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Abstract: Expansion of oil palm agriculture is currently one of the main drivers of habitat modification in Southeast Asia. Habitat modification can have significant effects on biodiversity, ecosystem function, and interactions between species by altering species abundances or the available resources in an ecosystem. Increasing complexity within modified habitats has the potential to maintain biodiversity and preserve species interactions. We investigated trophic interactions between Argyrodes miniaceus, a cleptoparasitic spider, and its Nephila spp. spider hosts in mature oil palm plantations in Sumatra, Indonesia. A. miniaceus co‐occupy the webs of Nephila spp. females and survive by stealing prey items caught in the web. We examined the effects of experimentally manipulated understory vegetation complexity on the density and abundance of A. miniaceus in Nephila spp. webs. Experimental understory treatments included enhanced complexity, standard complexity, and reduced complexity understory vegetation, which had been established as part of the ongoing Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Project. A. miniaceus density ranged from 14.4 to 31.4 spiders per square meter of web, with significantly lower densities found in reduced vegetation complexity treatments compared with both enhanced and standard treatment plots. A. miniaceus abundance per plot was also significantly lower in reduced complexity than in standard and enhanced complexity plots. Synthesis and applications: Maintenance of understory vegetation complexity contributes to the preservation of spider host–cleptoparasite relationships in oil palm plantations. Understory structural complexity in these simplified agroecosystems therefore helps to support abundant spider populations, a functionally important taxon in agricultural landscapes. In addition, management for more structurally complex agricultural habitats can support more complex trophic interactions in tropical agroecosystems. [ABSTRACT FROM AUTHOR]

California sage scrub (CSS), a native ecosystem type of low-elevation areas of Southern California, is increasingly threatened by urban development, altered fire regimes, and vegetation-type conversion to non-native grasslands. Using pitfall traps, we examined how suburbanization, type conversion, and fire influence ground-dwelling spider assemblages in eastern Los Angeles County, CA, by surveying spiders in three habitats (CSS, non-native grasslands, and suburban areas) before and after a fire that occurred in a small portion of our study site. Spider assemblages in the suburban habitat differed from those in CSS and non-native grassland habitats, but CSS and grassland assemblages did not significantly differ. This suggests that the urban development, but not vegetation-type conversion to non-native grasslands, has significant effects on ground-dwelling spider assemblages. Fire had no observable effect on assemblages. Because ground-dwelling spiders were not impacted by fire and type conversion, increased fire frequencies, which often result in the establishment of non-native grasses, may not deleteriously influence this animal group, a differing pattern from other taxonomic groups. However, the rapid urban development occurring in low-elevation areas of Southern California means that species requiring nonsuburban sites for their survival (15 species, 24.1%) may be threatened and require conservation assessment. [ABSTRACT FROM AUTHOR]