Influence of herbivores

The size and vigour of particular herbivore populations at a given time is a major influence on forest trees and other plants. Many examples could be chosen to illustrate this point; here we consider deer and elephants.

Deer have increased in number and expanded their range in many parts of the world over the last few decades, becoming a particular problem (Section 5.7.1). This is partly due to our previous management. Historically game managers strove to augment and protect deer populations, and hunters learned to limit takes and favour bucks. Today, such precepts are outmoded, but unlearning old lessons and reversing this cultural momentum has proved difficult (Cote et al., 2004). The result in many places is overbrowsing with consequent reductions in plant cover and implications for carbon storage and nutrient cycling. As importantly, selective browsing by deer can greatly influence the plant species composition of forests with consequences for animals needing different species, and affecting the species composition of young trees growing up to form the future canopy.

Elephants exert a strong pressure on the ecosystems in which they live, as is evident in the Chobe National Park, northern Botswana, which together with adjacent parts of Zimbabwe and Namibia, is home to more than 100 000 animals, the largest population of elephants in the world. Every day during the dry period they have to visit the Chobe river. In the wet season they spread out and disperse southwards drinking from temporary water pans. They browse heavily on the trees and shrubs of the river forest and also break down trees, few of which regenerate. Studies of the many ecological effects and history of this increasing population of megaherbivores are of great importance: indeed Skarpe et al. (2004) conclude that while the present population level is ecologically acceptable it may be necessary for social and economic reasons to cull elephants which persistently invade areas used for agriculture and housing.

A hundred years ago elephants here were rare due to excessive hunting, primarily for ivory. Moreover, rinderpest, a disease spread by cattle that had come to Africa in 1887, reached Botswana in 1896-1897 causing heavy mortality in both wild and domestic ungulates (hoofed animals). After restrictions were imposed on hunting, elephant numbers began to increase during the 1930s. There is now concern at the loss of scenic acacia woodlands along the riverfront. Many of these huge old acacia trees probably started life around a hundred years ago when populations of elephant and several other browsers were low due to the combination of hunting and sickness already mentioned. These trees are known to be unable to regenerate when impala densities are high. However, browsing by large herbivores is obviously not the only ecological factor at play: trees such as Croton megalobotrys and Combretum mossambicense have increased in density in recent years while Faidherba (formerly Acacia) albida and Garcinia livingstonei have decreased. Smaller and middle-sized browsers/grazers, particularly impala Aepyceros melampus, also consume tree seedlings and saplings, especially at sites close to the river. Grazing by ungulates leads to dominance of either fast-growing palatable species, which enhance the rate of nutrient cycling, or of slow-growing unpalatable species - often with physical or chemical defences - which diminish it. Small mammal populations varied greatly over short distances, being influenced by trampling, digging, defecation and urination by large herbivores.

Studies of many other facets of this ecosystem, including that of the lion population - several of which were radio-collared - are of considerable interest. In one 4-year study an incoming pride male killed the cubs of some of the female groups it took over; it was the only immigrant lion observed.

6.2.2 Beak size, guilds and resource partitioning

The evolution of herbivores is strongly influenced by the food available to them, a point well illustrated by comparing relative beak sizes of the birds of temperate forests with those of tropical forests, whose beaks are considerably larger. The diversity of tropical birds is also larger, a fact related to the greater diversity of foods available, particularly of large insects (Terborgh, 1992). Species utilizing the same pool of resources are referred to as a guild. These are often (but not always) made up of closely related species, such as the parrots in the Amazonian forests that form a guild of arboreal seed eaters. The smallest of these parrots is the size of a sparrow - the green-rumped parrotlet (Forpus passerinus), with the rather larger parakeets, the full-sized parrots, and finally the majestic macaws, following in a sequence of birds able to deal with successively larger seeds. The diminutive parakeets feed on fig seeds, discarding the pulp, whereas the beaks of the larger birds enable them to get at resources, such as hard nuts, that are much better protected. The evolution of these birds seems to have been influenced by Hutchinson's 'law of limiting similarity', that implies that two species cannot co-exist and utilize a common pool of resources unless they differ by some significant degree.

The packing of species in tropical forest guilds, with many species with only slightly different ecological requirements, is really quite remarkable. Such guilds routinely contain twice as many species, often many more, than in temperate forests. The neotropical antwrens (Myrmotherula species), which are basically similar to the warblers of the temperate zone, provide a good example of how specialization within the towering height and structural complexity of the tropical rain forest has led to speciation. As many as ten species of antwren can be found in a single locality. Similar in morphology and superficial appearance, all of them feed on insects but search for them in different ways. In the Amazonian forest the foraging zones of Myrmotherula species can be arranged vertically above each other with relatively little overlap; M. haematonata, M. axillaris, M. menetriesii and M. brachyara occupy successively higher zones, finally reaching the tops of the emergent trees.

