What the index asks
The Priority Species Index asks one question of a whole landscape: are the species that need
the most room doing better or worse than when counting began? It is built for species that are
rare, threatened and area-sensitive — the ones a landscape either keeps whole or loses.
How it is computed
1Each species against its own baseline
A species' abundance in a year is divided by its abundance in the landscape's first survey
year, times one hundred. Holding steady scores 100. Doubling scores 200.
2The geometric mean, never the arithmetic
An arithmetic mean lets one recovering species drown out four declining ones: a rise from 100
to 500 outweighs four falls from 100 to 50. The geometric mean treats a halving and a doubling
as equal and opposite, which is what a reader assumes an index does. Every figure here is a
geometric mean, as the current Supplementary Material defines the index.
3Uncertainty by simulation
The workbook states no confidence interval on any landscape’s index series, so none is drawn
on an index line anywhere. Where it does state intervals — the inside- and outside-protection
series for Ndoki-Likouala (eight points) and Rio Juruá (four) — the landscape records carry
them unchanged. The interval the paper reports on Madre de Dios’s index itself (its Figure 2e)
is not on any page: the workbook carries bounds per species-year only. Nothing anywhere is a
recomputed interval.
PSIi = exp[ (1/Si) · Σ ln(PRs,i) ]
PRs,i = (Ns,i / Ns,1) × 100
Three things the index is not
Not a population count. It is a ratio to a baseline, so a landscape that
began badly and improved can outscore one that began well and held.
Not one method. Across these seven landscapes the underlying numbers are
population estimates, camera-trap detection rates, line-transect densities, occupancy values,
and in one case what experienced local fishers and hunters report having seen over twenty years.
Each landscape's record states its own method and a caveat on reading it. That wording is this
page's, drawn from the paper's figure captions and Appendix A; where the paper states a limit
itself, the record quotes it and says so.
Not comparable between landscapes. Two indices sit on the same axis here
because they are the same kind of quantity, not because 140 in one place means what 140 means in
another.
Where the boundaries come from
The frame the paper screens from
The formula, and what is drawn around the seven
What is withheld, and why
Provenance
Priority Species Index