Other grazing mammals and birds

Small herbivorous mammals and birds play an equally important role in forests as the ungulates, and conservationists concerned with particular groups such as dormice (e.g. Briggs and Morris, 2002) have done much to investigate this interaction. Their effect is based not so much on the overall amount they eat but on what they eat, especially when it comes to seedlings and seeds. A 5-year study in New York State by Manson et al. (2001) used 40 x 40 m partial enclosures to manipulate the density of meadow voles Microtus pennsylvanicus and white-footed mice Peromyscus leucopus along the junction between a hardwood forest and a set of abandoned fields. They found seedling predation by meadow voles played a more important role than seed predation by white-footed mice in the invasion of trees. At the end of the study, there were an average of 19 tree seedlings ha-1 under low vole densities and 8 ha-1 under high density. The authors conclude that the main effect of the vole is to slow down tree invasion considerably, both directly by herbivory of seedlings, and also indirectly by reducing the establishment of shrubs which act as bird perches so the number of bird-defecated tree seeds reaching the site is less. This also means that the species composition of the developing tree cover can be changed as different tree species vary in their susceptibility to herbivory.

Other rodents can have even more significant effects on woodland. The ability of squirrels to climb and their continuously growing, extremely sharp incisor teeth, whose front surface consists of hard enamel and the rear of more easily eroded dentine, enable them to reach the seeds and cones of trees more effectively than any other group ofmammals. In Virginia, eastern USA, native grey squirrels Sciurus carolinensis feed on acorns of white and red oaks, which are normally produced in very great quantities every year. The acorns of the red oak are slightly darker in colour than those of the white, but the squirrels are colour-blind and differentiate between the two by smell. Acorns of white oaks germinate immediately they are shed, whereas those of red oak remain dormant until the spring. In normal years the squirrels feed on white oak acorns in the autumn and store those of red oak for consumption during the winter. If only white oak acorns are available the squirrels bite out the embryo of each acorn before storing it, preventing the growth of any that might be forgotten. Since the death rate of young seedlings is high, this extra loss might have long-term consequences for the oak.

The importation of the eastern grey squirrel S. carolinensis into Britain in 1876 and elsewhere in Europe may be one of the main causes for the very great losses of the smaller and less aggressive Eurasian red squirrel Sciurus vulgaris through much of the British Isles and northern Italy (Gurnell et al, 2004), as this species is badly affected by a virus (parapoxvirus) which the grey squirrel carries though itself immune to it (Tompkins et al., 2003). Fortunately, the red still exists in the Scottish Highlands and parts of Wales, besides persisting in such outposts as the Isle of Wight. As importantly, grey squirrels were estimated in 2003 to cause about £50 million worth of damage to UK trees annually where they often occur at much higher densities than in North America. Major damage is mainly caused by juvenile animals and involves bark-stripping in mid-July or a little later. Maples and beech are very badly affected, though very few tree species are immune from attack. Complete ringing of the upper parts of the main trunk of young trees may make them useless for forestry purposes. The most effective method of control is by using specially designed poison feeding-hoppers.

This is a clear example of globalization, the introduction of organisms into territories from which they are normally absent. In other cases such organisms may cross-breed with native species, as when the Russians cross-bred the American and European bison. Unfortunately they then released some of the resulting hybrids into the wild Caucasian herds.

Seed movement by small herbivores is also important. For example, seeds of the yew tree (Taxus baccata) in Europe are dispersed primarily by birds (mainly thrushes, various Turdus species) that void the seeds once the fleshy red aril has been consumed. Seeds end up concentrated under the bushes that the birds perch in, giving the seed a favourably moist microhabitat and protecting the seedling from large grazing animals. However, the seeds have to run the gauntlet of small predators attracted to the food supply. Of the 2-6 million seeds ha-1 produced by the yew, at least 60% that end up under bushes are normally eaten (Thomas and Polwart, 2003), predominantly by small rodents but helped by rabbits, squirrels and birds (mainly finches, great tits and woodpeckers). On the other hand, some of the left-over seeds carefully hoarded by rodents may be well placed for subsequent germination.

