World Ecology is an attempt to look at Earth as a single ecosystem in which everything interacts to some extent with everything else. World Ecology is important in International Affairs as it provides a reason for different nation-states to have to deal with each other.
Evidence of the relevance of World Ecology to international public policy comes up for example in the Law of the Sea Treaty which addresses, among other things, declining world ocean fisheries, efforts to address Global Warming, for instance in the Kyoto Treaty to which the U.S. is not a party, the inceasing direct scientific evidence that pollution from China impacts air quality in the United States, and in efforts to address international microbiological issues such as AIDS and SARS.
In order to be high functioning in the domains of world and national policy, leaders need to factor in the importance of factors that can motivate high impact changes in the environment. Deforestation in a distant country may threaten far more than the market value of U.S. timber, so neglect of such issues may have high costs at home, e.g., in higher rates of devastating storms, increases in summertime temperatures leading to large numbers of casualties among the unprotected (as happened a few years ago in Chicago). So far, among our political leaders only Al Gore has shown the capacity to think clearly beyond our national borders.
One of the more controversial theories in the field of World Ecology is the Gaia Hypothesis which argues that the world ecology is not merely a passive recipient of human ecological impacts, but instead actly works to preserve itself in an approximation of consciousness -- a refinement of some of the early ideas of Malthus an early modern population scientist and economist who noticed the linkage between overpopulation and phenomena such as War, disease and starvation. These ideas have relevance today as seemingly separate conditions in some of the most troubled places in the world and show strong correlations. For instance, in Rwanda repeated waves of genocidal massacres are strongly linked to fertility rates (children per woman in a lifetime). Such ideas of quasi-conscious reaction are explored, for example, in the science fiction writings of Greg Bear whose award winning series beginning with Darwin's Radio posits the existence of a species level process that trigger genetic change towards species wide mutation and possible harm to the existing species in times of high population stress.