Abstract
ContextEarly-season immigration into arable fields by natural enemies is key for effective biocontrol, but little is known about the mechanisms underlying immigration processes.ObjectivesHere we test the mass action hypothesis for ballooning spiders, stating that local immigration rates are positively related to the amount of spiders in the surrounding landscape.MethodsImmigration rates of spiders were assessed by sticky traps in remnant vegetation, in arable land 25–125 m from remnant vegetation, and in arable land further than 400 m from remnant vegetation. The experiment was conducted at 18 locations across two landscapes and repeated three times in a 2-week period in 2007 and 2008. Spider densities in crop and non-crop habitats were assessed by beat sheet sampling and used to calculate spider loads in landscape sectors around the experimental locations at five spatial scales.ResultsRegression analysis indicated that immigration rates were influenced by meteorological variables and landscape context at 2 km and possibly beyond. Regression models that included spider load at relevant spatial scales received more statistical support from the data than models with the proportion of remnant vegetation and crops. Regression analysis further indicated that wheat and—to a lesser extent—remnant vegetation are important habitats for the recruitment of ballooning spiders.ConclusionsOur study provides support for the mass action hypothesis by showing that a combination of land-use variables with habitat specific spider densities allows the generation of functional cover types with greatly improved explanatory power.
Highlights
The model indicated that the number of spiders in, adjacent to and far from woody remnant vegetation were distinctly different in the North and South landscape, with the highest spider density in remnant vegetation in the south and in arable fields in the north landscape (Fig. 2)
To understand the landscape drivers of spider immigration we compared variables based on land usebased metrics and spider load, a biologically meaningful variable informed by assessment of spider densities in crop and non-crop habitats
Key findings of our study are that (i) the spider load model is superior to the land use model at relevant spatial scales, indicating that the habitat specific spider density values are useful in predicting spider immigration, (ii) wheat and woody remnant vegetation are important habitats for the recruitment of ballooning spiders, while the contribution of barley, chick pea and sorghum is limited, and (iii) immigration into arable fields is influenced by meteorological variables, and landscape context at 2 km radius and possibly beyond
Summary
We test the mass action hypothesis for ballooning spiders, stating that local immigration rates are positively related to the amount of spiders in the surrounding landscape
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