Acridid (Orthoptera: Acrididae) abundance in Western European Grasslands: sampling methodology and temporal fluctuations
Résumé
The estimation of Acridid (Orthoptera: Acrididae) density is the goal of a wide variety of conservation studies to monitor the impacts of grassland management or habitat change. The first aim of this paper was to study the influence of surveyors, time of the day and size/form of the box quadrat method on density values of Acridids in grassland habitats in Western France. The main taxa were Calliptamus italicus (L.) and Gomphocerinae sub-family. Surveyor and day-time (early morning vs. mid-day) were not significantly influencing the density values of Acridids produced using a 1 m2 box quadrat. A smaller box quadrat of 0.5 m2, square or rectangular, produced underestimates of Acridid density, such as square 1 m2 box quadrat should be preferred. Population densities change through time. Thus, data-based inferences about density are tied to the time periods during which data are collected. This was the main focus of this paper to provide a description of Acridid density fluctuations from egg hatching to adult's death in grasslands with differing vegetation structures and grasshopper densities. Acridid densities were monitored weekly in 23 grassland plots over three seasons from 2004 to 2006. Weibull formulation was used to model density fluctuations for the two main taxa in each field with the Julian date as time. Phenological parameters of density were quite similar for the two taxa in the three years. This could be explained by similar meteorological conditions in the three years. However, these parameters exhibited a great variability between fields in the same year. Divergences between the fields could be attributable to different field ecological characters and to different grasshopper associations in the case of Gomphocerinae sub-family. This study provided data which may be useful in determining the optimum survey dates.