Herbivorous mammals and birds

Mammals have played a major role in the world's forests over a very long period. Their numbers are small compared with insects and other invertebrates, but their individual biomasses are large so collectively they have made a major impact on forest ecosystems. In Pleistocene times, for example, when even Australia had a marsupial elephant, elephants even larger than those of today were widespread. These enormous animals, which could break down large trees (Rackham, 2002), appear to have been exterminated in North America around 11 000 BC by Palaeolithic people employing spears tipped with flint Clovis points.

The Asiatic elephant Elephas maximus is the less formidable of the two contemporary elephants, but at 60 a plains male African elephant Loxodonta africana can weigh 6 tonnes; many reach 4.5 tonnes at 30. Despite their size and the damage they can inflict on vegetation by trampling and feeding, studies have shown that they benefit trees in a number of ways, including spreading seeds. As an example of their more indirect effect, Goheen et al. (2004) working on African acacia savannas found approximately twice the number of tree seedlings in areas with elephants, despite higher seedling death due to desiccation. In areas where elephants were excluded, the greater number of rodents and invertebrates led to ever-higher seedling death.

Elephants, which have rather few sweat glands, maintain a steady temperature by losing heat from their ears. Typical African elephants living in the relatively open savanna absorb a great deal of heat from the sun's rays and have very large ears, while those of forest type African elephants (see Frontispiece), whose total height is no greater than the shoulder of a savanna elephant, are relatively smaller. The ears of Asiatic elephants, whose whole skulls are different, are considerably smaller than either type of African elephant.

Large forest animals exert a very considerable influence on the trees they live among; they are amongst the agencies which may have led to the evolved ability to sprout from cut or broken shoots. Windblow, self-coppicing and fire are others. Humans have utilized this sprouting ability in pollarding and coppicing (the deliberate felling of trees above or at ground level, respectively, to stimulate long, straight growth). The history of coppicing is of too short a duration to have caused an evolutionary response itself.

It has been suggested (Redford, 1996) that many forests, particularly in the tropics, while appearing intact are actually devoid or depleted of large animals and consequently face long-term problems of pollination, seed dispersal, a detrimental build-up of herbivore numbers, etc., and therefore face severe long-term problems. Conversely, increasing numbers of some herbivores are causing equally severe problems, as discussed in the next section.

5.7.1 Megaherbivores and ungulates: deer and others

Without question, large herbivores have an important effect on forest dynamics. They are also often the most likely animals to go extinct when forests change rapidly such as when humans appear (this is because they are usually the longest lived and so less able to adapt quickly by natural selection -see Brook and Bowman, 2005). The ungulates, herbivorous mammals with hooved feet and a herbivorous diet, have a particularly important role in forests. Those with an even number of toes, the artiodactyls, include the insectivores, pigs, hippopotamuses, camels and giraffes. These tend to have a larger global impact on plants than the perissodactyls (with uneven number of toes) such as the horses and rhinoceroses. The most successful modern artiodactyls are the ruminants (Fig. 5.12), which include the deer (Cervidae) and the cattle, sheep, goats and antelopes (Bovidae), which have flourished only since the Miocene (5-24 Ma). Ruminants all possess a stomach with four chambers, of which the first two (rumen, reticulum) receive the food after it has just been swallowed. Here it is mixed with mucus and acted upon by a mixture of bacteria and ciliates which rapidly break up the celluose chemically. Food is then returned to the mouth when the animal 'chews the cud'. Following this it passes to the two latter chambers (omasum, abomasum) where useful material is absorbed. This feeding strategy enables the ruminants to feed very rapidly and then digest their food after retiring to a place of safety. It also enables them to utilize materials that are intrinsically difficult to digest, making them very effective grazing machines.

Deer are a most successful group of ruminants whose keen eyesight and sense of smell largely enable them to evade their enemies. They often graze extensively in small areas, yet being very mobile can spread their grazing over large areas, including even isolated woodlands. In the northern hemisphere, deer populations

Rainforest Herbivores

Figure 5.12 Large herbivores indigenous in Europe together with the omnivorous wild boar, classified according to their feeding strategy. Domesticated species are merely outlined. Indigenous species of the lowlands of central and western Europe include the aurochs, tarpan, European bison, red deer, elk, roe deer and wild boar. (Redrawn from Hoffman 1973, 1976, 1985, and Van der Veen and Van Wieren 1980. From Vera, 2000. Grazing Ecology and Forest History. CABI Publishing.)

