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In some systems there is a high diversity of herbivores and carnivores. Nearly all are

associated with tropical ecosystems. Whether a herbivore species is limited by pred-

ators is determined by its place in the hierarchy of herbivores. In African savanna

there are as many as 10 coexisting canid or felid carnivores feeding on ungulates,

lagomorphs, and rodents. They vary in size from the 200 kg lion (Panthera leo) to

the 10 kg wild cat (Felis sylvestris). The larger the carnivore the greater is its range

of prey sizes. Thus, the lions’ diet ranges from buffalo (450 kg) to dikdik (Madoqua

kirkii), a small antelope (10 kg), whereas that of the 16 kg caracal (Felis caracal) ranges

from duiker (15 kg) to 100 g rodents. The consequence of this is that smaller

ungulates have many more predators than larger ungulates. Thus, smaller ungulates

experience more predation and top-down regulation (Sinclair et al. 2003).

Regulation of herbivore populations through their food supply has profound con-

sequences on the ecosystems where they occur. Mammals may not be numerous com-

pared with other animal groups but their impact is considerable. Perhaps more than

any other group they can determine the physical structure of the habitats, alter the

rates of ecosystem processes such as nutrient flow, growth rate, or decomposition,

and dictate species diversity. These large-scale effects – at the level of ecosystems,

watersheds, and biomes – can be thought of as ecological landscaping(Sinclair 2003).

Plants determine the physical structure of habitats, the particular type being a

function of the abiotic conditions. Some periodic environmental effects, such as fire,

hurricanes, and floods, can interrupt the normal succession of plant species towards

a climax. In savanna, fire typically impedes the succession of trees to produce a “fire

disclimax” of grassland and fire-tolerant herbs, shrubs, and trees. Herbivorous

mammals can have analogous effects to fire in savanna systems (Hobbs 1996) and

so produce a “mammal disclimax.” Plant succession is held in a different state as a

result of the restructuring imposed by mammals. Such impacts are evident in most

terrestrial biomes where mammals are abundant. However, mammals have their great-

est impacts in the tropical savannas, particularly through feeding by megaherbivores

(Owen-Smith 1988); and in grasslands throughout temperate and tropical regions due

to grazing and browsing by ungulates (Sinclair 2003).

In recent times, mammal herbivores have had little structuring effect in the high-

latitude tundra biomes. However, the Pleistocene tundra supported a substantial biomass

of mammoths, woolly rhinos, and bison that fed upon the shrubs and sedges.

Herbivorous mammals also do not substantially alter tropical forest, although mam-

mals do influence the dispersal of tree seedlings ( Janzen 1970). In both arctic tundra

and tropical forest the low impact of herbivores may be due to the top-down effects

of mammal carnivores that limit herbivore densities in these systems (Terborgh 1988;

Oksanen 1990).

Mammals and birds influence the rates of nutrient cycling in addition to altering

physical structure. High densities of mammals and birds can influence the soil pro-

cesses through their deposition of feces and urine. Before the arrival of Pacific rats

(Rattus exulans) in New Zealand some 700 years ago, billions of shearwaters lived

on the forest floor and provided a considerable nutrient input. The extinction of

shearwaters from mainland New Zealand due to the rats has altered the nutrient dynam-

ics of that country (Worthy and Holdaway 2002). The volcanic Serengeti plains have

ECOSYSTEM MANAGEMENT AND CONSERVATION 373

21.8.4High-diversity
communities

21.9 Ecosystem consequences of bottom-up processes

21.9.1Vegetation
structuring

21.9.2Ecosystem
rates