Thill (1984) points out the advantages for multiple use management derived from
the diet partitioning. As forest practices intensify, forest ages decrease and the young
stands become impenetrable without artificial clearing. They are also poor areas for
deer forage. If cattle were used to graze these sites they could be kept open and so
benefit deer by improving accessibility and increasing production of the second growth
deer food plants. This presumes that increasing deer numbers is the management
objective. We should recognize that deer can have negative impacts, particularly
on rare plants and birds (McShea et al. 1997), so management of deer needs to be
carefully evaluated.
Hobbs et al. (1996) manipulated elk densities in randomized block experiments
in sagebrush grassland to study the effects of competition with cattle. The effects of
elk on cattle exhibited a threshold where low densities of elk had no effect but above
a certain density there were both competition and facilitation effects. Food intake
declined in direct proportion to elk density because elk reduced the biomass of stand-
ing dead grass in winter. There were some weak facilitatory effects of elk grazing
through an increase in digestibility and nitrogen content of the remaining grass avail-
able to cattle.
Cattle can also have indirect competitive effects by altering habitat structure. In a
study of bird communities using the riverine shrub willow habitats in Colorado, Knopf
et al. (1988) found that cattle grazing altered the structure of the shrubs but not the
plant composition. Areas with only summer grazing contained larger bushes widely
spaced and with few lower branches when compared with those areas that experi-
enced only winter grazing. The difference in structure affected migratory bird species
according to how specific their habitat preferences were. Densities of those with wide
habitat preference (e.g. yellow warbler (Dendroica petechia), song sparrow (Melospiza
melodia)), did not change between the sites. Those with moderate niche width
(American robin (Turdus migratorius), red-winged blackbird (Agelaius phoeniceus)),
were three times more numerous on the winter-grazed sites; while those with
narrow niches (willow flycatcher (Empidonax traillii), white-crowned sparrow
(Zonotrichia leucophrys)), occurred only on the winter-grazed sites.
Hayward et al. (1997) found from a 10-year exclosure of cattle in riparian habitats
of arid zones in New Mexico that small mammals were 50% more abundant in areas
where cattle were excluded. Similarly, kangaroo rats (Dipodomys merriami) were more
abundant in semi-desert shrubland where cattle grazing was reduced (Heske and
Campbell 1991), and reptiles were also more abundant (Bock et al. 1990).Exotic species, those that do not normally live in a country, are introduced for a
variety of reasons, and very often they become competitors with the native wildlife.
Rabbits in Australia are perhaps the most conspicuous example of this, for they have
been implicated in the decline of native herbivores through either direct competition
or apparent competition by supporting exotic predators such as foxes (Short and Smith
1994; Short et al. 2000; Robley et al. 2001). Dawson and Ellis (1979) measured the
dietary overlap between the rare yellow-footed rock-wallaby (Petrogale xanthopus),
the euro (Macropus robustus), which is a kangaroo, and two introduced feral species,
the domestic goat and European rabbit. During periods of high rainfall the rock-
wallaby’s diet was mostly of forbs (42–52%) but the proportion of forbs in the ground
cover was only 14%. Under drought conditions they still preferred forbs (there was
13% forbs in the diet when forbs were hardly detectable in the vegetation) but treesCOMPETITION AND FACILITATION BETWEEN SPECIES 161
9.10.4Introduction
of exotic species