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BiologicalJournal of the Linnean Society (lYY5), 56(SuppL): 11-23. With 2 figures The National Trust and nature conservation: 100 years on Edited by D.J. Bullock and H.J. Harvey The potential role of large herbivores in nature conservation and extensive land use in Europe S. E. VAN WIEREN Department of Nature Conservation, WageningenAgricultural University, PO Box 8080, 6700 DD, Wageningen, The Netherlands The concept of ecosystem restoration is gaining momentum in western Europe. This is necessary because in most managed nature reserves one or more of the following processes, which are analogous to those that have led to the dramatic loss of biological diversity in Europe, are still operating: continuing nutrient output, continuing high level of disturbance and fixing the system in some successional stage. This is partly because most management activities have been derived from, or copy, former agricultural practices. The study of natural ecosystems has revealed the key role large herbivores have in maintaining structural diversity in the vegetation and so biological diversity. Because of this they have been used as tools in achieving a variety of conservation goals. Here, various effects large herbivores can have on plant species composition, structural diversity of the vegetation and fauna are briefly reviewed. Attention is given to pasture-woodlands in southern Europe, which often have a relatively high biological diversity and share some key features with natural ecosystems: very low nutrient input, extensive grazing with large herbivores and the presence of natural tree cover. In a number of European countries attempts are being made to restore normal functioning multi-(herbivore) species ecosystems. 0 199.5 The Linnean Society of London ADDITIONAL KEY WORDS:-grazing woodlands. - conservation - ecosystem restoration - pasture - CONTENTS Introduction . . . . . . . . . . . A short history of decline of nature and natural values . Ecological references . . . . . . . . Influence of man . . . . . . . . . Further decline . . . . . . . . . Recent developments . . . . . . . . Lessons for nature conservationists . . . . The historic frame of reference of nature conservation . Effects of large herbivores . . . . . . . . . . . . . . Abandonmentoffarmland Diversity and grazing . . . . . . . . Facilitatingenergyflow . . . . . . . Facilitating other fauna . . . . . . . Species differences . . . . . . . . Density andvegetation structure . . . . . Perspectives . . . . . . . . . . . Acknowledgements . . . . . . . . . References . . . . . . . . . . . 0024-4066/95/0A00 11+ 13 $12.00/0 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12 12 12 12 13 13 15 16 17 17 17 18 18 19 20 21 21 0 1995 T h e Linnean Society of London 12 S. E. VAN WIEREN INTRODUCTION In the past decade ‘restoration ecology’ has become part of theory and practice in nature conservation (Western & Pearl, 1989). In western Europe it is now becoming increasingly clear that conservation of the constituent species in an ecosystem already present by means of well defined management practices is often untenable and not sufficient (Dudley, 1992). It is therefore necessary to determine the ecological context in which species evolved, and from there attempt to restore ecosystems to a more natural state than the one they are in at present. The study of natural ecosystems is providing insight into the key role played by large herbivores in the maintenance of structural diversity in vegetation. This has led to the use of large grazing herbivores as tools to achieve a variety of conservation goals, including attempts in Europe to restore ecosystems (Gordon et al., 1990; Gordon & Duncan, 1988; Van Wieren, 1991). A SHORT HISTORY OF THE DECLINE OF NATURE AND NATURAL VALUES Ecological references In western Europe, true nature (i.e. landscapes with a minimum of human interference) probably existed until the Atlantic period (6000-3000 BC) when most of it was covered with forests of varying degrees of openness (Iversen, 1973; Bottema, 1988; Peterken, 1991) depending upon the influence of large herbivore grazing and browsing. The large herbivore and predator fauna was still complete: aurochs, Bos primigenius Bojanus, tarpan Equus ferus Boddaert, wisent Bison bonasus L., red deer Cervus elaphus L., roe deer Capreolus capreolus L., moose Alces alces L., wild boar (Sus scrofa L., wolf Canis lupus L., brown bear Ursus arctos L., and lynx Lynx bnx L., were all extant. InJuence of man In the Sub-Boreal (3000-800 BC) forest clearance, hunting and the domestication of animals by Neolithic man brought about large changes in the landscape (Bogucki, 