Disturbance And Biodiversity

Disturbance has both positive and negative effects on species richness. Disastrous ecosystem destabilization through frequent burning, logging, and overgrazing can be found in every part of the world. The logging of tropical rainforests, the destruction of African savannas by elephants or fire, the denudation of Iceland from overgrazing by domestic livestock, and the destruction of arctic salt marshes by large populations of nesting geese, are but a few examples where repeated disturbance, from whatever cause, eventually prevents the establishment of perennial species and frequently leads to drastic soil erosion.

In the modern world overgrazing is usually due to human intervention and frequently leads to a reduction in species diversity. Despite the obvious calamitous situations where overgrazing and other forms of disturbance can lead to total habitat destruction there are many examples where plant diversity is improved and ecosystems renewed by periodic fires, physical erosion, and grazing. All these forms of disturbance have a history that predates the arrival of human beings as an ecological force. Plants have evolved in dynamic relationship with disturbance both by physical forces and herbivory and therefore it should not be surprising that a lack of disturbance can lead to a reduction in species richness.

Exclusion of grazers from fenced sites in the Arctic can lead to the dominance of mosses to an extent that inhibits the regeneration of flowering plants. Removal of winter grazing from maritime sedge-heath communities in Orkney deprives the rare endemic Scottish primrose (Primula scotica) of the bare patches caused by sheep's trotters on wet soils which facilitate seedling establishment (Fig. 2.3). Forest and heathland regeneration can benefit from burning, provided that the frequency and extent of the fires does not entirely remove the possibility of reseeding. Even bog vegetation can be rejuvenated by fire as it removes invading trees, while the carbon particles from the fire wash into the soil, impede drainage and thus help the growth of bog (Mallik et al, 1984).

Whatever the situation, the basic principle underpinning plant species richness is that all persistent species (species recognized as being part of a given plant community) must be able to increase when rare (Crawley, 1997). This is a fundamental property for plant survival which is particularly relevant to the survival of plant communities in marginal areas where windows of opportunity for reproduction may be intermittent, and the maintenance of species diversity can be impeded by a few dominant life-forms. Community impoverishment in this way is particularly noticeable in peripheral areas where a single dominant species or a group of species with similar life-forms can prevail and prevent regeneration and invasion by a more diverse flora. Marginal and oceanic habitats

(see Chapter 7), as well as extensive areas of the arctic tundra (see Fig. 6.13) can become dominated by bryophytes to the exclusion of other species if there is no disturbance from periodic fires and grazing is absent.

2.4.1 Grazing

Grazing is a form of disturbance that has long been recognized as one of the fundamental forces that maintains biodiversity by controlling aggressive and potentially dominant species. As a factor shaping plant communities grazing probably predates human pastoral activity. The highly noticeable impact ofNeolithic settlements on the species composition of natural vegetation, and especially on forests, has caused the influence of pre-landrnm (pre-agricultural settlement) grazing to be ignored or else assumed to be negligible. Nevertheless, in Europe the early Holocene had an extensive herbivorous fauna. Grazing mammals included aurochs (Bos primigenius; Fig. 2.4), tarpan or European wild horse (Equus przewalski), bison (Bison bonasus), red deer (Cervus elaphus), elk (Alces alces), roe deer (Capreolus capreolus), beaver (Castorfiber), and the omnivorous wild boar (Sus scrofa), all of which maintained a presence in Europe from the end of the Pleistocene (c. 12000 BP) to the early Middle Ages and some until the present day. In historical times the effects of the larger grazing mammals in creating marginal brushwood regions around forests is well-documented, and there is therefore no reason to

Auroch Skeleton
Fig. 2.4 Skeleton of a young aurochs. This specimen, now in Roskilde Museum, dating from 10 000 BP was found in a bog near Himmelev (Denmark).
Greenland Vegetation And Wildlife

Fig. 2.5 Musk oxen (Ovibus moschatus) in north-east Greenland. One of the last remaining large mammals of the Pleistocene mammoth fauna still found with viable populations in Greenland, Arctic Canada, and Alaska. Grazing is essential for maintaining nutrient levels and floristic diversity especially in the High Arctic.

