very high nutrient turnover rates due to the large herds of ungulates grazing them
in the wet season and the plethora of dung beetle species that act to bury the dung.
This process leads to high protein and mineral content of the grasses eaten by the
grazing herds. In essence, ungulates fertilize their own food, and so create a positive
feedback increasing their own density (Botkin et al. 1981; McNaughton et al. 1997).
In low-nutrient granitic soils of southern Africa, the vegetation is also low in nutri-
ents and only the very large mammals such as elephants can feed in those wood-
lands. Nutrient recycling is slow (Bell 1982).
On the Canadian arctic shorelines, high densities of snow geese (Chen
caerulescens) influence the growth of their food plants. Moderate grazing promotes
growth through fecal nutrient cycling. Recent population increases of geese have resulted
in overgrazing that has overwhelmed the positive effects ( Jefferies et al. 2004). In
boreal forests moose decrease nitrogen mineralization of the soil by decreasing the
return of high-quality litter: their browsing on deciduous trees reduces their leaf fall
while promoting low-quality white spruce inputs (Pastor et al. 1993). In contrast,
soil nitrogen cycling in Yellowstone and other prairie areas of the USA is increased
by large mammal grazers (Hobbs 1996; Frank and Evans 1997).Herbivory alters not only structure but also the type of plants that can withstand
such impacts. On the North American prairies, rodents such as black-tailed prairie
dogs (Cynomys ludovicianus) live in large colonies. These species graze grasses to
a low level (a few centimeters) around their colonies. Grazing changes the grass
species composition to low-growing forms, and many dicot species survive due to
reduced competition from grass. American plains bison preferentially graze these short
grasses, and pronghorn antelope (Antilocapra americana) feed on the dicots (Huntly
and Inouye 1988; Miller et al. 1994).
Rabbits maintain short grasslands with many dicots on the South Downs of
Sussex, England. When the epizootic myxomatosis removed rabbits in 1953, plant
species composition changed to one of tall tussock grasses with few dicots, and there
were subsequent changes in ants and lizards dependent on these plant forms (Ross
1982a,b). A whole range of plant species evolved in New Zealand with special struc-
tural defenses against moa browsing not seen elsewhere (Bond et al. 2004).Ecosystems, left undisturbed, will change through succession to a plant community
dominated by good competitors with their associated fauna. A few wet tropical forests
may exhibit this situation. However, it is rare that ecosystems experience such con-
stancy of environment. Disturbances disrupt this succession and the community reflects
this history of disturbance (Pickett and White 1985). If disturbances are severe
and frequent then a few hardy plants that can tolerate these stressful environments
characterize the community. The boundary between the alpine tussock grasslands
and nival (snow) zone of the southern alps of New Zealand is characterized by a
few plants adapted to steep loose scree (tallus) that moves frequently through heavy
rain, earthquakes, and trampling or feeding by exotic mountain ungulates, the tahr
(Hemitragus jemlahicus) and chamois (Rupicapra rupicapra).
More commonly disturbances are both less frequent and less extreme, allowing a
combination of good dispersing plants and good competitors (see Section 21.12). Forests
experience tree-falls that create canopy gaps, letting in light and opportunities for
light-seeking species. This disturbance is particularly important in tropical forests where374 Chapter 21
21.9.3Plant species
composition
21.10 Ecosystem disturbance and heterogeneity
21.10.1Degrees of
disturbance