Alain Canard

Spiders (Araneae) useful for pest limitation and bioindication

Abstract In northern Europe at least, extensive knowledge of the systematics and ecology of spiders leads the authors to consider them as a very suitable group for pest limitation and for biodiagnostic purposes. An examination of both the qualitative and quantitative aspects of perdition by spider populations and communities is discussed as well as the evolution of some human factors occurring in agroecosystems that are likely to induce changes in spider predation such as chemical spraying and cultural practices. Studies addressing the recolonisation of agroecosystems by spiders, taking into account their dispersing abilities and habitat selection are summarised, followed by a discussion of the global efficiency of spiders as predators in such environments, the risks associated with their use and how to maximise their efficiency. The bioindicative value of spiders is presented by referring successively to population level and community level. The growth rate or the reproductive rate observed in natural populations can be correlated with the amount of prey ingested in the field. Thus, these parameters give an indirect estimation of the habitat quality. Two specific field experiments are presented to illustrate this ecological concept. Moreover, the role of spiders as indicators of heavy metal pollution (atmospheric or soil pollution) integrated by organisms living close to sources of pollution is discussed by reference to a set of laboratory and field experiments. Due to the close correspondence between the vegetation architecture and the composition of the associated spider community, it is argued (with a list of examples) that fluctuations in the spider community structure allows the bioevaluation of human disturbances. Based on the composition of the spider communities, methods of ecological classifications of natural habitats in several European countries are presented.

Spider biodiversity in connection with the vegetation structure and the foliage orientation of hedges.

Abstract The relationship between the structure of spider communities and an index of hedge ecological quality (based on an analysis of vegetation architecture using vegetation diversity and foliage cover) was investigated. The comparison deals with six hedges each of low, medium and high ecological value. The species richness and species composition of dominant spiders was the same for hedges of different quality. Thus it is concluded that these two simple parameters cannot reflect the diversity of the hedge foliage. Indicating species of the differences between ecological quality of two hedges could be required among the groups of species absent from one type of hedge. However, the foliage orientation of the hedges may induce substitution of spider species; thus special attention must be paid to the foliage orientation when comparing the spider communities inhabiting the hedges.

Improving occurrence-based rarity metrics in conservation studies by including multiple rarity cut-off points

Abstract.  1. This study aims to develop a new method for assigning rarity weights to species in evaluations of the relative rarity of arthropod assemblages in conservation/monitoring studies.

ARE SALT MARSH INVASIONS BY THE GRASS ELYMUS ATHERICUS A THREAT FOR TWO DOMINANT HALOPHILIC WOLF SPIDERS

Abstract As a result of the Elymus athericus (Poaceae) invasion in the last ten years, a major change in vegetation cover has occurred in salt marshes of the Mont Saint-Michel bay (France). In this study, we investigated if the high conservation value of invaded salt marshes is preserved. Abundances, densities and flood resistance abilities of the dominant halophilic species Arctosa fulvolineata (nocturnal lycosid) and Pardosa purbeckensis (diurnal lycosid) were compared in both natural and invaded habitats. Elymus invasion involved both positive and negative aspects with respect to the conservation value of the salt marshes invaded: the P. purbeckensis population was clearly reduced in invaded habitats, whereas A. fulvolineata seemed to derive high benefits from the invasion. We supposed that abiotic parameters of the new habitat (mainly vegetation and litter characteristics) affected the two species differently with respect to their aut-ecology and their flood resistance abilities. Furthermore, food resources (estimated by different macrofauna density measurements) were likely to be reduced for P. purbeckensis in invaded habitats and unchanged for A. fulvolineata. Lastly, we hypothesize that individuals of P. purbeckensis are subject to increased interspecific competition (measured as intra-guild densities), whereas spiders from the same guild as A. fulvolineata have not increased in invaded habitats, resulting in an unchanged competition level.

How Do Alterations in Habitat Structure by an Invasive Grass Affect Salt-Marsh Resident Spiders?

In European salt marshes, recent invasions by the grass Elymus cithericus raise questions regarding subsequent habitat modification and its effects on the resident biota. In this study, we studied the effects of modified salt-marsh structure on occurring spider assemblages, especially on salt exposure and flood resistance. We showed that locations having tall and deep-rooted invasive vegetation contained more species able to resist tidal floods as compared with locations with natural vegetation. Dominant species displayed different temporal trends between habitat types after flooding, with some species being less affected by floods either in invaded or in natural habitats. Some cursorial species were able to (re)colonize salt marshes after a spring flood as salinity levels decreased. Finally, stand characteristics of E. athericus did not affect the distribution of halotolerant spiders. As a habitat structure relates to flood resistance and/or risk-avoiding strategies, our results indicate that the status of salt-marsh resident species should be considered according to their habitat affinity.