparing a footpath with adjacent vegetation, found that some species were present at much greater frequency on the footpath, but there were no species exclusive to the footpath. However, sometimes the vegetation changes to the extent that species enter the footpath community which are absent in adjacent untrampled vegetation. Liddle and Greig-Smith (1975) found two such species on tracks through a sand dune complex. These changes can be related to differences in species' trampling tolerance, which in turn can sometimes be related to the species' morphology (Sun and Liddle 1993; Bates 1935).

Once such vegetation change has occurred, the path will be more clearly marked, and the vegeta- tion will comprise prostrate species, which will fur- ther encourage pedestrian traffic, creating a posi- tive-feedback mechanism - a switch.

The two elements of the track-trampling switch are: (i) The shorter track vegetation encourages tram-

pling. This also attracts walkers from other areas;

(ii) Trampling keeps the vegetation of the tracks short and encourages trampling-resistant, pros- trate species. Lack of trampling keeps other areas tall. Both effects reinforce element (i), completing the positive feedback loop that is intrinsic to a switch.

The process represents a Reaction (i.e. Type 2)

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Fig. 2. An informal track, maintained by a trampling-mediated Reaction switch, producing a Stable mosaic situation. Kaikorai Valley, Dunedin, New Zealand. A path across waste ground, used by factory workers fetching lunch, etc.

switch (Wilson and Agnew 1992), since track vege- tation attracts walkers, and in the process diverts them from other areas. The outcome is that a stable mosaic is produced in a previously undifferentiated environment (category A outcome of Wilson and Agnew 1992).

Similar processes could operate in tracks formed by humans in other ways, e.g. informal tracks made by 4-wheel drive vehicles, by bicycles, by motor bi- cycles, etc. Similar switches are operated by other animals, such as sheep (Bates 1950), emphasising the unity in concept between switches mediated by humans and those mediated by other animals.

Other human-mediated switches

behaviour is then modified by the created/ sharpened boundary in a direction that tends to sharpen the boundary further, creating the positive- feedback that is intrinsic to a switch process. Weed- killer application could function by a similar mechanism to mowing, since workers might apply weedkiller to young plants in areas where large shrubs had already been removed or controlled, thus reinforcing the difference in vegetation and reinforcing their discrimination between areas. When broad hedgerows are left unploughed for conservation (Marshall 1988; Bond 1987), a similar switch could operate. Many of these processes will be very similar to those mediated by non-human animals: some examples are given in Wilson and Agnew (1992).

Switches are possible whenever humans create a boundary, or sharpen a boundary, and when their

Conclusion

Human interference is often seen as unnatural, a process ('noogenesis') that is not related to the laws of ecology, and which creates a system ('noo- sphere') that is replacing the natural biosphere (Vernadsky 1945). However, humans are animals as much as any others, and are subject to the same laws of nature (Odum 1971). Seeing some human behaviour as a switch mediator enables us to see at least some human activity as falling into the same patterns as traditional non-human ecology.

The 'unnatural' character of abrupt edges has played an important role in garden/landscape de- sign. The Renaissance 'Italianate' style developed into the formal gardens of France, reaching a peak at Versailles. The style is characterised by hard edges, most dramatically in the parterres. Such gardens were seen as symbolic of human control over the environment, of safety from enemies and relief from stress (Glacken 1967; Clifford 1966).

Renaissance values in garden design were over- turned by moves towards a more 'natural' land- scape, making more use of natural forms (Clifford 1966). The natural approach was espoused and executed by Kent, Bridgeman, Brown and Repton. Its primary characteristic was the elimination of straight lines, but often edges were softened too, as in Brown's work at Stowe. Some modern garden design has also emphasised the softening of edges: 'Nature abhors hard edges' (Ward 1912).

An alternative trend in modern garden design, the counterpart to the modernist movement in ar- chitecture, has involved a reversion to hard edges, as in the lines and grids of trees, and the square mono-specific beds of the Miller House garden, midwestern U.S.A. (Frankel and Johnson 1991). In the garden of the Wright House, Seattle, the hard straight edges of the terraces contrast with the created 'natural' background which has deliberate- ly softened edges (Frankel and Johnson 1991).

Ecology can interpret the hard/soft edge dicho- tomy in garden/landscape design in three ways. Firstly, the action of humans on the landscape can be seen as artificial and hence undesirable; from this point of view a human switch is a vicious cycle that exacerbates the undesirable influence of hu-

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mans. A second approach is to see hard edges as artificial, but to welcome this, to see the formality of, e.g. , the Miller House garden as the victory of mankind and its machine over nature (Gutkind 1952). The third viewpoint is that whilst gradual change is common in nature, hard edges are also present, caused by sharp environmental changes or by natural switches (Wilson and Agnew 1992; Agnew et al. 1993); human switches are an example of these edge-sharpening processes, and the hard edges they produce are therefore not to be es- chewed.

Not all human influences on the landscape oper- ate via a switch; probably only a few do. In order for a switch to operate, the crucial elements are that not only does human behaviour affect the vegeta- tion, but the vegetation also affects human be- haviour, and this feedback is in the direction that reinforces the behaviour and vegetation differ- ences. When such a process is operating, the con- cept of the switch can bridge the gap between hu- man behaviour, the study of vegetation processes and landscape ecology.

Acknowledgements

We thank Chris Bycroft, Louis Leland, Stephen Roxburgh, John Steel, Susan Walker and Anni Watkins for ideas and discussion, and the Editor, Frank B. Golley, for stimulating suggestions. JBW thanks the Biology Department, Open University, UK, for facilities whilst on study leave.

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