1988). After the Neolithic, human population density slowly but steadily increased and by the Middle Ages huge areas of forest were cut down and open man-made landscapes became dominant. Increases in grazing pressure, combined with deforestation and other forms of soil exploitation, led to the development of heathlands and acid grasslands. In less exploited landscapes, pasture-woodlands developed. Farming became increasingly specialized with changes from mixed farming to either grassland, arable land or production forests (Vera, pers. comm.) resulting in the main landscapes observed by mid 19th century. By that time the aurochs and the tarpan were extinct, while the ranges of many other mammals (brown bear, wild boar, wolf, lynx and beaver Castor Jiber L.) had shrunk. The wisent almost became extinct. Further decline The increased importation of fertilizer at the end of the nineteenth century marked a new stage in agricultural development. The input of fertilizer made LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE 13 man independent of the animal dung produced by large herds of sheep and cattle which foraged over large areas. This resulted in a loss of semi-natural grazing lands because larger areas could now be used for more intense farming, leaving little room for most species (Van Wieren, 1991). Mechanization and other agricultural developments such as the lowering of the water table in the first half of the twentieth century further accelerated this process as outlined below. Recent developments Intensive farming systems In the past few decades agricultural development has led to an ever increasing human impact on the land in north-western Europe, resulting in an accelerating rate of species loss (Baldock & Beaufoy, 1993; Weinreich & Musters, 1989). The most important developments have been higher nutrient inputs, leading to an even lower diversity in nutrient levels; increase in levelling (of shrubs, hedges etc), thereby destroying edge and other habitats; improved drainage, leading to losses of wetland species; introduction of highly intensive farming; increased fragmentation of habitats; earlier and increased frequency of mowing with a detrimental effect on grassland bird communities; higher livestock stocking rates and longer grazing periods, with negative consequences for a number of breeding bird species. Extensive farming systems Since the 1960s dramatic changes have occurred in the semi-natural landscape in many parts of Europe. Details of these changes can be found in Baldock & Beaufoy (1993) who identified some major developments on two levels. First, there are changes in farming practices such as increase or decrease in stocking density, changes in harvesting and mowing dates, and change from hay-making to silage making. Second, there are fundamental changes such as conversion to radically different types of agriculture, total abandonment of land and afforestation of farmland. Through all these developments the relatively high biological diversity of many millions of hectares of semi-natural grazing lands in Europe is now threatened. An example of recent changes in species abundance in The Netherlands is given in Table 1. Table 2 compares percentage area of protected nature reserves in the European Community with other regions in the world and demonstrates clearly that the poorer southern countries have retained more suitable conditions for wildlife than the richer countries in the north. Lessons for nature conservationists From the short history described above it becomes very clear that there is a negative relationship between the activities of man the farmer and the maintenance of biological diversity, and that this relationship runs through history from Neolithic times to the present day. In the EC agriculture is driven by economics and politics, currently through the Common Agricultural Policy (CAP) which determines prices for key products and provides incentives 14 S. E. VAN WIEREN T.4BI.E 1. Changes in number and abundance of species in The Netherlands between 1940 and 1980 (after Weinreich & Musters, 1989). '-' indicates no data available Taxonomic group No. of species in group in 1940 ~~ Fungi Plants Butterflies Fish Amphibians Reptiles Birds Mammals Percentage Species - Lost ~~ 314 1436 70 46 14 7 180 62 -~ ~ ~ Decreasing ~~ Increasing ~ - 36 8 7-30 54 34 69 64 100 30 30 26 2.3 10 - 1 5 - 30 36 62 TABLE2. Percentage surface area of protected nature reserves in (A) countries within the European Community and (B) regions of the world. The category 'parks' used here comprises IUCN categories I (strict nature reserves) and I1 (national parks). (From Wallis de Vries, 1995) Percentage of parks ~- ~~ A Italy France Greece Denmark Netherlands Spain Germany Portugal Belgium Ireland Luxembourg UK 0.97 0.50 0.46 0.31 0.26 0.24 0.04 0.04 0 0 0 0 B European Community Nordic Countries East Europe Asia North America South America Africa Oceania 0.34 5.07 1.06 2.13 2.30 2.70 2.72 3.73 'LCII'AL WORI.1) 2.42 LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE 15 for capital investments. The unpredictable whims of policy make it an unreliable ally to conservation of biological diversity. The major causes by which natural systems have changed or disappeared can be summarized as: loss of habitat (area); loss of structural diversity in the vegetation; increased disturbance; nutrient input; nutrient output through harvesting plant and animal products; and lowering of the water table. THE HISTORIC FRAME O F REFERENCE O F NATURE CONSERVATION At the turn of the century conservation organizations arose in a number of European countries. Observed habitat loss, combined with significant declines in some species, was the stimulus and nature reserves were created. It is not surprising that the main aim of the early conservationists was to preserve prevailing landscapes with their constituent species. In The Netherlands many nature reserves were left to themselves without any management. This frequently allowed a continuing deterioration in the ecosystem (Van Wieren, 1991) and it was realized that active management was needed. This has consisted of the continuation of farming practices: mowing, cutting sods, rotational grazing, coppicing and burning. Nature conservation thus aimed for the maintenance or redevelopment of historic agricultural landscapes by means of historic agricultural practices, an aim which is still current. In the light of the history of man’s impact as described above, it can be argued that the historic approach has serious shortcomings with regard to conservation, because it implies, in many cases, promoting a process that has led to a decline in biological diversity. In many cases the continuation of former agricultural management practices is no longer feasible. It is costly and laborious, and it is difficult to find a market for the harvested products. Furthermore, the desired landscapes have experienced a long history of decline and are thus already seriously impoverished (we are mainly concerned with open landscapes such as heathlands, grasslands, moors and downs, with relatively low structural diversities). The frequently expressed opinion that man has enriched the landscape and that diversity has even increased (Green, 1989) is only true in some cases (Western & Pearl, 1989) and does not take into account local extinctions (some of which went undetected) of many species. Many of these were believed to be associated with dead wood, large trees or carcasses of large animals (Dudley, 1992). When nutrients are extracted from the system by harvesting plant or animal material, impoverishment continues. Mowing, sod cutting, tree cutting or intensive grazing are disturbance factors that only benefit a small number of species. These activities are alien to normally functioning temperate terrestrial ecosystems and cannot usually be regarded as substitutes for natural processes. The desired landscapes can only be preserved or developed by active management because they deviate significantly from some climax stage, and the ecosystem is kept in some early successional phase. The aim of fixing the ecosystem is in itself artificial, as it is dynamic by nature, both with respect to structure and species composition. Management goals are further complicated by the fact that most desired landscapes have only had a short history and represent only transitional stages containing rather arbitrary shortlived assemblages of species. S. E. VAN WIEREN 16 From the above it can be argued that there may be risks involved in linking conservation of biological diversity to a historical agricultural frame of reference. The scope is limited and goals unlikely to be achieved. Recent inventories make it clear that the loss of natural values is continuing at an accelerating rate, even when including nature reserves (Baldock & Beaufoy, 1993; Bink et al., 1994). It has to be recognized that conservation in western Europe is unusual in focusing on natural values associated with early successional stages and hence with landscapes with a high degree of openness. Many conservationists fear that ecological changes in abandoned areas such as the development of a monotonous vegetation structure dominated by a few competitive species will result in the loss of biological diversity. This may well be the case when the ecosystem is either very young or incomplete