Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some global natural resources stocks to your portfolio, but don't have the time or expertise to handpick a few, the SPDR S&P Global Natural Resources ETF (GNR -0.36%) 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 SPDR ETF's expense ratio -- its annual fee -- is a relatively low 0.40%, and it recently yielded close to 2%.

This ETF is too young to have a sufficient track record to assess. (It has underperformed the world market over the past year.) 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.

Why global natural resources?
Our global economic slump won't last forever, and there are already signs of life here and there. Thus, companies specializing in global natural resources are poised to prosper as construction and infrastructure projects get under way, and manufacturing kicks into a higher gear. And food-related companies can rely on our planet's continually growing population.

More than a handful of global natural resources companies had strong performances over the past year. Archer Daniels Midland (ADM -0.10%) jumped 33%, with its last quarter featuring revenue slightly up, but earnings down, in part due to last year's droughts. The company remains a solid dividend payer, though, and is looking to expand in Asia via its purchase of GrainCorp, Australia's leading agribusiness. ADM is considering selling its cocoa business, amid falling cocoa prices and shrinking margins.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Brazil-based Vale (VALE 0.49%), the world's largest iron-ore concern, fell 22%. Its 5.6% dividend yield is appealing, as is its forward P/E near 7, about half of its five-year average of 14. The stock looks undervalued to many, though some are not convinced, sensing continued pressure on commodity prices. Vale has been tackling soft demand by cutting its costs, and its CEO is focused on disciplined capital spending. Protests in Brazil aren't helping matters, as some call for improved infrastructure and services instead of massive spending on upcoming Olympics and World Cup games.

Fertilizer giant PotashCorp (POT) dropped 15% and yields 3.5%. (Its dividend has been hiked 25% this year and some 700% over the past few years.) With its current and forward P/E ratios well below its five-year average, the stock looks interesting. Bulls like its low-cost structure and solid profit margins. Some of its fate is tied to massive developing economies, such as China, where growth has slowed, and India, where there is reportedly a potash oversupply.

Freeport McMoRan Copper & Gold (FCX -0.75%) shed 8% and yields about 4.5%. The world's largest publicly traded copper producer has been hurt by falling copper and gold prices, and a deadly mine disaster in Indonesia led to the closing of one of its mines (also one of the world's biggest mines). That mine has resumed production, though not yet at full capacity. Meanwhile, slowing growth in China and labor strikes haven't helped. The company diversified its operations considerably when it bought a pair of oil and gas producers. The stock seems rather undervalued.

The big picture
Demand for global 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.