I think the personal nature of the main characters is an important theme in "Force of "Nature." For example, it is Alice's nature to be smart, aggressive, mean, and bossy. The third sentence in the book says, "Alice had a mean streak so sharp it could cut you," and Alice's actions during the hike are often mean and bossy, especially to Beth and often to Lauren.
On the other hand, Lauren's nature is to lack confidence in her abilities: at work, as a mother, and as a hiker/camper. Even when she thinks Alice is wrong, she is slow to challenge her.
Alice's meanness and Lauren's lack of confidence collide at a dramatic moment in the book, when Alice and Lauren confront each other from p. 294 on. Lauren says to Alice, "I have changed. You're the one who's still the same...You were a bitch at school, and you're even worse now."
Alice laughs and responds, "You are kidding yourself...You are who you are. It's just your nature...Your daughter is just like you."
At that, Lauren is goaded into a shoving match with Alice that ends in Alice's injury.
Lauren's lack of confidence and Alice's mean criticism of Lauren lead to a tragic result. The different natures of the two women is the crucial factor leading up to this confrontation.
To me, the title has this dual meaning: the force of the wild setting in shaping the events of the book, and the important differences in Alice's and Lauren's individual natures that lead to their disastrous fight and Alice's injury. In a way, the two meanings mirror each other (and, of course, mirrors are another theme). Overall, this book is full of rich meaning and rewards close reading.