A model for amphipod (Talitrus saltator) population dynamics
Introduction
Talitrid amphipods constitute one of the predominant arthropod groups in sandy beach fauna (Dahl, 1945, Dahl, 1952, Lagardere, 1966, Palluault, 1954, Weslawski, Stanek, Siewert, & Beer, 2000), exhibiting a dynamic equilibrium with the environment. Due to their ecological 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, Williams, 1979, Williams, 1985, Williamson, 1951) and population dynamics (Lagardere, 1966, Scapini, Chelazzi, Colombini, & Fallaci, 1992, Weslawski, Kupidura, & Zabicki, 2000, Weslawski, Stanek, Siewert, & Beer, 2000, Williams, 1978, Williams, 1995). These studies are fundamental 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, Morritt, & Spicer, 1998) and recent data from Marques et al. (2003) clearly show geographical differences in population biology of T. saltator. Moreover, a large number of papers have been published on several aspects of talitrids behaviour and some of them linked behaviour with ecological conditions (e.g. Fallaci et al., 1999, Morritt, 1998, Nardi, Persson, & Scapini, 2000, Scapini, 1997, Williams, 1983).
All this data permit the construction of a model. Ecological models lead to a higher degree of awareness of the gaps in our knowledge. Moreover, by building a population dynamics model of Talitrus saltator a tool for future predictions under various scenarios is built. This can be used in order to understand or mitigate the impacts that environmental changes (e.g. human impact) may have over one of the most common supralittoral species of sandy beaches.
It is the purpose of the present study that the model will accomplish the following goals: (a) a correct simulation of the population dynamics of Talitrus saltator; (b) consideration of the most important ecological processes involved in the population dynamics of the species; and (c) provide indications for further research, namely on the relative importance of each parameter or process on the dynamics of the population.
Section snippets
Study site
Lavos sandy beach in the central region of Portugal is approximately 25 km long, between the mouth of the Mondego River and the Ervedeira coastal lagoon, close to Pedrógão, a small village. It is a relatively undisturbed beach, which receives a moderate number of summer visitors. The site chosen for the study, Cabedelo (40°07′32″N 8°51′49″W), was located at about 1 km south from the Mondego River mouth, constituting a very exposed beach. The eulittoral zone was about 60 m in width, with an
Results
Stella (version 5.1.1) with Euler integration method and a time step of 1 day were used in order to run the model. The choice of the time step was based on the need to be able to operate the switches that would start the reproduction. Larger time steps jump over the dates for the onset of reproduction. Time steps of 0.1 days had no impact (zero sensitivity) on any of the simulations at a short or longer term (1–5 years) and were computationally heavy.
The simulation results for all model
Discussion
Five model versions (a)–(e) were built before finding the adequate structure. Versions (a), (c) and (d) should not be considered validated, and were unable to replicate the abundances of Talitrus saltator observed in the study site. Only the model versions (b) and (e) passed all the statistical tests. Model versions (a)–(c), regardless of their statistical validity, were insufficient for the model purposes but provided clues on which direction to move. In these versions recruitment was simply
Acknowledgements
This work was carried out in the scope of the research project ‘Bases for the integrated sustainable management of Mediterranean sensitive coastal ecosystems’, funded by the European Union (IC18-CT98-0270) and by the project POCTI/CTA/36553/00 funded by FCT. The authors are indebted to Dr Isabella Colombini for supplying a summary of data regarding Talitrus saltator bioecology. This was a strong push forward for the beginning of the construction of the model.
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