Elliot,

I'll address the criticisms in your first comment below.

"You need to actually have enough valid data, of the right types, not fudge your numbers to represent what you guess the data would say if you had it. Guessing is less accurate, and less scientific, than using real numbers."

They do use real numbers. They have a pretty sophisticated and in depth analysis. What they do:

1. Identify the resources that require land area to produce. For example, charcoal, wood for construction, paper, firewood, etc. all are resources the use of which can be quantified. So are agricultural products. For CO2, they calculate the amount of land that would be required to naturally absorb the emissions.

2. find out how much of those resources were consumed per country. Easy enough - look at various economic reports.

3. figure out how much land was required to produce the resources consumed.

Blammo! Instant ecological footprint. It's very straightforward actually.

Now for your next criticism:

"they said their data about biodiversity over-represented whatever species people liked to study. So they just counted those less. How much less? Well, something about dividing the world into regions which they assume to be equally important, and then assuming that the convenient already-collected data for each region really is representative."

Um, are they going to go into the field and collect data on every single species in each bioregion? Not with the funding they've got currently they won't. They use data that has already been collected and verified, and they track it over time. If they can they use multiple datasets. I've already pointed out that their conclusions jive with the accepted science.

"And people study vertebrates more than invertebrates. So how can they make conclusions about invertebrates, without nearly enough data? Easy. Just assume the trends for vertebrates apply."

Vertebrates have a much bigger impact on the environment than your typical invertebrate. They eat more, move faster, etc. If invertebrates are declining we have every reason to assume that biodiversity in general is in decline. The index uses data about nearly 1000 species (including invertebrates, by the way) and determined that their populations are in decline. That's bad news, even if, say, Jellyfish and mosquito populations are on the rise.

For a closer look at the methodology: A download can be found here about the methodology of the 2005 report. Another is here about the methodology used in the 2000 report.

More information about the methodology behind the living planet index can be foundhere, pdf.