The demography of space occupancy: measuring plant colonisation and survival probabilities using repeated pin-point measurements.
Résumé
1. The study of plant demography has long been an important component of plant ecological studies. However, in plant communities, e.g. grasslands, where individual plants are not easily distinguished and often vary in size, a convenient method of describing the demography and its ecological consequences has been lacking. 2. The aim of this study was to discuss the potential for using the change in the probability of space occupancy as a measure of ecological success in plant population biology. This will be done by demonstrating how the change in the probability of space occupancy depends on the processes of colonization and survival, and demonstrating how colonization and survival probabilities may be estimated from repeated pin-point recordings at the same pin-position. Furthermore, it will be demonstrated how to calculate the sensitivity and elasticity of the change in the probability of space occupancy to colonization and survival probabilities. 3. The method is applied to a case of repeated pin-point data of the perennial grass Brachypodium phoenicoı¨des that underwent different grazing regimes from 2001 to 2008 on xero-halophytic grasslands. 4. It was demonstrated that grazing affected both plant survival and colonization probabilities so that the estimated colonization and survival probabilities of B. phoenicoı¨des were highest in the nongrazed regime. Furthermore, using the calculated elasticity of the change in space occupancy, we found that survival was more important than colonization events in determining the ecological success of B. phoenicoı¨des in the non-grazed regime, whereas colonization events were more important than survival in the grazed regime. 5. Synthesis and applications. We expect that the relatively simple method may be widely used on existing and future repeated pin-point recordings from the same pin-position and, consequently, that plant demographic questions may be addressed with larger precision in plant communities where individual plants are not easily distinguished.