In selecting habitat animal species require food, water and adequate cover. When these criteria are taken into account it appears that habitats utilized by terrestrial wildlife along the Grande Ronde River change longitudinally from mixed conifer forests to ponderosa pine forests in the headwaters and middle reaches to grassland and shrub-steppe in the lower reaches. Species diversity of mammals, birds and reptiles within these habitats will also decrease towards the lower reaches to some degree as habitat changes along the Grande Ronde River. A potential decrease in species diversity as habitat changes may indicate that habitat specialist species are more likely to be found in the headwaters of and middle reaches while habitat generalists are likely to be found in the middle and lower reaches of the Grande Ronde River. This concept of decreasing species diversity in a downstream gradient indicates that the River Continuum Concept, traditionally an aquatic hypothesis, may have some applicability to distributions of terrestrial wildlife in the context of riverine ecosystems.
Species diversity of terrestrial wildlife is also heavily influenced by the presence of riparian vegetation along the river. Riparian vegetation is likely to be found within the headwaters and middle reaches of the Grande Ronde River and in patches pertaining to locations of physical complexity along the river such as confluences with tributaries. Riparian vegetation provides complex habitat for mammals and birds that live along the river and those that utilize riparian habitat for foraging, water requirements, migration or other activities necessary for survival. Amphibians also depend on these patches of physical complexity to provide slower waters needed to complete their lifecycles. Wildlife species diversity will be higher in the headwaters and middle reaches of the Grande Ronde River and in areas of high physical complexity where greater diversity and structural complexity of habitat will support large numbers of wildlife species along the Grande Ronde River.