8Economic Dynamicsenvironmental economics already had a body of literature. What happened in the1970s and 1980s was that it became a recognised sub-discipline.Economists who had considered questions in the area had largely confinedthemselves to the static questions, most especially the questions of welfare andcost–benefit analysis. But environmental issues are about resources. Resourceshave a stock and there is a rate of depletion and replenishment. In other words,there is the inevitable stock-ﬂow dimension to the issue. Environmentalists havealways known this, but economists have only recently considered such issues.Why? Because the issues are dynamic. Biological species, such as fish, grow anddecline, and decline most especially when harvested by humans. Forests declineand take a long time to replace. Fossil fuels simply get used up. These aspectshave led to a number of dynamic models – some discrete and some continuous.
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