TheImprovement ofTropical and Subtropical Rangelands

SITE EVALUATION 111
Animal 7Jpe. All types of avian and terrestrial fauna (including
soil biota) are part of an ecosystem. Difficulties arise, however, in
defining the geographic boundaries when more-mobile animals are
evaluated. In practice, ecosystems are commonly based on plant
communities, soil classification units, or other abiotic features, or
combinations thereof, and animals are then -incorporated" into the
delineated ecosystems 88 consumers and secondary users. Mobile
animals generally roam over several ecoeystems.
Wild animals have varying effects on the ecosystem. Earthworms,
arthropods, and ground-dwelling mammals play major roles
in the decomposition oforganic material. In their absence, the nutrient
cycles of a site can be adversely disrupted. Birds are important
agents of seed dispersal for many individual plant species. This dispersal
activity can be beneficial or harmful to reproductive strategies.
Mammals, especially rodents, can also be important agents of seed
dispersal in many plant communities.
The grazing activities of the larger ruminant herbivores, both
wild species and domestic livestock, have already been mentioned,
and are covered in more detail in chapter 6. Grazing is commonly
considered destructive, although it can benefit the desired vegetation
on a site by removing competitive plants that otherwise may use
limiting water and nutrient resources. Grazing activities also prevent
the buildup of coarse, unpalatable plant parts and stimulate the
growth and tillering of more plant materials.
Techniques of enumerating animal types and their respective
numbers, including the censusing or sampling of animal populations,
are described in Child et al. (1984), Conant et al. (1983), and
Schemnitz (1980).
DUtribution and Migration PatteN" In addition to knowing what
types of animals occur, knowledge of their distribution and patterns
of migration can also be important in site evaluation. Uniformly
distributed animal populations generally tend to exert uniform effects
on a site. On the other hand, a population of animals that is
unevenly distributed will frequently have uneven effects upon a site
(for example, animals clustered around a wellhead). The distribution
of animals is also inftuenced by their migratory patterns. In general,
migration (whether seasonal, yearly, or indeterminate in response
to unknown stimuli) can result in cycles of impacts that should be
known when planning a range improvement program.
Information regarding the distribution of animals can often be