Abstract
Edges have become prevailing habitats, mainly as a result of habitat fragmentation and agricultural expansion. The interchange of functionally relevant organisms like insects occurs through these edges and can influence ecosystem functioning in both crop and non-crop habitats. However, very few studies have focused on the directionality of insect movement through edges, and the role of crop and non-crop amount has been ignored. Using bi-directional flight interception traps we investigated interchange of herbivore, natural enemy, pollinator and detritivore insects between native forest fragments and soybean crops, simultaneously considering movement direction, forest cover in the landscape and crop phenology. In total, 52,173 specimens and 877 morphospecies were collected. We found that, within most functional and taxonomic groups, movement intensity was similar (richness and/or abundance) between directions, whereas a predominantly forest-to-crop movement characterized natural enemies. Insect movement was extensively affected by crop phenology, decreasing during crop senescence, and was enhanced by forest cover particularly at senescence. Mainly the same herbivore species moved to and from the forest, but different natural enemy species predominated in each direction. Finally, our analyses revealed greater forest contribution to natural enemy than to herbivore communities in the crop, fading with distance to the forest in both groups. By showing that larger amounts of forest lead to richer insect interchange, in both directions and in four functional groups, our study suggests that allocation to natural and cultivated habitats at landscape level could influence functioning of both systems. Moreover, natural enemies seemed to benefit more than pests from natural vegetation, with natural enemy spillover from forests likely contributing to pest control in soybean fields. Thus consequences of insect interchange seem to be mostly positive for the agroecosystem, although consequences for the natural system deserve further study.
Highlights
Agricultural expansion drives loss and fragmentation of natural ecosystems, creating large expanses of abrupt boundaries between crop and non-crop habitats
Hymenoptera moved from the forest to the crop all along the season (Table 1; Fig 1B; S3B Fig) and, only at crop senescence when they were rarer, their abundance increased with forest cover
There was no difference in the numbers of pollinator species moving in each direction (Fig 1A), but they increased with forest cover at soybean senescence, when fewer species were observed (Table 1; S5A and S6A Figs)
Summary
Agricultural expansion drives loss and fragmentation of natural ecosystems, creating large expanses of abrupt boundaries between crop and non-crop habitats. Insects are an important taxonomic group for agriculture, including herbivore species and among them some of the most harmful crop pests, while on the other hand, contributing a wide variety of natural enemies that help controlling pest populations [6]. Pest control provided by natural enemy insects, including parasitoids and predators, is an essential ecosystem service valued in $400 billion per year worldwide [7]. The value of the pollination ecosystem service has been estimated to be between $5000 and 14000 million per year in the US alone [9,10], from which a large proportion is attributed to native insect pollinators [11]. Detritivore insect species are relevant for crops because of their contribution to nutrient cycling [6]; services provided by dung beetles alone may amount to $380 million per year [11]
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