Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect natural resources companies around the world to perform well as global economies recover, developing economies build out their infrastructure, and energy demands increase, the SPDR S&P Global Natural Resources ETF (NYSE: GNR) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The natural resources ETF's expense ratio -- its annual fee -- is a relatively low 0.40%. The fund is fairly small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

This ETF doesn't have much of a performance history to evaluate, as it's very young. Still, it's the future that matters most, so think about how bullish you are on the prospects for natural-resource companies. Meanwhile, remember that as with most investments, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
Several natural resources companies had strong performances over the past year. International Paper (NYSE: IP), for example, gained 20%. You might worry about its future, as our society cuts back on paper in many ways, thanks to electronic advancements. But IP is involved in packaging and recycling, too. And many items, such as disposable cups for coffees, have seen their volume grow over the past decade or two. The company is also operating in China and India and other developing economies, and cutting costs, as well.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Fertilizer giants PotashCorp (NYSE: POT) and Mosaic (NYSE: MOS), for example, shed 24% and 31% over the past year, respectively. But their managements seem comfortable enough with projected earnings that they've hiked dividend payouts by 100% and 150%, respectively. These companies and peers had cut back on production recently, due to flagging demand, but it looks like demand is heating up again.

Meanwhile, Freeport McMoRan Copper & Gold (NYSE: FCX) shrank by 29% as its major Grasberg facility in Indonesia suffered labor unrest. But that situation is stabilizing and global copper demand is expected to outstrip supply in the coming year, so the company seems poised to perform better soon.

The big picture
Demand for natural resources isn't going away anytime soon. A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

Learn about the 5 ETFs That Could Soar in 2012. And if you're looking for some great investments beyond ETFs, consider these 12 Dividend Stocks for 2012.