Interacting disturbances reshape bird assemblages via divergent community trajectories across southeastern Australia

Rahil Jasminkumar Amin , Jessie C Buettel, Barry W. Brook 

Species counts can remain stable even as ecological communities collapse. This paradox exposes a critical blind spot in biodiversity monitoring: richness metrics miss the compositional upheaval that defines modern ecological change. As human pressures intensify, specialists decline and generalists proliferate, creating numerically similar but functionally different communities. Using three decades of citizen-science data on 272 bird species across southeastern Australia's fragmented eucalypt woodlands, we analysed how woodland conversion, livestock grazing, and dominance by the native noisy miner (Manorina melanocephala) reshape species richness and β-diversity. By partitioning β-diversity into turnover and nestedness, we distinguished distinct community change processes. Land conversion and noisy miner presence reduced species turnover whilst increasing nestedness, indicating directional filtering and systematic loss. Lightly grazed woodlands showed the opposite, with higher turnover and lower nestedness, indicating resilience through species replacement. When disturbances co-occurred, communities followed novel trajectories as tolerant and introduced species assembled into unique configurations, boosting turnover at broader scales. Species-level analyses revealed that widespread generalists drove homogenisation while rare, range-restricted species generated local distinctiveness. Critically, high β-diversity was not always ecologically valuable, as some unique sites harboured disturbance-adapted taxa. Consequently, community responses to disturbance followed no single rule. Depending on the combination of stressors, communities underwent filtering, replacement, or novel divergence. Conservation strategies must therefore go beyond simple species counts and aggregate indices. Understanding whether ecosystems are degrading, restorable, or shifting toward alternative stable states requires metrics that capture these underlying processes. In an era of inevitable ecological transformation, the challenge is not only to halt biodiversity loss but to manage the nature of ecological change.