ESTIMATING POPULATION CHANGE AND DISPERSAL ACTIVITY OF SPIDERS IN AN AGRICULTURAL LANDSCAPE

Abstract

The principal aims of this work were to investigate dispersal activity of spiders commonly found in agricultural crops, and to study the influence of both crop type and farm operations on spider assemblages and populations. Work was also conducted to develop and test an improved trapping method for samplmg aerially dispersing (ballooning) spiders. Objectives were to determine whether common farmland species exhibited species-specific differences in dispersal activity. Behavioural data from wind chamber studies were also related to field data to establish i f seasonality in dispersal was related to seasonal changes in dispersal motivation or variation in ground population densities. Ground population data were used to determine if field-scale differences in spider assemblage were related to crop type, and if crop-specific management was associated with variable impacts on spider populations. The improved trapping method ('stick and bottle' trap) was observed to increase catch sizes significantly (F(i,i8) = 30.11, P < .0001) compared to climbing-sticks with trapping adhesive. Total catch size over an eleven day period was 564 spiders. The use of an 'interception net' increased catch sizes threefold. Average loss of spiders from the traps was 9.1% ± 7.7% for daylight hours between 09:00 and 17:00. 5 The common Imyphiid spiders Engom atra, Oedothorax fuscus and Tenuiphantes tenuis were observed to display different patterns of dispersal over time. Patterns were similar for closely related species. Under wind chamber conditions, ballooning related activity in E atra comprised almost one third of total activity time yet was vitually absent in O. fuscus. Erigone atra was observed to balloon more frequently than O. fuscus m the field - it is suggested that factors influencing the tendency to balloon are different for these species. Ground populations related positively to balloonmg activity for Oedothorax spp. females and dentipalpis males. For other species the high efficiency of the trapping method may have reduced applicability to local ground populations Seasonal differences in ballooning motivation observed in E atra females in the wind chamber were likely related to differences in seasonal temperature affecting activity and not ballooning motivation Variation in spider assemblage composition was observed for fields under different management and crop production. Correspondence analysis suggested vegetation structure may mfluence the abundance of some species Set-aside was observed to have a higher proportion of non-lmyphiid species than other fields Oedothorax fuscus was dominant in almost all crops which could relate to its affinity for grass leys which predommate m the landscape. Harvesting m cereals and grass were seen to negatively impact spider populations with declmes of 96% and 83% observed respectively withm nme days of harvestmg Post harvest emigration was thought to contribute to these reductions Harvesting in maize however had a neglible impact on spider populations with relative high densities of adults overwmtering in maize stubble