Migration patterns and functional groups of spiders in a desert agroecosystem

Abstract

1. Arthropods living in annual crops suffer mortality caused by agricultural practices. Therefore, migration from surrounding habitats is crucial to maintain populations of natural enemies of insect pests in crops. In desert agroecosystems there is a pronounced contrast between managed and unmanaged habitats, where irrigated and fertilised crops are islands of productivity in an arid matrix. This contrast could either enhance or inhibit movement of natural enemies between the landscape components.2. The importance of the surrounding arid habitats as a source for spiders in crops was examined in the Negev desert of Israel. Spiders were sampled in both arid natural habitat and adjacent wheat fields using pitfall traps and visual searching. In addition, spiders in wheat fields were sampled throughout the winter cropping season using emergence traps at increasing distances from the field edge. Stationary and movable emergence traps were used to distinguish between residents and migrant species.3. The spider assemblage in the wheat was dominated by three families: Linyphiidae, Theridiidae, and Gnaphosidae. Spider sampling in both natural arid habitat and adjacent wheat fields enabled four functional groups to be recognised that differed in habitat preference, movement patterns, and population dynamics. Thirty‐three per cent of collected individuals were classified as crop residents whereas more than 50% were classified as migrants from the surrounding habitats. These findings suggest that the surrounding habitats influence spider assemblage composition in the fields, in spite of the marked contrast in habitat structure and productivity.4. Spider assemblages in the wheat fields were dominated by migrant species arriving from the surrounding arid habitats. Migrant spiders inhabited the crop throughout the cropping season. The combined contribution of resident and migrant functional groups may act to prevent insect pest outbreaks in this desert agroecosystem.

Non-crop habitats in the landscape enhance spider diversity in wheat fields of a desert agroecosystem

“…In Europe, Linyphiidae benefit from a high percentage of non-crop habitats in the landscape, which serve as their winter refuge SchmidtEntling and Dö beli, 2009). In contrast to these findings, Linyphiidae in the northwest Negev clearly preferred cropland over natural semi-desert (Pluess et al, 2008;Gavish-Regev et al, 2008). Nonetheless, they did not benefit from higher percentages of crops in the landscape matrix investigated here.…”

“…In our study system, no landscape effect was found for Linyphiidae, and Theridiidae responded most strongly at small to medium scales, suggesting that aerial dispersal in these families is less common in the Negev Desert. Nevertheless, in the study by Gavish-Regev et al (2008), theridiid colonisation of a wheat field was consistent with dispersal by ballooning. This highlights the need for studies of spider ballooning in semi-desert regions (Bell et al, 2005).…”

“…This may at least be true for Lycosidae, which occur in similar activity-densities in crops and semi-desert, while Gnaphosidae clearly prefer semi-desert to arable land (Pluess et al, 2008). Thus, Gnaphosidae should depend on semi-desert for recolonisation (see also Gavish-Regev et al, 2008), but what affects their migration patterns will need further investigation. Significant effects of the surrounding landscapes at the largest scales were found for the cursorial Thomisidae .…”

“…Spider populations in arable land however are reduced due to the sowing of the winter crop early in November and thus immigration of spiders from the less disturbed surroundings could be important for pest suppression purposes. Gavish-Regev et al (2008) found that immigrants from non-crop habitats contribute considerably to the spider fauna of wheat fields in the northwest Negev. This suggests that the composition of the landscape around wheat fields can influence within-field densities of spiders also in this climate zone.…”

Landscape-scale mass-action of spiders explains early-season immigration rates in crops

“…1), the model estimates suggest that a higher proportion of the spider population in wheat are recruited for crop immigration than in remnant vegetation. This result is in line with observations that crops are often dominated by spider species that have the ability to disperse by ballooning (Pearce et al 2005;Gavish-Regev et al 2008, Pluess et al 2008 and that the ballooning propensity is higher in spiders originating from disturbed habitats, such as crops, than from undisturbed habitats, such as remnant vegetation (Entling et al 2011). Furthermore, increased spider ballooning from wheat crops could have been triggered by wheat harvest, which coincided with the time that the experiment was conducted.…”