My authorial "thing" is to gaze at the underbelly of an alien character's lovelife and poke fun at it. And, you might not have guessed it, but of all the sciences in science fiction, Biology is my favorite.
OK. "Survival-of-the-species romance" isn't the same thing as "Survival Romance". If it were, most alien romance authors could be said to write survival romances. It seems to be a plot staple of alien abduction romances that the hero --for whatever reason-- has no choice but to abduct healthy human females of reproductive age in order to save his world/civilization/species.
The downside is scientific credibility. Unless The Great Originator could do better than us, and did (but with the same basic ingredients), extra-terrestrials ought not to be able to breed productively with us.
There are plausible ways around the problem.
Survival of the species is interesting, on Earth, in the wild, too. (I seem to be unfashionable in that I like to capitalize the proper name by which we call our planet, to differentiate it from the soil beneath our feet.)
Do you know the ins and outs of a crab's sex life? I do. For most of the year, a female crab's body armor gives new meaning to the word "impregnable". But there is the moult. A mature male crab who is eager to mate, has to wait. And wait he does, very patiently, very protectively, close by the female as she enters her vulnerable time when her outgrown shell comes off.
He crouches over her, and protects her from natural predators who would like to eat a soft shelled crab -- I don't know much about the delicacy called soft shell crabs. Are we eating pregnant females?-- and his reward is that he is in a position to father her offspring.
She pumps water into the space between her body and what will be her new shell, so that her shell hardens a little on the large side, giving her room to grow.
These musings are a bit off topic, and not much to do with how one's novel's hero and heroine survive if they are stranded on a deserted island, set adrift in a small boat, shot down onto an alien planet, left for dead in an icy/arid/forested wilderness.
However, I am looking forward to my imminent summer holiday on an island overseas. While not harming the wildlife I observe, I do enjoy studying tidepools or shorepools, and considering the ecosystems in them as a microcosm of possible alien life.
Have a wonderful summer.