This article is part of our Rising Star Portfolio series.

America's long-standing love affair with the automobile may be fading into something closer to a marriage of convenience -- occasionally comfortable and necessary, but nothing close to daily romance.

The fact that the way we view transportation may be changing dramatically is just one of the reasons I'm purchasing shares of car-sharing company Zipcar (Nasdaq: ZIP) for my socially responsible Rising Star portfolio.

The business
Cambridge, Mass.-based Zipcar boasts "Wheels When You Need Them." Founded in 2000, Zipcar provides self-service vehicles to 650,000 members known as "Zipsters" in 15 major metropolitan areas and 250 college campuses in the U.S., Canada, and the U.K.

Zipcar started off providing its temporary car use service in major metro markets Boston, New York, and Washington, D.C. Since then, it's expanded to San Francisco, Chicago, Seattle, Philadelphia, and other major markets.

First-mover advantage gives Zipcar an edge over other car-sharing services. So far, it has more than 9,500 cars ready and waiting in its major metro markets, and in its IPO prospectus, it boasted that no other service provides the same size, diversity, and market penetration that its fleet represents.

A mission that emphasizes sustainability gives Zipcar a comfortable spot in this portfolio, too. Zipcar estimates that each Zipcar shared removes 15 personally owned vehicles from the roads. The company also says that with 10% of the population expected to ditch car owning and commit to car sharing, billions of gallons in gas and oil will be saved.

Why I'm buying
Zipcar's stock price has been halved since its April IPO. That reduces some of the risks inherent in its historic lack of profits and the fact that it's in the very early stages of its business. I felt wary of this IPO in April, but I'm much more comfortable at the current price.

Zipcar has consistently increased its revenue over the last several years. Meanwhile, in November it reported a surprise quarterly profit of $0.02 per share (although it cut revenue estimates for the full fiscal year).

Clearly car sharing, as opposed to car owning, is catching on with more people than conventional wisdom might deem possible. What's caused this gradual but real change of heart?

Congested urban areas make it difficult, and occasionally even annoying, to own a car. Automobiles' value depreciates the moment they're driven off dealerships' lots. Add in factors such as high gas prices, parking fees, personal property taxes, state inspections, and regular maintenance, and cars could be described as a money pit.

Zipcar estimates that its members save an average $500 per month compared to car owners. Regardless of the green factor, many urbanite consumers could catch on that this method of only accessing a car when it's truly necessary could save them some serious cash.

This upstart has ample room for growth. If its business accelerates according to plan, its growth could be absolutely stunning. Zipcar has identified more than 100 major markets across the globe as attractive. Zipcar is also tackling Europe aggressively, having recently named a new president of European operations.

And now, the risks
Zipcar could face some serious speed bumps, and investors shouldn't take the risky nature of this investment lightly.

It may be the first mover with a brand that's synonymous with car sharing, but it hasn't lacked hitchhikers latching onto its idea. Not only does Zipcar compete with other car-sharing networks and traditional car rental companies like Hertz (NYSE: HTZ) and Avis (NYSE: CAR), both of which have launched car-sharing products, but Zipcar also competes with grassroots, peer-to-peer car-sharing services, which are also gaining traction.

Furthermore, Zipcar is up against an engrained theme: Americans adore their automobiles. As far as sustainability goes, the major car companies are cranking out green automotive alternatives that certainly didn't begin and end with, say, Toyota's Prius. Tesla's (Nasdaq: TSLA) entire business revolves around sustainable electric vehicles, in fact. (Read why I didn't purchase Tesla for this portfolio here, although it was a contender at one point.)

When I first purchased shares of Solazyme (Nasdaq: SZYM) for this portfolio, I noted one safety measure: The biofuels company hadn't taken on a hair-raising amount of debt before it went public, a common issue with IPO companies and start-ups. Zipcar, on the other hand, had $95 million of debt outstanding as of December 2010; one of the reasons it went public was to reduce its indebtedness.

This leads us to the fact that Zipcar's business is very expensive, requiring heaps of capital. Part of Zipcar's advantage could be described as a double-edged sword, in fact. While many consumers are clearly realizing that if they ditch car ownership and utilize Zipcar's services, they're saving big money, that fact works against Zipcar's profitability as it builds out its fleet of automobiles and attempts to keep them well-maintained.

Foolish bottom line
I know other Fools like Zipcar, too; Foolish analyst David Meier bought shares for his Trends and Trades portfolio, and my colleague Annie Feldman recently pointed out its promising attributes for a greener portfolio. The Foolish community members who visit my discussion board have brought up Zipcar's merits several times, too.

Zipcar is positioned to capitalize from a major shift in American consumer spending attitudes and habits, as well as the growing sustainability trend. Its first-mover advantage gives even more fuel for its plans. Fools, start your engines.

Share your thoughts on Zipcar in the comments box below, or add these companies to your watchlist to track the trends: