How does interest in particular ecosystem services change over time?

Although many ecosystem services benefit people whether or not they are recognized as such (e.g. oxygen is essential for life but was only discovered in the 18th century), peoples' awareness of and interest in ecosystem services can tell us a lot about the relative value placed on particular services at specific points in time. As an example, the plot below tracks the frequency with which several terms related to ecosystem services appear in English-language books from the years 1800-2000. Two terms relate to provisioning services, "timber" and "farming"; one term relates to a regulating service, "flooding"; and two terms relate broadly to cultural services, "hiking" and "wildlife". As you can see, the terms associated with provisioning services were more frequently referenced throughout the entire time series. Starting in around the 1940s, however, these terms began declining in importance as the terms "wildlife" and "hiking" began appearing more frequently (both were almost unused prior to 1920). This is possibly a result of the post-war boom in urbanization and industrialization in much of the English-speaking world. As people became more disconnected from the rural economy and had greater opportunities for education and leisure, interest shifted from ecosystems as sources of material goods to ecosystems as sources of cultural services - despite the obvious fact that people still required both food and wood products. The use of the term "flooding" also increased in importance after WWII, suggesting that society may have also begun placing a greater emphasis on the risks associated with reduced ecological function during this same time period. The increase is less dramatic, however, and the term had been of greater importance than "hiking" and "wildlife" in the pre-war years (although still substantially of lesser importance than "farming" or "timber"). This suggests that benefits associated with regulating services (or damages associated with their loss) were seen as being of greater importance in the public dialogue than outdoor recreation, wildlife viewing, and other cultural activities that require more leisure time. Data were derived from the digitized archive at Google Books, using Google's Ngram Viewer.