A model for amphipod (Talitrus saltator) population dynamics P.M. Anasta´cio a, * , S.C. Gonc¸alves b , M.A. Pardal b , J.C. Marques b a Department of Ecology, IMAR—Institute of Marine Research, University of E ´ vora, Rua Roma ~ o Ramalho 59, 7000-671 E ´ vora, Portugal b Department of Zoology, IMAR—Institute of Marine Research, Faculty of Sciences and Technology, University of Coimbra, 3004-517 Coimbra, Portugal Received 23 October 2001; accepted 31 July 2002 Abstract This study provides a population dynamics model for one of the most common species (Talitrus saltator) in sandy beaches. Sensitivity analysis showed that in the model the abundance of T. saltator is deeply affected by variations in the minimum recruitment day length, the minimum temperature at recruitment, and the period between recruitments. The mortality rate, as expected, also had a profound effect on model performance. The abundance (IST, ind m ÿ1 ) of T. saltator was most sensitive to the initial number of individuals in age class 5 (ca. 4 months old) and least sensitive to the initial number in age class 9 (ca. 8 months old). Data used for the calibration of the model were obtained in the Lavos beach in central Portugal. The model is based on differential equations and it was constructed using Stella simulation software. Five different model versions were tested corresponding to different modes of recruitment that could fit the data. It was found that the best model was obtained when considering reproduction dependent on temperature and photoperiod and occurring on a semi-lunar basis. Two out of the five different model versions tested gave a good statistical performance. Only one of these two model versions makes an effective use of causality mechanisms. This was also the model with the best intercept and slope of the simulated vs. observed regression equation. The other statistically satisfactory model used continuous reproduction between two dates. These dates do not match a photoperiod threshold. The final model version (e) may serve as a useful tool, when used alone or when coupled with other models. One of its uses could be in assessing the effects of human actions upon a sandy beach ecosystem. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: sandy beaches; sandhopper; Crustacea; recruitment; photoperiod; temperature; Atlantic Ocean; Portugal 1. Introduction Talitrid amphipods constitute one of the predomi- nant arthropod groups in sandy beach fauna (Dahl, 1945, 1952; Lagardere, 1966; Pallualt, 1954; Weslawski, Stanek, Siewert, & Beer, 2000), exhibiting a dynamic equilibrium with the environment. Due to their ecolog- ical importance, talitrids have been studied worldwide. Talitrids include key species such as the sandhopper Talitrus saltator, which is widely distributed and generally abundant where it occurs. The oscillations of T. saltator population size, its reproductive strategies, its efficiency in using available energy and its productivity, are important for understanding not only the bioecology of the animal, but also to its ecology and role in the sandy beach ecosystem. In the literature, several studies address Talitrus saltator reproductive biology (David, 1936; Williams, 1978, 1979, 1985; Williamson, 1951) and population dynamics (Lagardere, 1966; Scapini, Chelazzi, Colom- bini, & Fallaci, 1992; Weslawski, Kupidura, & Zabicki, 2000; Williams, 1978, 1995). These studies are funda- mental but with few exceptions (e.g. Williams, 1985) the causality of the processes affecting population dynamics is not thoroughly studied. This becomes even more relevant considering that behavioural and physiological adaptations may differ throughout the distribution range of T. saltator. This has been observed in other species (Kevin & Spicer, 1998; Morrit & Spicer, 1998) and recent data from Marques et al. (2003) clearly show * Corresponding author. E-mail address: anast@uevora.pt (P.M. Anasta´cio). Estuarine, Coastal and Shelf Science 58S (2003) 149–157 0272-7714/03/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(03)00041-6