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. The v21 submission carried
arithmetic figures and every one of them is superseded.
3Uncertainty by simulation
Where the paper reports confidence intervals they are carried here unchanged. Nothing on this
page 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 states its own method and its own caveat, in the paper's words.
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
Every drawn boundary is a specific OpenStreetMap object, fetched by id and listed on each
landscape's record. Nothing is hand-drawn. Where a boundary could not be sourced the landscape
is marked by position only and the page says so — Greater Yellowstone and Germany are both in
that state, for reasons given on their records.
What is withheld, and why
Provenance
Priority Species Index