Invasive species undermine the bioark hypothesis in a low-latitude urban biodiversity hotspot

Abstract

Urbanization often reduces local biodiversity, yet cities can maintain surprisingly high total species richness at broader spatial scales. This paradox has led to the "cities-as-bioarks" hypothesis, which proposes that protected remnant habitats within cities can function as refuges for native species. However, most evidence supporting this hypothesis comes from higher latitudes, and it remains unclear whether remnant habitats in low-latitude cities can serve the same role. We surveyed ant communities across 60 sites in Atlanta, Georgia (USA), spanning streetscapes, manicured parks, and forested parks, to test whether relatively undisturbed urban forests support the highest native diversity. Contrary to the bioark prediction, native species richness was lowest in forested parks and highest in habitats with intermediate disturbance. Community composition varied with habitat structure, but surface temperature was not a significant predictor of richness or composition. Instead, native abundance and richness declined strongly with increasing abundance of the invasive Asian needle ant, Brachyponera chinensis, a forest-adapted invader capable of dominating relatively intact habitats. In contrast, two other invasive species, the red imported fire ant Solenopsis invicta and the Argentine ant Linepithema humile, were largely restricted to more disturbed habitats and had comparatively weaker associations with native diversity loss. These findings refine the bioark hypothesis by demonstrating that habitat protection alone may be insufficient to conserve insect diversity in warmer regions, and instead must be paired with active invasive species management.