Assessing spatiotemporal variation in abundance: A flexible framework accounting for sampling bias with an application to common pochard ( Aythya ferina )
Résumé
Assessing trends in the relative abundance of populations is a key yet complex issue
for management and conservation. This is a major aim of many large-scale censusing
schemes such as the International Waterbird Count (IWC). However, owing to the
lack of sampling strategy and standardization, such schemes likely suffer from biases
due to spatial heterogeneity in sampling effort. Despite huge improvements of the
statistical tools that allow tackling these statistical issues (e.g., GLMM, Bayesian in-
ference), many conservationists still prefer to rely on stand-alone turn-key statistical
tools, often violating the prerequisites put forward by the developers of these tools.
Here, we propose a straightforward and flexible approach to tackle the typical statis-
tical issues one can encounter when analyzing count data of monitoring schemes such
as the IWC. We rely on IWC counts of the declining common pochard populations of
the Northwest European flyway as a case study (period 2002–2012). To standardize
the size of sampling units and mitigate spatial autocorrelation, we grouped sampling
sites using a 75 × 75 km grid cells overlaid over the flyway of interest. Then, we used
a hierarchical modeling approach, assessing population trends with random effects at
two spatial scales (grid cells, and sites within grid cells) in order to derive spatialized
values and to compute the average population trend at the whole flyway scale. Our
approach allowed to tackle many statistical issues inherent to this type of analysis
but often neglected, including spatial autocorrelation. Concerning the case study, our
main findings are that: (1) the northwestern population of common pochards experi-
enced a steep decline (4.9% per year over the 2002–2012 period); (2) the decline was
more pronounced at high than low latitude (11.6% and 0.5% per year at 60° and 46°
of latitude, respectively); and, (3) the decline was independent of the initial number of
individuals in a given site (random across sites). Beyond the case study of the common
pochard, our study provides a conceptual statistical framework for estimating and as-
sessing potential drivers of population trends at various spatial scales.
Fichier principal
Ecology and Evolution - 2022 - Folliot - Assessing spatiotemporal variation in abundance A flexible framework accounting.pdf (1.17 Mo)
Télécharger le fichier
Origine | Publication financée par une institution |
---|