Acridids represent a key taxon in the trophic chain because of their herbivorism and their status as prey, notably for plains birds, whose decline is partly attributed to the rarefaction of their food resources, in the same way as reptiles. Estimating the abundance of Acridids in an environment has thus become the subject of numerous studies focused on the preservation, management and valorisation of biodiversity.
As with many invertebrates, part of the biological cycle of Acridids occurs in the soil. For this reason, disturbances linked to tillage and crop rotations cause the extinction of populations in plots used for annual crops. Thus the maintenance of Acridids in the landscape can only be achieved via the intermediary of perennial habitats or grasslands. Unfortunately, such environments are few in number and are frequently destroyed. Thus populations subjected to local extinction can only be sustained if new habitats become available and accessible to colonisation. For this reason, only species that can disperse themselves sufficiently will survive.
Researchers at the CEBC focused on studying the functioning of Acricid communities present on the Sèvre Plain and Valley (Plaine et Val de Sèvre) Workshop site (450 km²) with respect to farming practices and landscape structure. In this context, the general objectives were to assess the effects of habitat management measures on Acridid abundance, and to establish a relationship between the abundance of predators requiring protection and Acridid resources.
In order to standardise the studies performed by researchers and naturalists, the CEBC team first of all studied the reliability of the Acridid capture method used to estimate their abundance. They were thus able to establish that the technique that consisted in throwing a 1 m2 cage in front of them could be used under any meteorological conditions, on grasslands of any type and by numerous observers. The optimum size of the case was 1m²; this could not be reduced because of the effects of a proportionally large frame at smaller sizes.
Secondly, the scientists showed that the abundance of Acridid species at the study site displayed marked variations in terms of both season and space. Thus, for Gomphocerinae, maximum annual densities were found to vary from fewer than two individuals per square metre in 2007 to more than seven in 2004. Another species, Calliptamus italicus, was found at very low densities during all years (a maximum of 0.5 individuals/m²). The presence of Acridids also depended on the type of habitat found in the agricultural landscape. Although the abundance of C. italicus was not linked to plant cover, that of Gomphocerinae was structured by cover type (adjusted means in wild grasslands: 3.1 individuals/m², in artificial grasslands: 0.8 individuals/m²). The density of Gomphocerinae increased with the age of grasslands up to a period of 4 to 8 years when maximum abundance was observed, declining thereafter.
The researchers then modelled evolutions in the abundance of dominant species at the study site – the Calliptamus italicus species and the sub-family of Gomphocerinae – from egg hatching until the death of adults in the autumn. To achieve this, they performed weekly sampling campaigns from the end of May to mid-October every year from 2004 to determine the Acridid density in 23 grassland areas. Statistical analysis consisted in modelling the kinetics of the two dominant taxons using Weibull's law. They were thus able to characterise the fluctuations in seasonal abundance of the two species using a mean annual design that described 70% of the plots studied (see Figure).
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Curves representing the seasonal abundance dynamics of the two Acridid taxons studied: Calliptamus italicus and the sub-family of Gomphocerinae. The "Julian" date was the number of days elapsing since 1st January.
This modelling made it possible to establish that during one season, populations fluctuated very rapidly over time (bell curve). In addition, for a given Acridid species and a given year, it was possible to determine the date on which the populations of juveniles or adults reached their peak, as well as the duration of juvenile development. These dates varied from one year to another. Access to this information may be of great interest as it could be correlated with climatic data and thus contribute to assessing the impact of global warming. At a more applied level, it may enabled the choice of the optimum period for sampling, depending on the objectives of a study.
References:
Acridid (Orthoptera: Acrididae) abundance in Western European Grasslands : sampling methodology and temporal fluctuations. Journal of Applied Entomology, 133 (2009) 720-732. I.Badenhausser1, P.Amouroux1, J.Lerin2 & V.Bretagnolle3.
1Centre d’études biologiques de Chizé, INRA-UPR 1934 CNRS, F-79360 Beauvoir sur Niort, France. 2INRA-URP3F, F-86600 Lusignan, France. 3CNRS-CEBC IPR 1934? F-79360 Beauvoir sur Niort, France
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