In the opening act, for the most part, individuals act contrary to nature, giving rise to a great deal of grief as well as the play's central conflicts. We learn in the very first act that Duke Frederick has usurped his older brother by taking over his dukedom and driving him into exile. Since Duke Senior is the eldest brother, by nature, or by natural rights, the dukedom belongs to him and not to Duke Frederick. Duke Senior claims to be content in the forest, but of course being in exile creates hardships, particularly the hardship of having to survive during harsh weather conditions. This conflict between the two brothers is not resolved until Frederick also travels into the forest and discovers religion, transforming him into a changed man who relinquishes the dukedom back to Duke Senior. Hence we see that nature drives the plot forward when characters try to act contrary to nature.
Also in Act 1, Duke Frederick soon exiles Duke Senior's daughter Rosalind, and Frederick's own daughter Celia decides to go with Rosalind. In order to survive traveling to the Forest of Arden, Rosalind devises a plan to disguise herself as a boy, which would also be contrary to nature. Rosalind's own act of defiance of nature drives the plot forward by not only getting her, Celia, and Touchstone to the forest but also later creating conflict between Rosalind and Orlando. Orlando must also hide in the forest because his brother is trying to kill him, which also contradicts nature, and while in the forest, proclaims his love for Rosalind by attaching love poems addressed to her on trees. Rosalind responds by addressing him still in disguise as Ganymede, thinking that disguised as a boy, she can have the chance to test the strength of his love. This creates a conflict because Orlando wants to win Rosalind, but Rosalind is denying him this chance until she is more certain.
The best place to see in Act 1 exactly how acting contrary to nature is serving to drive the plot forward, creating a central theme, is in Orlando's opening speech. Oliver's treatment of Orlando certainly is cruel, unloving, and thus unnatural. Oliver is even treating the estate animals better than Orlando, which best characterizes the unnaturalness of Oliver's actions, as we see in Orlando's lines:
His horses are bred better; for, besides that they aer fair with their feeding, they are taught their manage, and to that end riders dearly hired: but i, his brother, gain nothing under him but growth. (I.i.10-14)
Hence we see that nature plays a role in Act 1 to develop the plot because many characters act contrary to nature, which creates the play's central conflicts.