You’re right that there has been significant growth in number of publications about ecosystem services, and that is a noteworthy trend. Although it does not directly map onto the assessment you did, Elizabeth Corley and I recently conducted a study of recent trends in ecology based upon an analysis of ecology publications,
Neff, M. W., & Corley, E. (2009). 35 years and 160,000 articles: A bibliometric exploration of the evolution of ecology. Scientometrics, 80(3), 657-682. (DOI: 10.1007/s11192-008-2099-3)
so I felt compelled to offer my insights. The way you and I did our searches differed, but perhaps you’ll be interested in our findings.
The field of ecology (as defined by the ISI ‘ecology’ journal classification, which includes your top five ‘ecosystem service journals with the exception of PNAS) has grown significantly over the past couple of decades, from 914 articles in 1970 to 10,488 in 2005. Assuming you searched for those terms in the ‘Topic’ field of the ISI WOS database, the results identify all articles with those terms in the title, abstract, author keyword, and indexer assigned keywords. You would have to normalize by the total number of words in all of those things in indexed publications to identify an increase relative to the number and length of indexed publications generally (and the number of journals, publications per journal, and number of words in titles and abstracts are all increasing).
Just to give you an idea, the total number of words in titles of articles in ecology journals – which takes into account the increased number of articles and increased title length – grew over 300% between 1990 and 2005. Also, what ISI indexes (keywords, abstracts, etc) has changed over time and is not consistent across journals. All of these things really complicate attempts to see trends in ecology over time.
The most reliable way I found to analyze trends in the discipline using the publication record is to limit your search to article titles because ISI has been consistent in the way it indexes them (of course, this introduces a suite of problems itself). Then, you have to normalize by the total number of words in titles to get an idea of the relative growth in that area compared to the rest of ecology.
If you search only in titles and normalize for the total number of title words each year, the graph of trends for ‘ecosystem’ and ‘services is unremarkable compared to others. Most notable is the increase in molecular genetic terms and topics like climate change, tropical forestry, and biodiversity. I’ve included one graph comparing the normalized trends in ‘ecosystem’ and ‘services’ to molecular genetic terms show you how the growth in that topic compares.
Our 2009 paper contains more graphs of recent trends in ecology.
Perhaps the biggest trend is the sheer growth in the field, but I have no idea how that compares to the growth of the scientific enterprise writ large.