An allegory is a literary device in which a character, place or event is used in order to deliver a much broader message. Allegories allow readers to illustrate or convey complex ideas in simple, imaginative ways.
The central theme of this poem is life and death, and this is conveyed through the use of allegory. In this poem, the allegory is build up through the comparison of man and the swan. Rilke talks about how the swan is awkward when he walks on land, always unstable. This is compared to how man is throughout his life. Rilke contniues the allegory by explaining how a swan looks when he lets go of land and jumps into the water. On water, the swan is pleased to be carried, and needs little effort to be at peace. This is compared to the transition of man to death.
The author might compare "the awkward waddling walk of a swam to the torture of life that humans life on this planet". This might be considered a metaphor for the "release of death and the grace" in comparison to life itself. The reader might perceive that life is awkward and death is sweet. So it could be said that the theme of the poem might be "the release of the burden of life in death".
the exact definition of natural vegetation is a cover of plants that can grow alone with little to no human interference, so i believe this fits your description best!
