Thanks for your comments. My statement was too simplified and I should have been more careful, so thank you for pointing that out. You are correct that mercury can be emitted in volcanic gases. However, human sources of mercury emissions, including coal combustion, which is one of the largest sources, are many times higher than volcanic sources. Importantly, the study from Utah State University, referred to in my article, found that there were significantly elevated concentrations of mercury but this could not be attributed to a volcanic ash source because if the source was volcanic, other metals would also be expected to be at elevated levels, and they were not. The combination of metals and their concentrations were, however, consistent with a coal source.
Regarding your questions about the Living Planet report, the report is based on the Living Planet Index, which is a dataset based on scientific studies of 21,256 populations from 4,270 species from around the world. The data is gathered from scientific journals, online database, and government reports. You can access the data they use, references to the published studies, and information on how the index is calculated here: http://www.livingplanetindex.org/data_portal
They do not include humans in the Living Planet Index. And humans are one vertebrate species (not the only one) that has increased its population, with current models predicting a peak and then a declining population from sometime around 2050. However, this does not alter the overall picture of vertebrate population decline.