Invasion impacts vary across the diel cycle: hemipterans supercharge ant ecosystem functions and restructure local invertebrate communities

Mark Wong , Zohara Scott, Farhan Bokhari, Julian Kalau, Raphael K. Didham, Melissa Thomas

Biological invasions threaten biodiversity and ecosystem functions, often causing cascading effects across trophic levels. Yet how these impacts vary over the fundamental day–night cycle remains largely overlooked. On Barrow Island, a high-conservation-value reserve off northwestern Australia, we examined how infestations of the non-native scale insect Saissetia miranda (Hemiptera: Coccidae) on a dominant native tree species – the fig Ficus brachypoda – influenced native arboreal invertebrate communities and ant-mediated ecosystem functions in a natural matched-pairs experiment, comparing scale-infested versus uninfested sites. We observed the ecologically dominant native meat ant Iridomyrmex sanguineus collecting carbohydrate-rich honeydew from the non-native scale insects on fig trees. Although the meat ants are primarily diurnal, their nocturnal foraging activity increased by 164% at infested sites. Scale-infested sites supported higher abundances of arboreal invertebrates (~28% higher overall). In particular, scale infestations boosted the nighttime abundances of moth and cockroach species, which were observed feeding on excess honeydew, as well as spiders, possibly preying on these groups. Moreover, we uncovered knock-on effects on ant-mediated ecosystem functions in the brown and green food webs of infested sites. On the ground, the higher nocturnal ant activity contributed to a 207% increase in the mass of protein baits scavenged by ants during the night. In the canopy, there was a nearly fivefold increase in ant attacks on insect herbivores, as well as a 35% reduction in leaf damage across diel periods. Our findings show that non-native hemipterans can trigger carbohydrate-rich pulses in ecosystems, reshaping ant behaviour and amplifying both ant–herbivore interactions and higher-order effects on multitrophic communities. Critically, these impacts are not uniform across the diel cycle, but disproportionately affect nocturnal assemblages. Recognising the temporal dimension of invasion impacts is crucial for understanding and managing their full ecological consequences.