Figure 5.12 Large herbivores indigenous in Europe together with the omnivorous wild boar, classified according to their feeding strategy. Domesticated species are merely outlined. Indigenous species of the lowlands of central and western Europe include the aurochs, tarpan, European bison, red deer, elk, roe deer and wild boar. (Redrawn from Hoffman 1973, 1976, 1985, and Van der Veen and Van Wieren 1980. From Vera, 2000. Grazing Ecology and Forest History. CABI Publishing.)

have been expanding in size and distribution over recent decades. Red deer Cervus elaphus (the red deer of Britain and the elk of north-western American forests - Fig. 5.13) is a case in point. Although the claims are disputed, the population of red deer in the Scottish Highlands is thought to have risen from 300 000 in 1989 to 450 000 in 2002. If heather cover and woodland regeneration are to be protected here it will be necessary to reduce numbers of sheep as well as deer, and to protect sensitive woodland with fences. Similar problems are being experienced through most of its North American range as well.

Skull Canis Lupus

Figure 5.13 The carnivore/herbivore relationship. The wolf Canis lupus and the skull of a red deer Cervus elaphus, known as elk in North America, which it had killed earlier. These deer defend themselves with their antlers, which the males also use in competitive battles with rivals. (Drawn by Peter R. Hobson.)

Figure 5.13 The carnivore/herbivore relationship. The wolf Canis lupus and the skull of a red deer Cervus elaphus, known as elk in North America, which it had killed earlier. These deer defend themselves with their antlers, which the males also use in competitive battles with rivals. (Drawn by Peter R. Hobson.)

In Eurasia Alces alces, the largest deer in the world, is called moose or elk, or just moose if you are in North America. It is superbly adapted for snowy climates with long legs (males stand around 2 m tall at the shoulder, females 1.7 m) and wide shovel-like antlers on males used to help dig through snow to reach food. Males can reach 2.8 m in length and their antlers commonly have a spread of 120-150 cm. Much of its summer fodder comes from aquatic and wetland plants, but in the winter it spends most time feeding in forests. Inevitably, an animal that can reach 630 kg (in the male) requires a lot of food and its impact can be high through grazing, trampling, thrashing shrubs to remove velvet from the antlers and debarking trees. In Sweden, where it damages commercial forests, numbers of moose rose to very high levels from 1970 onwards. In the 1980s approximately 1.5 million were killed in Sweden alone (Cederlund and Bergstrom, 1996).

As well as the red deer, one other species of deer is native to Britain - the roe deer Capreolus capreolus, of which the largest bucks weigh only 20-30 kg. However, a number of deer have been introduced, adding to the deer problem. The fallow deer Dama dama, a native of southern Europe and Asia Minor, was introduced into Britain probably by the Romans. Males are renowned grazers rather than browsers. When kept in parks fallow deer are gregarious, but in the feral (wild) state they go about in small parties.

Deer which have reached Britain much more recently include the sika Cervus nippon, the Siberian roe Capreolus pygargus, the black-tailed deer Odocoileus columbianus from western North America, the Chinese water-deer Hydropotes inermis and the Indian muntjac Muntiacus muntjak. The Chinese muntjac Muntiacus reevesi, or barking deer has spread from Woburn Park. It stands less than half a metre tall and the antlers are very short, ending in a single spike. The upper canines of the males project from the mouth as small tusks. Though it is so small and primarily a grassland animal, the foraging of this species, like those of its larger relatives, is causing a great deal of damage, particularly to the herbs of the woodland floor.

Reindeer Rangifer tarandus are the only deer to have been truly tamed by humans, and are unusual in that both sexes have antlers. Although they have only recently been introduced into North America, there are several domesticated races in northern Europe. Within the Arctic Circle they are used in much the same way as cattle are further south. The caribou of North America is so similar that it is now placed in the same species, though it has never been tamed. Many caribou spend their lives on the vast treeless arctic tundra, while others are found mainly in the taiga (northern boreal forest). They gather in late winter in groups of 10 000-100 000 before the spring migration. In winter they feed mainly on lichens, in summer they have a much wider diet and consume mushrooms, fruit, green plants, twigs of trees and even discarded antlers.

