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Fifteen species richness estimators (three asymptotic based on species accumulation
curves, 11 nonparametric, and one based in the species–area relationship) were compared
by examining their performance in estimating the total species richness of epigean
arthropods in the Azorean Laurisilva forests. Data obtained with standardized
sampling of 78 transects in natural forest remnants of five islands were aggregated
in seven different grains (i.e. ways of defining a single sample): islands, natural areas,
transects, pairs of traps, traps, database records and individuals to assess the effect of
using different sampling units on species richness estimations.
2.
Estimated species richness scores depended both on the estimator considered and on
the grain size used to aggregate data. However, several estimators (ACE, Chao1, Jackknife1
and 2 and Bootstrap) were precise in spite of grain variations. Weibull and several recent
estimators [proposed by Rosenzweig
et al
. (
Conservation Biology
, 2003,
17
, 864–874),
and Ugland
et al
. (
Journal of Animal Ecology
, 2003,
72
, 888–897)] performed poorly.
3.
Estimations developed using the smaller grain sizes (pair of traps, traps, records and
individuals) presented similar scores in a number of estimators (the above-mentioned
plus ICE, Chao2, Michaelis–Menten, Negative Exponential and Clench). The estimations
from those four sample sizes were also highly correlated.
4.
Contrary to other studies, we conclude that most species richness estimators may be
useful in biodiversity studies. Owing to their inherent formulas, several nonparametric
and asymptotic estimators present insensitivity to differences in the way the samples are
aggregated. Thus, they could be used to compare species richness scores obtained from
different sampling strategies. Our results also point out that species richness estimations
coming from small grain sizes can be directly compared and other estimators could give
more precise results in those cases. We propose a decision framework based on our
results and on the literature to assess which estimator should be used to compare species
richness scores of different sites, depending on the grain size of the original data, and of
the kind of data available (species occurrence or abundance data). | |
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