What the ecological perspective on the human habitation of the planet has to offer people is a box outside of which they cannot think. Nature as box may seem like an unnecessarily rigid confinement of it's apparent boundlessness and the adaptibility of it's creatures, but whenever that question of it's limits is broached, it is with the place of human life as a part of nature which we are truly concerned. Nature will continue with or without us, to be sure. The question of nature's boundlessness, however true, does not mean it is always useful to think about it in that way. There are completely natural habitats humans could never hope to survive in, let alone flourish, so the question of infinite human adaptibility has to be put to rest.
The human struggle to overcome the crueler aspects of nature and the large measure of success achieved lends a lot of historical credibility to the view that humans have been liberated from the animalistic constraints imposed by ecology. But what may be true in a relative sense is not necessarily true in an absolute sense. The hard part is knowing how to measure the limits of our achievements and to determine what is possible and what is impossible. This is where ecological concepts, principles, and laws are very useful to judge the human place in the natural world.
One broad and useful way to measure human's ultimate limit is the ecological footprint, an idea which sprang from the concept of carrying capacity used in ecology to understand an ecosystem's population limit. The ecological footprint puts the weight of human needs against the regenerative capacity of nature. It is specific to humans and is used to determine the sustainibility of a given level of economic activity. As a way to illustrate in a hopefully not simplistic way, if you use X number of trees to build a house and it takes Y number of years to regrow the trees, then Z number of houses can be built in some number of years sustainably. This can be applied to aquaculture, agriculture, energy use, and anything else that cycles through the human economy. All of this can be scaled up to the planetary level to give a grand total of how many humans can live at what economic level indefinitely within some margin of error.
Barring problems in methodology, it is a work in progress after all, the notion that it is possible to measure the impact of human civilization on the natural world, and to measure both the utmost human habitation and a desirable human habitation on the entire planet is entirely logical. There is not much that is mysterious about how people live or what people use to live and where they get it from. It really comes down to getting good data, which isn't always easy. The good news is the ecological footprint, like it's offspring the carbon footprint, can be used to guide public policy and personal behavior. The bad news is what it tells us about the human situation.
According to at least one version of the footprint (the main one), humanity surpassed the carrying capacity of the Earth sometime around 1980. As of 2006, as it says in the wikipedia article, humanity was living at 20% beyond carrying capacity. This means that humans are in overshoot of their global ecological support system. Currently human civilization needs 1.4 Earths to sustain it's current way of life. Barring an intragalactic raw materials trade opening up soon, the amount of basic natural stuff for human use will soon become scarce. And if there is any doubt about the legitimacy of the ecological footprint concept, I would point to two clear examples of resource overuse. Soil erosion is happening nearly everywhere there is agriculture. The recent flooding in Iowa showed that millions of tons of topsoil can be lost in a single thunderstorm. But soil erosion is happening at slower but still relentless rates all the time. The midwestern U.S. farm belt has lost half of it's natural topsoil since European settelment. Topsoil is still eroding faster than it is replenished in most places. Obviously, this state of affairs cannot continue for much longer. The other easily researched example of ecological overshoot is the overfishing of oceans. Several areas of the world's oceans have been overfished so much that the fish populations cannot recover by themselves. North Sea and Newfoundland fishing has seen the disappearance of makeral and Atlantic cod for commercial use and may be gone for good. Many other species are threatened with the same fate.
There are countless other indicators of human overshoot that are clear to anyone who studies the problem. Behind these examples lies a very potent and apparent fact: Humanity cannot sustain itself as it currently inhabits the Earth. The question is how far can we continue and what it means for us as we surpass the limits of natural bounty. It is possible for populations to continue to grow beyond the carrying capacity of an ecosystem for a while. What happens after that ecosystem is effectively tapped out is what should worry everyone.
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