DOI: 10.5061/dryad.kkwh70s5f
title: Capturing the dynamics of small populations: A retrospective assessment using long-term data for an island reintroduction

description: 1. The art of population modelling is to incorporate factors essential for
 capturing a population’s dynamics while otherwise keeping the model as
 simple as possible. However, it is unclear how optimal model complexity
 should be assessed, and whether this optimal complexity has been affected
 by recent advances in modelling methodology. This issue is particularly
 relevant to small populations because they are subject to complex dynamics
 but inferences about those dynamics are often constrained by small sample
 sizes. 2. We fitted Bayesian hierarchical models to long-term data on
 vital rates (survival and reproduction) for the toutouwai (Petroica
 longipes) population reintroduced to Tiritiri Matangi, a 220-ha New
 Zealand island, and quantified the performance of those models in terms of
 their likelihood of replicating the observed population dynamics. These
 dynamics consisted of overall growth from 33 (± 0.3) to 160 (± 6) birds
 from 1992–2018, including recoveries following five harvest events for
 further reintroductions to other sites. 3. We initially included all
 factors found to affect vital rates, which included inbreeding,
 post-release effects, density-dependence, sex, age and random annual
 variation, then progressively removed these factors. We also compared
 performance of models where data analysis and simulations were done
 simultaneously to those produced with the traditional two-step approach,
 where vital rates are estimated first then fed into a separate simulation
 model. Parametric uncertainty and demographic stochasticity were
 incorporated in all projections. 4. The essential factors for replicating
 the population’s dynamics were density-dependence in juvenile survival and
 post-release effects, i.e. initial depression of survival and reproduction
 in translocated birds. Inclusion of other factors reduced the precision of
 projections, and therefore the likelihood of matching observed dynamics.
 However, this reduction was modest when the modelling was done in an
 integrated framework. In contrast, projections were much less precise when
 done with a two-step modelling approach, and the cost of additional
 parameters was much higher under the two-step approach. 5. These results
 suggest that minimization of complexity may be less important than
 accounting for covariances in parameter estimates, which is facilitated by
 integrating data analysis and population projections using Bayesian
 methods. 13-Aug-2021 --
description: We surveyed the island 121 times over 26 years (April 1992 to
 January 2018) to obtain data on survival, abundance and habitat
 availability, and monitored most breeding pairs over the first 20 years to
 obtain data on reproductive success. We always
 conducted surveys in September (start of breeding season) and January
 (toward end of breeding season), and conducted additional surveys when
 needed to quantify factors likely to affect survival. The analysis
 presented here uses 76 surveys, including surveys conducted to quantify
 post-release effects, bykill caused by the poison drop, removal of birds
 during harvests, and changes in juvenile survival with age. Surveying
 involved walking through all areas of closed-canopy forest playing
 territorial calls to attract toutouwai and recording their colour
 combinations. We also counted any unbanded toutouwai detected in September
 surveys so that these individuals were included in the abundance estimates
 made at the start of each breeding season (unbanded individuals were
 easily distinguished because the birds are rigidly territorial at that
 time and most of the population was always colour banded). We mapped any
 extensions to the forest cover each September to measure changes in
 habitat area. Breeding monitoring involved checking all
 breeding pairs throughout the breeding season to determine numbers of
 offspring fledged (see Armstrong & Ewen 2002 for details). We
 colour-banded most of these offspring in the nest 9–14 days after
 hatching, but otherwise caught them ca. 4 weeks post-fledging when they
 could be lured under hand nets to eat mealworms. These birds were
 considered to enter the population as juveniles at the January survey and
 transition to adults at the September survey (Fig. 2). Offspring from
 unmonitored pairs were captured as independent juveniles or adults using a
 mealworm-baited clap trap, and entered the population as adults at the
 September survey if detected. We discontinued this intensive breeding
 monitoring in 2013 but continued to band a sample of nestlings and
 independent birds found. This monitoring resulted in
 559 observations of reproductive success (number of offspring fledged by a
 female over a breeding season) and encounter histories for 1458
 colour-banded birds, including the 58 birds released, 1234 birds banded as
 nestlings or fledglings, and 166 birds banded as independents.