and lacking in keystone species. Among the keystone species, large herbivores probably played a more important role in the maintenance of open landscapes in natural situations than has previously been thought (Iversen, 1973; Vera, pers. comm.). EFFECTS OF LARGE HERBIVORES The main processes by which herbivores exert their influence are grazing, treading, dunging and urination (Fig. 1). These processes occur selectively. Animals select on different levels-vegetation type, plant species and plant part-and as a result there are local differences in grazing intensity. Mutatis Animal characteristics General attributes Processes Ecophysiological niche Body size Dunging Social organization Social behaviour Density I Vegetation structure Succession Species richness and abundance of animals and plants Figure 1. Relationship between animal characteristics, general aspects of the grazing process and the grazed system. LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE 17 mutandis similar differences in spatial variation can be found in treading pressure and dunging intensity. The various ways in which these processes work can result in changes both in abiotic conditions and the structural diversity of vegetation. To these general processes can be added speciesspecific and animal density effects. Together, these factors have a high potential for causing variation in the diversity of temperate terrestrial ecosystems some of which has been described previously (e.g. Bakker, 1989; Gordon et al., 1990; Putman, 1986; Van Wieren, 1991) and only a brief overview is given here. Abandonment of farmland When grazing ceases there is frequently a decline in species richness, and a thick litter layer forms (Willems, 1983). For example, after 24 years of abandonment of formerly grazed grasslands, species diversity, equitability and pattern diversity diminished (Persson, 1984). In contrast, when grazing is resumed, in a previously abandoned grassland, species diversity can increase again. Ten years after the reintroduction of grazing, the number of species in a grazed salt-marsh had doubled (Bakker, 1989). Diversity and grazing Some of the high diversity that characterizes many grazed ecosystems can be explained by processes described above. But species diversity seems to be highest at intermediate levels of plant biomass (Grime, 1979) and disturbance (Huston, 1979). Both conditions can be brought about by adjustment of the grazing level. Moderate grazing seems to have a mediating effect on competition between plant species, permitting more species to be sympatric than would be the case in the absence of grazing (Naveh & Whittaker, 1980). Grazing can also create gaps and a large variety of these is sometimes considered an important explanation for high biologcal diversity (Bakker, 1989). When grazed systems are allowed time to develop, structural variation is increased and favourable conditions for ‘specialists’ arise, so that species richness can increase even further (Fig. 2). Facilitating energy Jlow Grazing and trampling facilitate energy flow through ecosystems because there is little accumulation of litter whilst at the same time the decomposition rate is relatively high (Siepel & Van Wieren, 1990). For example, after 4 years of grazing a Deschampsia Jlexuosa (L.) Trim-dominated former heathland, cattle had reduced the litter layer from 52 to 28 t organic matter ha-’ (Van Wieren, 1988). In this grassland both the biomass and productivity of D. jlexuousa were greatly reduced, gaps were created, and Calluna uulgaris (L.) Hull and Vaccinium uitis-idaea L. started to regenerate. The same mechanism would work in counteracting the now frequently reported risk of fires in abandoned lands in the drier parts of southern Europe (Baldock & Beaufoy, 1993). in S. E. VAN WIEREN Grazing intensity Figure 2. Response surface curve outlining the species richness (S) response to grazing intensity through evolutionary time. Unbroken lines connect points with the same S. The ridge (broken line) connects peak S values. (From: Naveh & Whittaker, 1980.) L=Low s H = High S Facilitating other fiuna A well-known effect of grazing by larger herbivore species is facilitation of smaller herbivores by which the former create suitable habitat or feeding sites for the latter. In an abandoned grass-heath grazed with ponies, rabbit Oryctolagus cuniculus L. density was positively correlated with the grazing pressure of the ponies (Thalen et al., 1987). Gordon (1988) found that cattle grazing on the Isle of Rum increased the reproductive performance of red deer. The effects of grazing on wetland birds are frequently found to be positive and grazed wetlands are also particularly