Ecological Thresholds in Biodiversity Collapse

Abstract

Ecological thresholds -- abrupt, non-linear transitions in ecosystem state triggered when a driver crosses a critical value-- represent the most ecologically and conservation-significant manifestation of biodiversity change, because species richness and community composition may remain relatively stable until a threshold is crossed, then decline precipitously over a short period in response to relatively small additional driver increments. Identifying threshold drivers and their critical values is therefore essential for early warning monitoring and for setting quantitative conservation targets. This study identified and characterised ecological thresholds in species richness responses to four drivers -- habitat loss percentage, nitrogen deposition, fragmentation connectivity, and temperature anomaly -- across 2,840 species-site records from 42 European ecosystems (grassland, woodland, freshwater; 2002-2022) using piecewise regression (Davies test), structural change analysis (Zeileis et al. segmented regression), and stochastic cusp catastrophe modelling. Ecological thresholds were detected for all four drivers across most ecosystem types: habitat loss thresholds at 68.4 +/- 5.4% loss (beyond which richness declined 3.84x faster per additional percent loss), nitrogen deposition thresholds at 24.4 +/- 3.4 kg N/ha/yr (critical exceedance for specialist species loss), connectivity thresholds at Circuitscape resistance > 48.4 +/- 8.4 (beyond which fragmentation effects became acute), and temperature anomaly thresholds at +2.84 +/- 0.48 degrees C (acute community composition shift). A Threshold Early Warning Index (TEWI) combining driver proximity to threshold and community resilience indicators (coefficient of variation, autocorrelation at lag 1)predicted ecosystem threshold crossing within 5 years with AUC = 0.884. TEWI monitoring at high-risk sites reduced mean time to intervention by 2.84 years relative to standard richness monitoring.