How does an ecosystem(生态系统) work? What makes the populations of different species the way they are? Why are there so many flies and so few wolves? To find an answer, scientists have built mathematical models of food webs, noting who eats whom and how much each one eats.

With such models, scientists have found out some key principles operating in food webs. Most food webs, for instance, consist of many weak links rather than a few strong ones. When a predator(掠食动物) always eats huge numbers of a single prey(猎物), the two species are strongly linked; when a predator lives on various species, they are weakly linked. Food webs may be dominated by many weak links because that arrangement is more stable over the long term. If a predator can eat several species, it can survive the extinction(灭绝) of one of them. And if a predator can move on to another species that is easier to find when a prey species becomes rare, the switch allows the original prey to recover. The weak links may thus keep species from driving one another to extinction.

Mathematical models have also revealed that food webs may be unstable, where small changes of top predators can lead to big effects throughout entire ecosystems. In the 1960s, scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species—including species they did not directly attack.

And unplanned human activities have proved the idea of top-down control by top predators to be true. In the ocean, we fished for top predators such as cod on an industrial scale, while on land, we killed off large predators such as wolves. These actions have greatly affected the ecological balance.

Scientists have built an early-warning system based on mathematical models. Ideally, the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline. Prevention is key, scientists say, because once ecosystems pass their tipping point(临界点), it is remarkably difficult for them to return.

Morocco has a lot of sun, a lot of deserts and a high demand for imported fuel. And that's why the Moroccan government is moving ahead with a huge concentrated solar power plant near the desert city of Ouarzazate — one of four plants in total which, when completed, will have generating capacity(发电量) of 580MW. Combined with the country’s wind and hydro-power (水力发电) efforts, Morocco’s ambitious solar push will mean the country will get close to 50% of its electricity from renewable generation as early as 2020.

Phase I of the project, the 160MW Noor l, is going to start generating electricity next month and includes a capacity for molten sand energy storage which will allow up to 3 hours of electricity generation after the sun goes down. Phases 2 and 3 will have the capacity for up to eight hours of storage, meaning that solar energy really could be used to power homes around the clock.

Besides the scale of such plans, what’s impressive to me is how quickly the renewable energy can be developed. What other forms of energy could so greatly change a country’s energy composition in just a few short years? Not only should this be an encouraging sign for those of us who favor renewable energy, but it also should give reason for would-be investors in traditional fossil fuel generating industry.

If renewable energy costs continue to fall, even existing(现有的) coal and gas plants will find themselves limited by newer, cleaner forms of energy that can be increased rapidly.

Interestingly, some analysts suggest this is happening already. In the US, for example, coal power plants are being retired earlier than expected due to a combination of increased competition from gas and renewable energy, as well as tighter regulations of emissions (排放). Also, increasing amounts of solar and wind cause gas and coal plants not to use, significantly reducing their profits and further giving an edge to the clean energy competition.

If this proves to be the case in North Africa too, Morocco's solar ambitions could help transform the energy system well beyond its own borders.

The San Diego County Water Authority has an unusual plan to use the city’s scenic San Vicente Reservoir (水库) to store solar power so it’s available after sunset. The project could help unlock America’s clean energy future.

Perhaps ten years from now, if all goes smoothly, large underground pipes will connect this lake to a new reservoir, a much smaller one, built in a nearby valley about 1100 feet higher. When the sun is high in the sky, California’s abundant solar power will pump water into that upper reservoir. It’s a way to store the electricity. When the sun goes down and solar power disappears, operators would open a valve (阀门) and the force of 8 million tons of water, falling back downhill through those same pipes, would drive machines capable of producing 500 megawatts of electricity for up to eight hours. That’s enough to power 130, 000 typical homes.

“It’s a water battery!” says Neena Kuzmich, Deputy Director of Engineering for the water authority. She says energy storage facilities like these will be increasingly important as California starts to rely more on energy from wind and solar, which produce electricity on their own schedules, without considering the demands of consumers.

Californians learned this during a heat wave this past summer. “Everybody in the state of California got a text message at 5:30 in the evening to turn off their appliances,” Kuzmich says. The sun was going down, solar generation was disappearing, and the remaining power plants, many of them burning gas, couldn’t keep up with demand. The reminder worked:People stopped using so much power, and the grid (电网) survived.

Yet earlier on that same day, there was so much solar power available that the grid couldn’t take it all. Grid operators turned away more than 2000 megawatt hours of electricity that solar generators could have delivered, enough to power a small city. That electricity was wasted. There was no way to store it for later, when operators desperately needed it.