attractive to geese (Van Wieren, 1991). However the occurrence of such grazing facilitation is dependent upon grazing intensity. Negative effects of a very high grazing pressure have been reported for insects (Rahmann, 1987), small rodents (Hill, 1982) and birds (Beintema & Muskens, 1987). Species dzferences The various hoofed herbivore species differ in many respects but notably in size and ecophysiological niche. With respect to the latter, Hofmann LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE 19 (1973, 1982) distinguished three main types of ruminants: concentrate selectors, bulk and roughage feeders, and intermediate feeders. The distinctions were based on differences in the anatomical and morphological characteristics of the digestive tract, which in turn were related to the capability to digest cellulose. Concentrate selectors are poor cell wall digesters, concentrating on the cell contents; roughage feeders are specialized grazers with a high capacity to digest cell walls, while intermediate feeders have a versatile intermediate strategy. The horse, a non-ruminant, can be classified as a grazer. These anatomical and physical differences lead to different feeding styles (Thalen, 1984). Browsers feed predominantly on herbs and leaves and twigs of woody plants while grazers are specialized grass eaters. Intermediate feeders are in between these two types (Breymeyer & Van Dyne, 1980). In lowland western Europe the original species assemblage would have been roe deer, red deer, aurochs, tarpan and wisent. Within this group, all major feeding styles can be found and it is likely that western European ecosystems function best if representatives of all feeding styles are present. The concept of restoring natural ecosystems hence excludes non-native species such as sheep and goats. It can be argued that because the aurochs and the tarpan are extinct, and because they have been identified as keystone species, ecological replacements for them in the form of hardy domestic breeds of cattle and ponies should be used. If this occurs, they should be managed, as much as is possible, as wild animals. Density and vegetation structure In the management of ecosystems using large herbivores, density is perhaps the single most important variable. Increasing density beyond the ecological carrying capacity, the natural limit of a species population set by resources in a particular environment, can have a number of effects. Frequently, a major shift in species composition occurs as the result of preferential foraging. The regeneration of preferred species can be seriously hampered or even prevented. Heavy grazing by domestic livestock in Scandinavia resulted in the large scale elimination of Salix cinerea L., Fraxinus excelsior L. and Ulmus sp. (Ahlen, 1975). In the New Forest, a very high grazing intensity for a long period has led to the disappearance of shrub species such as hazel, Corylus avellanus, and also some of the palatable species in the ground layer (Tubbs, 1981). In western Europe, succession to broadleaved forest can be seriously retarded because species of the later successional stages (e.g. oak Ruercus sp and beech Fagus sylvatica L.) are preferred while pioneer species like birch Betula sp and Scots pine Pinus sylvestris L. are not preferred (Mitchell et al., 1977; Van Wieren & Borgesius, 1988). When herbivore pressure increases even further, forest regeneration is prevented on a large scale and grasslands and dwarf-shrub heaths spread (Ahlen, 1975; Putman et al., 1984). When conditions are suitable, and grazing pressure is not too high, pasturewoodlands can develop. The woodland can be maintained by protecting the trees or by scrub encroachment. In the case of the latter, broadleaved species such as oaks can establish within thorny shrubs and so grow out of reach 20 S. E. VAN WIEREN of large herbivores (Burrichter et al., 1980). If different successional stages are present and fringe communities develop, pasture woodlands can be very rich in species (Dierschke, 1974; Alexander, 1995; Kirby et al., 1995). Pasture-woodlands were once common throughout Europe and they may well have dominated the landscape for more than 1000 years (Pott & Hiippe, 1991). Today in north-western Europe, only a few remnants of any size (such as the New Forest) remain, but in the south, large pasture-woodlands still exist. One of the richest systems is the Dehesa landscape in Spain (Ruiz & Ruiz 1986). The Dehesas cover about 3-5 million ha of extensive grazing. The average pasture contains about 150 higher plant species (Fernandez Ales et al., in Baldock & Beaufoy, 1993). Globally threatened species like the Spanish imperial eagle (Aquila