Forty people gathered in Porto Alegre, Brazil, during March 24-28, to study the theoretical background and some applications of hierarchical models for ecological data. The meeting took the form of a workshop with lectures and computer exercises led by Drs. Marc Kery, from the Swiss Ornithological Institute and Gonçalo Ferraz, from Brazil’s Universidade Federal do Rio Grande do Sul (UFRGS), with the support of four teaching assistants from three different institutions: Jérôme Guelát (Swiss Ornithological Institute), Heloíse Pavanato (Instituto Mamirauá, Brazil), Thiago Couto (Instituto Nacional de Pesquisas da Amazônia - INPA, Brazil) and Murilo Guimarães (also from INPA).
Because this was the first workshop of this kind in Porto Alegre, we started with a broad introduction to linear models, which emphasized how the various hierarchical models currently used for estimating biological population parameters are instances of generalized mixed linear models. Such models can be implemented in the maximum likelihood or the Bayesian statistical framework. We focused on the second framework, mostly for pragmatic reasons, highlighting the elegance and flexibility of the Bayesian approach for dealing with the complex reality of biological processes and imperfect data sets.
The ‘hierarchy’ of ‘hierarchical models’ refers to the formal consideration of the simultaneous contribution of biological and sampling processes for the generation of ecological data. In this contribution, the sampling process is conditioned on the biological process, and thus the later stands hierarchically above. Since the data is a result of both processes, we can only obtain a statistical assessment of biological quantities of interest if our models account for the implications of inevitable sampling faults. Towards the end of the workshop, we illustrated this accounting with examples of hierarchical models of detection/non-detection data and of capture-mark-recapture data.
Workshop attendants came from Argentina, Brazil, Chile, Colombia, and Switzerland. They had a wide variety of backgrounds and research interests spanning from fisheries management to plant community ecology. Being a first edition, this workshop was an organizational experiment. Everything turned out well thanks to the energy and dedication of Dr. Sandra Hartz, head of the UFRGS graduate program in Ecology and Silvana Barzotto, graduate program secretary. We are also thankful to Drs. Valério Pillar, Teresinha Guerra, and João Ito, all from UFRGS, who provided administrative support. Dr. Rosana Mazzoni and Eduardo Velez, from ABECO, provided precious funding management assistance. Institutional support for this event was provided by CAPES, CNPq, the Swiss Ornithological Institute, the UFRGS Instituto de Biociências, and the UFRGS Graduate Program in Ecology.