"Drought Year" is a Naturalist poem because it describes the environment with a distant, objective (and Realist) perspective. The speaker observes nature in its most brutal reality. 

In the first stanza, the images are a dried creek, plants that can not bear fruit, roots burned, dry air, and the dingoes' cry which is like a song lamenting the death caused by the drought. 

The river, Thirty-mile Dry, is also barren. And although there are some who benefit from the drought (the wagtail), Wright presents it in graphic detail: 

I saw the wagtail take his fill
perching on the seething skull. 

The wagtail (a bird) is eating the leftover scraps or maggots out of a dead animal's skull. 

In the third stanza, the speaker (Wright) shifts these pictures of death and decay to signs and warnings of death and decay. Perhaps, this is a statement about the awesome power of nature and/or a reminder of our own (human) mortality. In this stanza, the speaker hears the dead horse's bone "whisper" is if to warn the listener about the danger of the wilderness and/or the inevitability of death. The speaker commands (someone or nature itself) to prop up the horse to warn people/animals away from the Thirty-mile Dry. The poem ends with the haunting wail of the dingoes. 

Nature, death, and the wilderness are all themes in this poem. Since the poem ends with these images of death becoming signs, warnings, and lessons, it seems plausible that the poet wants to stress the power of nature. As Wright was a conservationist, perhaps this harsh display of the power of nature was meant to provoke humility in humanity, so that people would respect nature and thereby preserve it; if human actions destroy natural habitats, the effects could result in landscapes as bleak as those described in this poem.