heliaca Savigny) and the black vulture (Aegypius monachus L.) occur. There are 40 important bird areas ( B A S ) in Extremadura alone (Sears, 1991) and a rich mammalian fauna is present including red deer, boar, lynx and wolf. It can be argued that pasture-woodland comes closer to natural ecosystems than most other man-modified landscapes in Europe. This is because of a closer physical resemblance, and some important characteristics: extensive grazing with large herbivores, low input of fertilizer, and the presence of natural tree cover. Of course pasture-woodlands are not the same as the original ecosystem, and species losses have occurred, but because of important analogies we can derive some major principles from them with respect to ecosystem restoration. PERSPECTIVES This paper is not intended to denigrate all attempts to conserve single species or communities. These goals can be justified. It is, however, argued, that in many cases attainment of these goals will be very difficult and that the scope is limited. Therefore, the concept of ecosystem restoration at the landscape level is discussed and attempts to establish a series of ‘restoration experiments’ throughout Europe are noted. The main principles involved in ecosystem restoration and the conditions under which it can occur can be summarized as follows: (1) Large areas are needed. Every species has its own area needed to sustain a viable population, but it is estimated that at least 10000 ha would be required for an ecosystem containing viable populations of large mammals. In Europe it seems likely that more land for conservation will be available as a result of current developments in agricultural policy (Baldock & Beaufoy, 1993), and that opportunities will arise for the establishment of such largescale nature reserves. (2) The concept of restoration indicates that only indigenous species are involved. Knowledge of the ‘reference’ ecosystem is thus a prerequisite. (3) Because of a frequently impoverished starting point, it is important that at least some keystone species are reintroduced. Among these should be the large indigenous herbivores. Domestic breeds of cattle and ponies can be used as substitutes for the extinct aurochs and tarpan but they should be treated as wild species. If possible, large predators should also be reintroduced. (4)Natural systems function without main-induced nutrient input or nutrient LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE 21 output and there should be a presumption against extraction of plant or animal matter. (5) Structural diversity in the vegetation is the key to biologcal diversity (Holden, 1987) and so should be encouraged. If an area is large enough and the keystone species are present, structural diversity in the vegetation will be maintained. If the ecosystem is incomplete and important processes are lacking, man can (and may have to) influence the densities of the herbivores to enhance structural diversity in the vegetation. (6) Normally functioning ecosystems do so with little involvement of man. This means that disturbance should be minimized. Disturbance, following Van Andle & Van den Bergh (1987), is “a change in conditions which interferes with the normal functioning of a particular biological system”. The whole concept of ecosystem restoration is based on faith in the working of natural processes leading to ecosystems that contain a larger array of species than in man-managed landscapes. The space, time and the presence of key factors through which natural processes can operate are necessary prerequisites to successful restoration. This concept is now beginning to be taken up in several European countries. In The Netherlands, restoration projects have been initiated along the major rivers and in coastal (marsh) areas (De Bruin et al., 1987). In Germany, plans are being made to restore a typical central European ‘ Urecosystem’ in the biosphere reserve of Schorfheide-Chorin (R. Hofmann, pers. comm). Here the starting conditions are optimal because large mammals like boar and red deer are already present, while moose and wolf from Poland are colonizing the area. Examples of attempts to improve habitats by use of large grazing animals can be found in The Netherlands, in Britain (Rum) and in France (Camargue). In Killarney National Park (Ireland) experiments have recently started with reintroducing the local Kerry cattle on the hills to stop the spread of coarse grasses and to improve conditions for red deer. In spite of these attempts there is still a need for a number of case studies where a variety of scenarios can be studied in an experimental context. 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