Bees: Bees play a critical role in pollinating crops. Without pollination plants don't produce. Bees are easy forget about because they have always worked for free.
Colony collapse disorder reared its head in 2006 in North America as entire bee colonies disappeared without a trace. The possibility of losing the bees reminded people how important the bees are to modern food production. Food would become both scarce and expensive if it needed to be pollinated by hand. Fortunately the bees still do it for free.
Water: "We never know the worth of water till the well is dry." —Thomas Fuller. Although he was writing in the 1600s, his words are still relevant. Canadians have such a luxurious water system that we wouldn't know how to live without copious amounts of fresh potable water at our fingertips. As you read this chances are that you're within fifteen seconds of a tap that would, at a turn, dispense an endless supply of water. Given the distances that some people have to haul water, our instant access to virtually unlimited drinking water is quite the luxury.
Lots of the water we use is pumped up from underground aquifers. Some aquifers recharge slowly, others (fossil aquifers) don't recharge at all. Groundwater is a limited resource. The more people we have, the faster we will use the water, and the sooner we'll be forced to treat this scarce resource with the respect it deserves.
Time: Perhaps the scarcest resource of all. Time is simple until you try to define it, even though everybody knows what it is.
Unlike money you can't earn more of it. You spend each second as it comes, and when those seconds have passed, they are gone. Live the moment you are currently in as best as you can. These seconds can seem all too plentiful especially when you're bored or waiting for something, but they're ultimately limited. Nobody lasts forever.
What are you going to do with the rest of your seconds? What can you do in the next 60 seconds to make them really count?