In most natural populations, exhaustive counts are not possible and
estimates need to be derived from partial sampling by using analytical
methods that account for biological processes, sampling errors and
detection probability. The methods available have contrasting pitfalls
and payoffs in relation to the assumptions made but are seldom
contrasted on the same population.
We compared density estimates derived by different sampling methods.
Despite the real density being unknown, the ‘soft’ validation of density
estimates might help to better understand the possible pitfalls and
payoffs of each method. This was done in three closed populations and
with three different habitat typologies to disentangle the effects of
different capture-detection processes to those introduced by the method
We considered the problem of estimating population density of the endemic Balearic lizard, Podarcis lilfordi,
in three island populations. We compared estimates derived by distance
sampling (LT) in three types of habitat with those calculated from a
simultaneous 3-day capture–mark–recapture study. Capture histories of
marked individuals were used to estimate density using spatially
explicit capture–recapture models (SECR) and a capture–mark–recapture
model without spatial data (CMR). Moreover, we empirically assessed the
influence of survey duration by extending the survey in the largest
island to five occasions. The real population density was unknown and
absolute accuracy of each method cannot be assessed; nevertheless,
relative estimates might be informative.
LT estimates had the greatest coefficient of variation in vegetated
habitats, corresponding to possible departures from model assumptions.
SECR estimates differed among islands and were from 12% to 37% lower
than those derived by LT but only in the largest islands with high and
dense vegetation. CMR estimates depended on the number of occasions
whereas SECR did not and showed lower variance. LT and SECR estimates
showed differences across islets.
Line-transect and capture–recapture methods gave comparable results but
the interaction between recapture processes and habitat types should be
considered when inferring density to the whole area. We found density
estimates between 1500 and 2500 individuals ha–1, being a higher value than those found for lizards in continental regions.
Pitfalls and payoffs of each method are discussed to optimise experimental design in estimating population density.