Résumé
Quantifying metapopulation dynamics is a challenging task. Difficulties
particularly arise in species that possess unobservable resistance forms
that bias the estimation of colonization and persistence rates. Here, we
develop a general multistate occupancy model that allows estimation of
species persistence for both normal and resistant forms, even when the
latter are not detectable. We apply this model to an 11-year data set on
the tropical freshwater snail Drepanotrema depressissimum in a network of
229 ponds. These ponds frequently dry out, and the snails can persist by
aestivating in the ground, where they are not detected. Our model
adequately captures this feature because it infers a high persistence rate
for the resistant form, while models ignoring aestivation tend to
overestimate extinction and colonization. In addition, we find that,
surprisingly, colonization and persistence are even higher in sites prone
to desiccation and during years with low rainfall than in more humid sites
and years, suggesting that D. depressissimum favors unstable sites where
competitors are rare and where it can rely on aestivation to persist. Our
model has the potential to provide valuable insights into the
metapopulation dynamics of many species that otherwise could hardly be
studied because of the existence of undetectable life forms.