Canopy spiders rely substantially on prey other than herbivores across tropical rainforest and plantation systems

Conversion of tropical rainforest into agricultural production systems causes biodiversity loss and shifts in trophic functionality. However, little is known how land-use change and concomitant shifts in prey availability, basal resources and habitat structure affect arthropod trophic niches and functional diversity at higher trophic levels. We used bulk tissue 15N/14N and 13C/12C ratios to investigate changes in the trophic structure of canopy spider communities, major arthropod predators, following the conversion of tropical rainforest to rubber agroforests and rubber and oil palm monocultures. We examined the total community and the four most abundant families: the cursorial Salticidae and Clubionidae, the ambush hunting Thomisidae, and the sheet-web building Theridiidae. Across land-use systems, spiders showed little ¹⁵N enrichment relative to local tree leaves (low Δ¹⁵N), suggesting substantial consumption of prey with lower ¹⁵N/¹⁴N ratios than herbivores, including algae-microbivores such as collembolans and psocopterans, detritivorous blattodeans and herbivorous orthopterans. The lowest spider Δ¹⁵N values in rainforest indicated a greater contribution of algae-microbivorous prey, whereas herbivorous prey became more important with increasing land-use intensity. Despite similar basal resource use, trophic niche ranges and generalist predatory functioning across land-use systems, lower maximum Δ¹⁵N values suggested narrower trophic niches in some spider families in plantations. Higher isotopic divergence in rainforest and jungle rubber further suggested that species occupying extreme isotopic niches contributed disproportionately to community biomass in these systems. Cursorial and sheet-web-building spiders occupied similar trophic positions, suggesting overlapping prey spectra. However, higher maximum Δ¹⁵N values in Salticidae, Clubionidae, and Thomisidae in plantations than in rainforest and jungle rubber indicated a greater contribution of herbivory-linked prey and/or intraguild predation among cursorial spiders in plantations. Overall, canopy spiders functioned as generalist predators with widely overlapping prey spectra across land-use systems despite marked shifts in community composition, feeding substantially on prey other than herbivores, presumably including algae-microbivores and detritivores.