Deer are often regarded as being keystone species in forests, able to have larger effects on their habitat than their numbers would suggest. The problems caused by an almost worldwide expansion of the deer population are considered in detail by Rackham (2003), who has first-hand experience of the damage they cause in Europe, America and Japan. Large populations of deer graze trees so thoroughly that the height of the browse-line they can reach up to is an indication of the dominant species present. Ivy and holly are particularly palatable, as are elm and hawthorn. Under severe conditions forests may be devoid of seedlings and young saplings; even resilient species like ash are often deformed if grazing is later lessened. Rooney and Waller (2003) point out a strong linear relationship between increasing deer browsing pressure and decreasing seedling density of various conifers and deciduous trees in the forests around the North American Great Lakes, leading eventually to complete regeneration failure. White-tailed deer Odocoileus virginianus have so changed the Allegheny National Forest in Pennsylvania that black cherry Prunus serotina dominates much of its 800 square miles (its leaves contain hydrocyanic acid that releases hydrogen cyanide - cyanide gas - and so are little grazed) and every hemlock Tsuga canadensis has a strong browse-line, and none are younger than 70 years. Deer grazing also tends to lead to an increase in grasses, sedges, rushes and ferns at the expense of the herbs that produce the spectacular spring show. Rackham states that most Japanese woods are free of deer browsing, but Mount Odaigahara is an exception. This mountain has zones of different tree communities, but its top is a graveyard of dead nikko fir trees Abies homolepis. The fir is killed by the resident deer whose population exploded after 1970. The deer eat the tree bark which they prise off with their antlers. By 1998 the ever-expanding circle of dead trees had reached a diameter of some 14 km and the deer were working their way through other conifers. Their main food is probably the dwarf bamboos forming carpets or thickets in Japanese woods, but even the rhododendrons show a browse-line.

Changes induced by deer go beyond just the plants since other mammals or insects dependent upon the same plants will also be affected, creating a trophic cascade (i.e. effects cascading through other trophic levels). As an example, a study in the pinewoods of Scotland by Baines et al. (1994) showed that grazing by red deer reduced the height of bilberry Vaccinium myrtillus by half, which in turn led to a fourfold reduction in Geometrid moth larvae. These larvae form an important food supply for insectivorous forest birds such as the capercaillie Tetrao urogallus and black grouse Tetrao tetrix and so the red deer may indirectly be a major factor in affecting their numbers via this trophic cascade. Similarly, Flowerdew and Ellwood (2001) reviewed how deer affected small mammals in British lowland woodland and concluded that deer can have strong impacts on most small mammals through modification of habitat structure and food supply. However, not all effects are bad since the more open habitat maintained by deer can be beneficial: in the authors' words, 'the presence of some deer is a good thing, too many deer and too few are not'.

Deer are also host to the ticks that transmit Lyme disease, which is caused by bacteria that bring about skin changes, flu-like symptoms and joint inflammation in humans. Though normally found on deer (see Section 4.4.3), these ticks can also infest dogs and humans. Though most prominent in the USA and first recognized at Old Lyme, Connecticut, in 1975, Lyme disease is also a problem in many parts of Europe.

Amongst the other ungulates, sheep, goats and cattle are renowned for their grazing ability. Fortunately, feral populations with access to wooded areas are fairly limited around the world, but in these areas their effect can be equally if not more deleterious as deer. The forests of the world contain many species of wild boar Sus and their relatives. European wild boar Sus scrofa, from which the domestic pig was derived, is common through Europe, North Africa and through Asia to Japan, and is being seen again in southern England (after an absence of four centuries) following its escape from captivity (Fig. 4.14). In 2003 it was estimated that a population of over 1000 animals had built up. The wild boar is a highly intelligent and fast-moving animal which can weigh up to 180 kg and be 2 m long. Its lower canines rise straight up outside the mouth and point backwards towards the eyes; these formidable tusks can rip open the body of an adversary. Males join the herds, each of which is led by an old sow, in December and January. They leave again after the rutting season and the attractively striped young are born 3-4 months later. Though individual animals are good at concealing themselves, wild boar leave unmistakable traces in the forest. They are omnivorous (eating anything with food value), consuming seeds, fruits, bulbs, insects, earthworms, reptiles, small mammals, and the eggs and young of birds. Their rooting activities are important in creating areas of bare earth on which tree seedlings can establish. Venturing from the forest they attack many farm and garden crops, rooting through the ground with their formidable snouts. They also leave very distinct disturbances in adjacent muddy lakes or swamps, where they wallow to rid themselves of parasites, later making muddy rubbing marks on tree trunks and branches.

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