The part played by herbivores in spreading seeds is one of the most important of all biotic interactions. The cassowary, a very large flightless bird of ancient Gondwanaland lineage (Fig. 5.14), and said to distribute the seeds of some 250 species of understorey plants in its droppings, is probably a record breaker in this respect. Cassowaries belong to the ratites that all possess a flat sternum and have powerful legs for running. Cassowaries are now confined to New Guinea, parts of Polynesia, and the remaining tropical rain forest of North Queensland, the latter now largely cleared and used for growing bananas, amongst other crops. The cassowaries of New Guinea stand as tall as a person and are the largest animals in an extremely rich rain forest that contains roughly 20 000 species of trees and flowering plants. The forest is populated by agile marsupials like the spotted cuscus Phalangista maculata; there are no squirrels or monkeys.

Smaller mammals may also play a key role in herbivory of foliage. In Fig. 5.15 a browse-line can be seen that would be normal for a deer except this picture was taken near the treeline in Arctic Canada. In this case the cause was snow-shoe hares Lepus americanus, able to browse progressively higher as snow pack builds up over the winter, and able to remove as much foliage as an ungulate.

Beavers Castor canadensis, with their prominent orange incisor teeth and paddle-like tails, are the largest rodents in North America and have an intense if localized effect on forests by damming forest waterways and flooding low-lying areas, and by gnawing down trees. They construct stick-and-mud lodges with underwater entrances and inside platforms raised above water, emerging at dusk to forage for succulent plants or cut down trees and shrubs. Cuttings stored in late summer and autumn in the mud at the bottom of the ponds are eaten in winter and the young are born in spring. The European beaver C. fiber does not create large ponds, instead it burrows along the water's edge and makes small dams to slow streams that are too narrow or fast-flowing for its purposes. They also tend to be less aggressive in tree felling and less invasive of surounding waterways. These animals, which were in England hunted to

Figure 5.14 Cassowary (Casuarius). These birds all have a prominent horny helmet or casque on their heads; this is thought to protect the bird as it moves through the tangled undergrowth. Their feathers are highly modified and look like fur, those of the wings being reduced to quills hanging over the flanks. The birds defend their feeding territories and the males are particularly aggressive when tending their young; if aroused they strike out with both feet, which are each armed with three large spiked toes, and they sometimes disembowel human intruders. (Drawn by Peter R. Hobson.)

Figure 5.14 Cassowary (Casuarius). These birds all have a prominent horny helmet or casque on their heads; this is thought to protect the bird as it moves through the tangled undergrowth. Their feathers are highly modified and look like fur, those of the wings being reduced to quills hanging over the flanks. The birds defend their feeding territories and the males are particularly aggressive when tending their young; if aroused they strike out with both feet, which are each armed with three large spiked toes, and they sometimes disembowel human intruders. (Drawn by Peter R. Hobson.)

Northwest Snow And Forests
Figure 5.15 Browse-line on willow vegetation at head-height on the people, caused by snow-shoe hares able to browse to almost head-height by standing on the winter build-up of snow. Photograph taken north of Inuvik, Northwest Territories, Canada. (Photograph by Peter A. Thomas.)

extinction in the late Middle Ages, are now making a comeback after the European population had been reduced to very low levels after World War II, having been hunted for its meat, fur and the castoreum produced from its sex glands. Ten beavers released in Brittany in the 1960s have given rise to a population of 50, while individuals are now established in the UK at the enclosed Ham Fen nature reserve, near Sandwich, and a private estate in the Cotswolds. It remains to be seen whether free-living natural populations will again be allowed to flourish in the UK, although it is thought they are unlikely to develop into a widespread pest given their more sedentary behaviour compared with their North American cousins.

The above are examples of mammals that have significant effects on forests, but most mammals are less influential and often appear to have little day-to-day impact on their food supply. A prime case is the koala Phascolarctos cinereus, a marsupial which evolved in parallel with the eucalypts and is unusual in its ability to digest their nutrient-rich, but hard and chemically unusual leaves. These animals can live for 15 years; they sleep for up to 20 hours a day while they digest eucalypt leaves with the aid of a bacterium which is passed from generation to generation, the babies consuming some of their mother's excrement. The young animal is very small when born and is carried in its mother's pouch for 6 months, being later transported on her back until half-grown. So poor is the food supply that koalas can support only a very small brain (a very energy-demanding organ); offer it leaves lying flat rather than hanging and it has no idea what to do with them (Martin and Handasyde, 1999).

Other marsupials flourish on the floors of forests. The smaller ones exploiting the canopy include several arboreal tree kangaroos that flourish in the forests of Papua New Guinea where they spend half their time feeding on leaves, as they formerly did throughout Australia when it was still largely covered by trees.

Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment