Physical Fragility Versus Biological Stability And Diversity

The sight of major physical disturbance inevitably creates alarm and anxiety for the well-being of the biota. When widespread areas are devastated by fire or erosion these anxieties may be well founded. However, an area may be physically fragile and subject to periodic disturbance such as flooding, erosion, drought or insect attack yet nevertheless be biologically diverse as these episodic events often serve to reduce the presence of dominant species. Dune and slack systems are more diverse when prone to physical disturbance. A prolonged period of coastal accretion may appear on the surface to be beneficial in extending the terrain for coastal species, but as distance from the sea increases so can diversity be lost.

The Scottish National Nature Reserve at Tents-muir already referred to above was in serious decline in terms of its biodiversity during a period of rapid coastal accretion. Birch invaded the dune slacks and increased the process of eutrophication which led to the dominance of a few aggressive grass species (Crawford, 1996b). Fortunately, rising sea levels eroded the major dune front, and inundation with seawater killed the invading birch and restored the diversity of the halo-phytic vegetation.

Dune systems can look physically fragile and give an appearance of imminent coastal retreat. Such has been the case on the island of Vatersay in the Scottish Hebrides where the fragile nature of dunes on either side of a strip of much grazed machair has long given an impression of imminent disappearance. An early nineteenth century geological visitor described the island of Vatersay (Fig. 12.3) 'as two green hills united by a sandy bar where the opposite seas nearly meet. Indeed if the water did not perpetually supply fresh sand to replace what the wind carries off, it would very soon form two islands; nor would the tenant have much cause for surprise, if on getting up some morning he should find that he required a boat to milk his cows.' However, comparison of the front line of these apparently badly eroded dune fronts with photographs taken 80 years apart (Fig. 12.4) shows little sign of any imminent retreat (Crawford, 2001). The dissected dunes are constantly undergoing a cycle of erosion down to a level where the water table is high enough to enable regeneration and stabilize the dune system first with sand couch grass (Elytrigia repens) and then marram (Ammophila arenaria).

Salt marshes which consist of a number of small fragmented islands frequently give an impression of physical instability, suggesting that they are in imminent danger of erosion. Here again, as with the tombola beach on Vatersay, photographic records can demonstrate a

Fig. 12.3 The machair and the tombola beaches on the Island of Vatersay (Outer Hebrides). Despite fears that were first recorded nearly 200 years ago for the stability of the tombola, the natural cycle of machair regeneration (see text) has proved effective in prolonging its existence.

Fig. 12.4 Sandy beach on the east side of Vatersay, Outer Hebrides, photographed from the north (upper) on 20 July 1922 by Robert M. Adam, (lower) on 7 May 2000 by the author. Despite evidence of continuing erosion activity there has been no significant retreat of the position of the dunes. Cycles of erosion and regeneration appear to be operating. (Robert Adam photograph reproduced with permission from the Archives and Muniments of the University of St Andrews.)

Fig. 12.4 Sandy beach on the east side of Vatersay, Outer Hebrides, photographed from the north (upper) on 20 July 1922 by Robert M. Adam, (lower) on 7 May 2000 by the author. Despite evidence of continuing erosion activity there has been no significant retreat of the position of the dunes. Cycles of erosion and regeneration appear to be operating. (Robert Adam photograph reproduced with permission from the Archives and Muniments of the University of St Andrews.)

remarkable stability even of small fragments of marsh (Fig. 12.5).

Similarly, plant communities that live near areas with fluctuating water tables are typically diverse with zonations of various species of reeds, sedges, rushes, and flood-tolerant willow and alder scrub. Each of these groups differs in their ability to tolerate flooding in relation to depth, duration, and seasonality (Chapter 8). When the flooding stress is reduced by drainage then the floristic diversity of the regions is reduced. Reed beds (Phragmites australis) are normally found as monospecific stands at the edges of lakes and rivers and suggest a homogeneous type of vegetation. However, within large reed beds on larger lakes molecular studies have shown that there are both morphological and physiological variation between the plants in the centre of a reed bed as compared with those at the edge of the stand, with the former being more tolerant of anoxia than the plants at the periphery (Keller, 2000). Thus a marginal single species plant population can structure itself genetically so as to increase the physiological fitness of the stand as a whole. These selected examples and others mentioned in the foregoing chapters demonstrate an innate biological robustness in many marginal plant communities.

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