Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect companies involved in natural resources to thrive over time as our recovering and growing global economy demands more energy and commodities, the iShares S&P North American Natural Resources (NYSE: IGE) 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 a lot of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The iShares ETF's expense ratio -- its annual fee -- is a relatively low 0.48%.

This ETF has performed rather well, beating the S&P 500 over the past five and 10 years. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

With a low turnover rate of 11%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?
A few natural resources companies have performed well over the past year, despite the fact that the global economic recovery is not yet in full swing. Natural gas specialist Williams (NYSE: WMB) gained 30%. It may not have all the markings of a perfect stock, but it has more than doubled its dividend over the past two years, which signifies a lot of confidence in its earnings stream.

A lot of companies didn't do as well last year, but could see their fortunes change in the coming years. Oil-field services giant Halliburton (NYSE: HAL) shed 33%, but its recent earnings report was strong, with revenue up 30% and earnings up 23%. It's benefiting from a shift in many companies' focus from low-priced natural gas to more lucrative oil. My colleague Jason Moser sees Halliburton as "a steal," citing its expertise in fracking, deepwater drilling, and more.

Freeport-McMoRan Copper & Gold (NYSE: FCX) shrank by 29%, but its bulls see its labor issues being resolved, demand from the auto industry and elsewhere driving copper prices up, a valuable leadership position in molybdenum, and strong management running the company.

Suncor (NYSE: SU), down 27%, looks promising, with its oil sands reserves in Canada and rising production. Bears worry about rising costs and controversy over oil sands. The company is experiencing a CEO turnover, too, which can sometimes be less than smooth. The new CEO comes from within, though, having served as COO.

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.

Halliburton isn't the only company profiting from oil. Check out our special free report, "3 Stocks for $100 Oil," and meet some compelling contenders for your portfolio.