Short Stories vs. Novels
The skills involved are different. I know quite a few good novelists who are simply incapable of being succinct and focused enough to write a good short story. Likewise, I've met plenty of great short-story writers who couldn't finish a novel to save their careers. There is a widely held belief out there that you must start out writing short stories in order to pay your dues, or something like that. It's simply not true.
True, many of the now-great names started out writing short stories for the pulps, but that is mostly because that's where the market was in those days. Even The War of the Worlds was first published as a magazine serial. The paperback market didn't come into existence until the 1940s; the paperback originals market until the 1950s; and it didn't become possible for a significant number of writers to make an adequate living doing nothing but original novels until the 1960s.
In a way, aspiring SF writers in particular have been deluded by the popularity of books like The Martian Chronicles and I, Robot. A novel is not simply a bunch of short stories stapled together, even if they do share common characters and a common background. Speaking in sweeping generalizations now, a short story focuses on a single event that either changes or provides some insight into a single person, a small group of people, a situation, or an institution. A novel reveals a vastly wider and deeper story. (There are exceptions, of course. There are plenty of short stories that cover spans of thousands of years and casts of millions of people, but again, the emphasis is on one truth or insight. Likewise, there are novels — say, One Day in the Life of Ivan Denisovich — that focus with claustrophobic intensity on one character in one very brief or confined circumstance. But those are exceptional cases.)
The place where most novelists attempting short stories fall down is in keeping it brief and tightly focused. When writing a short story, the primary rule is economy. Every word, sentence, and paragraph must contribute to carrying the reader forward to the conclusion. When you're writing a 3,000-word short story, you simply cannot afford to spend 500 words describing the unusual batik pattern on a minor character's skirt, unless the whole story turns on that pattern and that skirt.
The place where most short story writers attempting novels fall down is in relaxing control enough to disregard a niggling attention to economy. In a novel, you can afford to let your narrative meander off into little baroque curlicues, provided the details revealed, while irrelevant, are interesting.
The most common sin of short story writers attempting novels is to approach each chapter as a separate short story. This leads to a jarring, jumpy narrative that reads like a ride down a very badly potholed road: say, I-94 between Madison and Milwaukee. The worst sin is to write a really great short story first, and then think, "Oh, if this is successful, I'll just keep spinning it out and make a novel." A well-written story has a strong ending, and a good ending by definition makes it difficult to pick up the thread later and resume the story. This is probably the reason why so many old SF novels start with a really great beginning —
And then, somewhere between chapters 1 and 3, there's a neck-snapping transition into an entirely new story. E.g., "Chapter 2: Forty years later, the descendants of the survivors gathered on the ridge overlooking the crater where their grandparents starship had crashed..."
As for the canard about paying your dues doing short stories: definitely not true. All writing is good exercise, of course, but being successful in the short story market no longer has much connection to selling an original novel. In fact, most experienced editors I've known have at least one really awful horror story about a novel they bought from a successful short story writer, based on a really terrific partial and outline, only to discover that the writer was simply incapable of finishing the blasted thing.
Writing short stories is good exercise. Writing short stories is fun. Many of my all-time favorite works of fiction are short stories.
But as an unpublished writer, the only way to prove you can write a novel is by writing a novel.