6.2.3 Evolution, diversity and activities of insects

The insects are by far the most diverse group of animals that have ever existed, accounting for well over half of all the species of living things so far named and described (Fig. 6.1). The next largest group of living organisms is that of the vascular plants, which Grimaldi and Engel (2005) list as amounting to 248 400 species. Although this is certainly an underestimate (there are at least 400 000

species in the angiosperms alone), there are far fewer vascular plants than insects. Insect variation and evolutionary history over the past 400 million years are considered in detail by Grimaldi and Engel (2005), who trace their development and relationships with other organisms, many of which are components of forest ecosystems.

Anatomically, insects are divided into three parts, the head, thorax and abdomen, and have six legs. The greatest number of extant insects belong to the Holometabola (83%; Fig. 6.1 - insects that have distinct larval and adult stages). Of these by far the largest group is that of the beetles (Coleoptera)

Insect Groups
Figure 6.1 Proportions of species in present-day insect groups. See the text for an explanation of the insect groups. The total number of named insect species now existing is believed to be over 926 000. (From Grimaldi and Engel, 2005. Evolution of the Insects. Cambridge University Press.)

whose forewings are modified to form protective elytra that meet along the mid-dorsal line and protect the membranous hind wings which fold beneath them. They have biting mouth parts and many of them, especially the death-watch beetle Xestobium rufovillosum, cause considerable damage to timber. The weevils (Curculionoidea), of which there are 11 families, are by far the largest major group of Coleoptera. One species, the spruce bark beetle Ips typographus, has killed millions of Norway spruce Picea abies trees. Next numerous are the Lepidoptera with less than half the number of species. Many of these four-winged moths and butterflies are extremely beautiful, but their larval stages often cause severe damage to plant populations. The hornets, wasps, bees, and ants of the Hymenoptera follow with 13% of species, while the two-winged flies of the Diptera have 12%. Various members of the Hemiptera (true bugs), whose mouth parts are used for piercing and sucking, attack both flowering plants and animals to obtain either sap or animal juices including blood and in so doing create entry paths for disease organisms. They form more than half of the remaining species, which account for 17% of the grand total. Amongst the remaining small groups are the wingless hexapods including the springtails and silverfish of the Collembola, the mayflies and dragonflies of the Paleoptera, the grasshoppers and crickets of the Orthoptera, the stoneflies of the Plecoptera, and the termites (Isoptera). The Paraneoptera include not only the bugs, but also thrips (Thysanoptera) and book-lice (Psocoptera). See Box 7.6 concerning the activities of colonial insects in attacking undecayed wood.

Jones and Elliot (1986) provide an excellent description of pests, diseases and ailments of Australian plants, together with a diagram (their page 100) showing the regions of trees affected by pests in the tropics. The importance of insects in causing and transmitting disease, as pests of trees particularly in the tropics, in defoliating trees, and in pollinating the flowers of a high proportion of the flowering plants (angiosperms) with which they co-evolved, is outlined below:

(a) Insects transmit fungal and viral diseases of herbs, shrubs and trees. They and other arthropods of medical importance also cause or transmit many important diseases of humans and other animals (Grundy, 1981). The two-winged slender flies of the mosquito subfamily (Culicini) that spread malaria, yellow fever, dengue fever and filiariasis, are amongst the greatest dangers of tropical forests.

(b) Insects attack many trees, particularly in the tropics (Speight and Wylie, 2001). The adverse influence of Hypsipyla shoot borers belonging to the Lepidoptera on the plantation growth of valuable timber trees in the Meliaceae is described in Section 10.6. Attacks by the sap-feeding yellow scale Aonidiella orientalis on the exotic neem tree Azadiracta indica growing in the Lake Chad region of sub-Saharan Africa were countered by use of residual contact insecticide, employing older transplants for re-establishment and using previously uncultivated sites. These measures greatly reduced losses.

(c) The role of insects in defoliating trees is considered in Section 5.3. Defoliation of pines in Vietnam and China by the pine caterpillar Dendrolimus punctatus is often very severe, being aggravated by the fact that this Lepidopteran can have up to four generations a year and that each female lays an average of 300-400 eggs with a possible maximum of 800. Other species of Dendrolimus are also involved and every year about 3 million ha of forest are infested. Records of these destructive attacks on Masson pine Pinus massoniana and also slash pine P. elliottii and other pine species go back to 1530 AD. On a recent occasion almost complete defoliation was followed by the death of almost 25% of the trees and the volume growth of the survivors was reduced to 31% of normal.

(d) Pollination by insects is far less wasteful than that by wind, particularly when individuals of a species are far apart (see Section 4.2.1). In return the pollinators are rewarded by nectar and often use a small proportion of the pollen, so this is a very effective symbiosis.

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