Fig. 2.5 Musk oxen (Ovibus moschatus) in north-east Greenland. One of the last remaining large mammals of the Pleistocene mammoth fauna still found with viable populations in Greenland, Arctic Canada, and Alaska. Grazing is essential for maintaining nutrient levels and floristic diversity especially in the High Arctic.

suppose that grazing disturbance in pre-Neolithic times did not exist and help to create the brushwood interface between forest and open land. It is in these marginal zones, created by grazers between forest and open land, that many tree species, such as pedunculate oak (Quercus robur), sessile oak (Q. petraea) and hazel, regenerate most readily. It has been suggested (Vera, 2000) that the presence of these tree species in the pre-landnam forest pollen record should be taken as evidence for the influence of the early indigenous grazing fauna on maintaining openings in the ancient forests of lowland Europe and thus contributing to their biodiversity. However, further examination, using as a control the situation in Ireland where ancient forests existed but large mammal herbivores were absent, has shown that oak and hazel (Corylus avellana) still regenerate successfully without the disturbance of large herbivores (Mitchell, 2005). Nevertheless, large herbivores, both wild and domesticated, can have a significant impact on forests, as can be seen in the present situation with grazing by excessive numbers of red deer in the Scottish Highlands.

In the Arctic the impact of grazing by wild geese (Anser spp.), lemmings (Lemmus lemmus), musk oxen (Ovibus moschatus) and reindeer (Rangifer tarandus) has a profound influence on the diversity of plant communities and the fertility of the soil (Holtmeier, 2002). The tundra has probably been grazed as long as this vegetation has existed. In northern Eurasia and in Alaska the mammoth fauna of Eurasian horses, the Mongolian ass (kulan), bison, yak, and musk oxen (Fig. 2.5), as well as mammoth, woolly rhinoceros and reindeer, grazed on the disappearing Pleistocene tundra-steppe. The abrupt reduction of their natural habitat started about 12 ka BP. The last mammoths became extinct in northern continental Siberia c. 9.6 ka BP. However, a dwarf mammoth population inhabited Wrangel Island from 7.7 to 3.7 ka BP (Kuz'min et al., 2000; see also Fig. 11.7). Caribou and reindeer, and to a lesser extent musk oxen, still remain widespread throughout the High Arctic (Klein, 1999). The length of the Holocene presence of reindeer at high latitudes is particularly notable (Fig. 2.6) and their presence in the past would have contributed, as it does

Fig. 2.6 Holocene occupancy of the High Arctic by reindeer (caribou). Island abbreviations: E, Ellesmere; S, Svalbard (Spitsbergen); FJ, Franz Josef Land; NZ, Novaya Zemlya; SZ, Severnaya Zemlya; NS, New Siberian Islands; W, Wrangel Island. (Reproduced with permission from Klein, 1999.)

today, to the turnover of nutrients. When there is little or no grazing the vegetation becomes relatively nutrient poor with an extensive accumulation of dead forage material which can shade the soil surface; this impedes the warming of the upper layers, causing the permafrost level to rise and form ice wedges, which limits the rooting capacity of vascular plants. The end result can be an increase in bryophytes and the suppression of many of the flowering plant species.

Overgrazing, where plant communities can suffer long-term damage, is also found in the Arctic, even in the remoter regions. In polar semi-desert communities of north-west Spitsbergen the reproductive potential of the keystone vascular species, Dryas octopetala, is currently constrained by low summer temperatures, resulting in the infrequent production of viable seeds. This is further depressed on the Brogger Peninsula by the summer foraging behaviour of a numerous population of reindeer (Rangifer tarandus platyrhynchus) on the flowering shoots (floral herbivory). Surveys of neighbouring coastal tundra areas with considerably less floral herbivory show just how severe the grazing pressure is on reproductive shoots of D. octopetala in this region. This situation is not unique to this area of Spitsbergen and also extends to other species of flowering plants (Cooper & Wookey, 2003).

In salt marshes both in temperate regions and in the Arctic there is a delicate balance between goose grazing and diversity of the salt-marsh vegetation. Overgrazing can be highly destructive and lead to catastrophic changes in the ecosystem. In arctic salt marshes along the western shores of the Hudson Bay large areas have been destroyed as a result of overgrazing by increasing populations of the lesser snow goose (Jefferies, 1997). The North American lesser snow goose has traditionally wintered in the coastal marshes of the Gulf of Mexico. The development of large fields of rice and other winter cereals with large nitrogen inputs has led to a large increase in their population as a result of reduced winter mortality. The high number of birds that now come to breed on the shores of the Hudson Bay has dramatically affected these arctic coastal marshes (Figs. 2.7-2.9). Studies over many years concentrating on the area around La Perouse Bay (Jefferies et al., 2004) have shown that the breeding colony there has grown from about 1300 pairs in 1967 to an estimated 44 500 pairs in 1997. Their preferred species are the stoloniferous

Fig. 2.7 Goose grazing in the Arctic. (Left) Location of studies on the effects of increased lesser snow goose populations on the western shore of the Hudson Bay. (Right) Satellite image of vegetation changes at La Perouse Bay from 1973 to 1993. Red refers to the areas that have lost vegetation over the period, green indicates no net change. (Reproduced with permission from Jano etal., 1998.)

Fig. 2.7 Goose grazing in the Arctic. (Left) Location of studies on the effects of increased lesser snow goose populations on the western shore of the Hudson Bay. (Right) Satellite image of vegetation changes at La Perouse Bay from 1973 to 1993. Red refers to the areas that have lost vegetation over the period, green indicates no net change. (Reproduced with permission from Jano etal., 1998.)

Fig. 2.8 Damage to coastal stands of willow by overgrazing by lesser snow geese on the shores of the Hudson Bay. (Left) Grubbing of intertidal marsh in early spring. (Right) The birds have removed the insulating mat of graminoid vegetation around the willows. This has exposed the willow roots and as a result of increased evaporation has led to hypersaline soil conditions (3 X oceanic water at 120 parts per thousand) thus causing the death of the willows. (Photos by Professor R. L. Jefferies.)

Fig. 2.8 Damage to coastal stands of willow by overgrazing by lesser snow geese on the shores of the Hudson Bay. (Left) Grubbing of intertidal marsh in early spring. (Right) The birds have removed the insulating mat of graminoid vegetation around the willows. This has exposed the willow roots and as a result of increased evaporation has led to hypersaline soil conditions (3 X oceanic water at 120 parts per thousand) thus causing the death of the willows. (Photos by Professor R. L. Jefferies.)

Fig. 2.9 Population index of the mid-continent light geese (mainly lesser snow geese). Data taken from winter counts from 1950 to 2003 as recorded in U.S. Fifth and Wildlife Annual reports. (Reproduced with permission from Jefferies et al., 2004.)

graminoids of the salt marshes, which are rich in soluble sugars and amino acids, of which Carex sub-spathacea, C. aquatilis and Puccinellia phryganodes are the principal species. The geese can remove 90% of the annual above-ground primary production as well as grubbing up the buried stolons. Once the vegetation has been lost, increased evaporation from the exposed sediment draws inorganic salts to the surface from the underlying marine clays that were deposited under the ancient Tyrell Sea. In summer these salts accumulate and give rise to hypersaline conditions which inhibit recovery of the graminoid vegetation and also kill the willows in the supratidal marshes (Jefferies & Rockwell, 2002).

In other marginal areas, salt marsh vegetation can benefit from moderate goose grazing by arresting succession and preventing the establishment of taller species (van der Wal et al., 2000). High brent goose (Branta bernicla) populations can in this way increase the areal extent of grazing lawns (Person et al., 2003). There can also be a benefit from grazing by both mammals and geese. On the Dutch island of Schiermonnikoog, tall-growing shrub species such as sea purslane (Atriplex portulacoides) can eventually make the marsh unsuitable for feeding geese. However, winter grazing by brown hares (Lepus europaeus) can retard vegetation succession and facilitate grazing by brent geese by preventing shrubs from spreading into younger parts of the marsh.

In many European marginal areas, free-ranging herbivores are now being used more widely in the interests of conservation, particularly in natural grassland areas, dune heaths and wetlands. Herds of cattle and horses, and to a lesser extent sheep and geese, are proving effective in preserving biodiversity. A mixture of cattle and horses owing to their different feeding strategies is particularly efficient in controlling invasive coarse grasses and woody species. In France, where grazing by horses and cattle is used for conservation management in the Camargue, free-ranging animals have been shown to be highly efficient in preserving biodiversity of this marginal habitat. Horses are particularly successful in plant management because they remove more vegetation per unit body weight than cattle and make greater use of the more productive plant species, especially the graminoids (Menard et al., 2002). The processes contributing to diversity in vegetation structure can therefore be linked to an alternation of positive and negative interactions which leads through competition to the creation of sequences of shifting mosaics.

