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Effects of forested strips on spider assemblages in adjacent cereal fields: dispersal activity of spiders

Kajak A.

Polish Journal of Ecology

2007

Vol. 55, nr 4

691-704

Spiders are important components of agricultural ecosystems as far as they affect markedly the abundance of crop pests. The objective of the study was to determine whether the planting of forested strips (shelterbelts) in agricultural area enhances the biomass of spiders active on soil surface of cereal fields and if the exchange of individuals between these habitats depends on the age of trees. The investigations were carried on in 2000 and repeated in 2003/2004 along transects across young shelterbelts (2 -11 years old) - ecotones - fields. In the second period the 150 years old forest strip was included into the study. So altogether in both periods the study was performed in the strip-managed area in forested strips 2, 6, 7, 11, and 150 years old, adjacent cereal fields and in the control field located in a deforested area. The intensity of patrolling (number of individuals captured per trap per day) the soil surface by spiders was investigated using pitfall traps forming lines parallel to the shelterbelt. The results of both investigation periods show, that forested strips increased the biomass of patrolling spiders (BP - biomass of spiders per trap per day) in the fields. It was higher in the fields located in the strip-managed area than in the deforested area by ca 70%. The BP decreased gradually with the increasing distance from the strips. Differences between the strip and the field at a distance of 50 m were significant (two-way ANOVA, post hoc Turkey test). The reason for the higher BP in the fields adjacent to forested strips was mainly the dispersal of large spiders from strips to adjacent fields. Dispersal is high between the very young strip and the field and low between the oldest strip and the field. The group of forest inhabiting species accounted for 0.3-0.5% of the total number of spiders in the field located in deforested area. In the fields adjacent to young shelterbelts this proportion increases with the strips age from 1 to 6%. But in the field bordering the oldest forest strip it is low again and accounted for 1.4%. In this field the proportion of agrobionts is almost as high (94%) as in the field located in deforested area (98%). Similarly the species diversity was very low. The mean individual body mass of spiders from all the fields located in the strip-managed area was 2.7 times higher than in the control field (P<0.001). It can be concluded, that in the old forest strips the number of species, which don.t disperse to surrounding, increases. This conclusion may be important for landscape management.