Per the title, the balance of nature refers to the theory that biological systems are usually in a stable equilibrium and a small change in some particular parameter will be corrected by some negative feedback that will bring the parameter back to its original "point of balance" with the rest of the system.
Barbara Rush (Barbara Matheson) previously played Leonora Edmond in The Forms of Things Unknown (1964). Along with Leonard Nimoy, David McCallum (who appeared with her in The Forms of Things Unknown (1964)), Cliff Robertson and Peter Breck, she is one of only five actors and the only woman to appear in both The Outer Limits (1963) and The Outer Limits (1995).