2.4.2 Fire

Fire, through natural causes, is a worldwide factor with a long history that predates human intervention in vegetation succession and regeneration. Consequently, plants have evolved a dynamic relationship that has operated on a large scale in the evolution of the life history traits of numerous species. The South African fynbos, the Brazilian cerrado and the heathlands of Europe are all examples of vegetation types that have evolved in association with fire and depend on fire for their existence (Figs. 2.10-2.11). The dependence of these species-rich areas on periodic conflagration raises the question of whether biodiversity is positively or negatively related to fire frequency. In common with other disturbance phenomena, a high frequency offires can eliminate many species while a low frequency leads to the dominance of competitive species and contributes to species loss. In general, within a single fire cycle of destruction, recovery and post-fire succession, species richness typically peaks in the first year after the fire and then declines as the community ages (Bond & van Wilgen, 1996). Thus, it might be expected that heaths with intermediate fire frequencies will have the greatest biodiversity. In the South African Fynbos fires occur with a frequency that varies between 5 and 40 years, while in the Brazilian Cerrado the fire frequency is between 1 and 3 years (Bond & van Wilgen, 1996). This high frequency appears to be essential for the preservation of the typical cerrado flora (see below), as protection from fire for two to three decades brings about an increase in woody plants and other fire-sensitive species (Moreira, 2000).

In Scotland large areas of moorland are burnt in order to regenerate the heather for sustaining viable populations of birds for game-bird shooting and for sheep grazing (Figs. 2.12-2.13). Depending on whether the management objectives are primarily for shooting or agriculture, muirburn frequency can vary. It is also possible to manage the intensity of the fire depending on the weather conditions at the time of burning and the age and size of the heather plants. Moors that are managed for shooting red grouse (Lagopus lagopus

Fig. 2.10 Land cover according to dominant functional types. The colour key represents from left to right: bare ground, C3 grasslands, C4 grasslands, angiosperm forests (deciduous and evergreen), gymnosperm forests, and crop lands. Land-cover types: 1, broad-leaved evergreen forest; 2, broad-leaved deciduous forest; 3, mix of 2 and coniferous forest; 4, coniferous forest; 5, high latitude deciduous forest; 6, wooded C4 grassland; 7, C4 grassland; 9, shrubs and bare ground. (Adapted with permission from Bond et al., 2005.)

bare C3 C4 ang gym crop

Fig. 2.10 Land cover according to dominant functional types. The colour key represents from left to right: bare ground, C3 grasslands, C4 grasslands, angiosperm forests (deciduous and evergreen), gymnosperm forests, and crop lands. Land-cover types: 1, broad-leaved evergreen forest; 2, broad-leaved deciduous forest; 3, mix of 2 and coniferous forest; 4, coniferous forest; 5, high latitude deciduous forest; 6, wooded C4 grassland; 7, C4 grassland; 9, shrubs and bare ground. (Adapted with permission from Bond et al., 2005.)

Fig. 2.11 Global distribution of fire in 1988 mapped by World Fire Atlas European Space Agency. Note that most fires occur in C4 grass ecosystems. (Reproduced with permission from Bond et al., 2005.)

scoticus), which are dependent on young nutritious heather shoots as their main food, are best managed by burning small patches at a time, especially if combined with a relatively high frequency of burning. How often these patches of moor are burnt can vary between 8 and 20 years depending on the productivity of the moorland.

In relation to the preservation of biodiversity the extent and frequency of burning, and the habitats in which this management occurs, are contentious issues as burning has both positive and negative effects on the biota and the moorland (Yallop et al., 2006). Blanket bogs lose their conservation value if burnt frequently with hot fires. Wild birds can be adversely affected by too frequent muirburn. Meadow pipit (Anthus pratensis) density has been observed to increase with moderate heather cover but falls if it is too high (Vanhinsbergh & Chamberlain, 2001). It appears that a mosaic of heather, bog and grassland may be the optimum habitat for meadow pipits.

Recent changes in upland management and high labour costs have resulted in moorlands no longer being regularly burnt. The disappearance or degradation of heather mosaics (Fig. 2.13) in some areas is having a deleterious effect on the biodiversity of this habitat.

Studies on the time sequence of species diversity in relation to fire on Scottish moorlands have shown that there is a decline in species number after long periods without burning (Hobbs et al., 1984). The maintenance of biodiversity in these marginal heath habitats will therefore depend in the future on a reassessment of the how and when muirburn is practised as climate changes and the use of the moorlands also alters. Upland sheep grazing is proving less profitable than it was in the past and an ecosystem that can produce one red grouse per hectare per annum may prove financially more rewarding, with a faster cash return than traditional methods of upland farming.

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