Sequence Index
The new SABAP2 website (sabap2.adu.org.za) has introduced some new data formats
and data terms. One of these is the Sequence Index. The sequence index is the
average position that a species normally occurs on a list, and is expressed as
a percentage on the SABAP2 website. Unlike the traditionally used reporting
rate (how often a species occurs on a set of lists, also usually expressed as a
percentage) where a high score is indicative of high abundance (and/or
detection), a high sequence index indicates lower abundance (and/or detection)
i.e. it took a long time to detect a species, indicated by the fact that many
other species were reported before it. Simply put, a rare species should have a
high sequence index and low reporting rate, while common species should have
high reporting rate and low sequence index. This is probably easiest to see for
a migratory species, e.g. Barn Swallow, which is absent in winter. Here the
sequence index (yellow line) goes up over the winter period, while reporting
rate (blue line) goes down, and then up again over summer.
We thus say that the sequence index is negatively correlated
to reporting rate. This is more clearly seen if we use a set of species, for
instance 58 of South Africa’s endemic bird species. Calculating mean reporting
rate across a species range, we see that species with high reporting rate have
a low sequence index, while species with low reporting rate have a high
sequence index.
While both indices thus give indications of apparent
abundance (and/or detectability), there is an important distinction in how
these are calculated: reporting rate is calculated based on the number of lists
where a species was present or absent; while sequence index is calculated from
IF a species was detected, what position on the list did the species hold? This
apparently subtle distinction does have implications though: for instance, with
the BirdLasser revolution lists now have fewer species compared to the period
before, for reasons we won’t explore here. This means there is a small, but
significant, probability of NOT reporting a species. This means that reporting
rate can appear lower but in this situation this does not reflect a change in
abundance; it is a change in reporting probability. It appears that the sequence
index on the other hand is robust to this change, given it is calculated
independently of the number of cards submitted. The sequence index is thus an
additional useful tool in the box of information that SABAP2 can deliver.
