Landscape, Local Management, or Abundance-Driven Richness? Disentangling Predatory Arthropod Assemblages in Indonesian Croplands

Abstract

Understanding the mechanisms structuring predatory arthropod assemblages in agroecosystems is crucial for effectively implementing biological control strategies. However, the relative impacts of landscape configuration, local management, and underlying abundance biases remain highly debated. To disentangle these drivers, we assessed the species richness and community composition of generalist predators (spiders, carabid beetles, and ants) in two highly contrasting Indonesian farming systems: flooded rice fields and intensive dryland cabbage fields. We sampled ground-active and low-foraging predatory arthropods across 24 sites (12 per crop type) using a standardized combination of pitfall and yellow pan traps. We also evaluated local management intensity and landscape metrics within a 500-m radius. Generalized linear models and multi-model inference revealed that total individual abundance overwhelmingly governed observed species richness across all taxa. This pattern suggests that apparent richness differences were largely driven by variations in local population size, although abundance may also represent an ecological pathway through which habitat quality influences species richness. In contrast, non-metric multidimensional scaling revealed that habitat identity strongly shaped community composition. Spider and ant assemblages diverged significantly between the two crop types: flooded rice fields were dominated by active hunters (e.g., Lycosidae) and moisture-dependent ants (Lophomyrmex), whereas cabbage fields favored disturbance-tolerant opportunists (e.g., Monomorium). Conversely, carabid beetles showed broadly overlapping community structures across both systems, suggesting weak habitat specificity in this highly mobile group. In conclusion, within the sampled predator strata, perceived richness variation was largely driven by abundance, whereas community structure was strongly linked to local habitat identity. Therefore, agronomic conservation strategies should prioritize enhancing the baseline carrying capacity of dominant predators to strengthen assemblages relevant to biological control, rather than assuming that ecosystem services can be secured solely by maximizing species richness or altering landscape structure.