Allee effects in corals during fertilisation

Gerard F Ricardo , Peter J Mumby, Russell Babcock, Elizabeth Buccheri, Taison Ka Tai Chang, Christopher Dorpoulos

Allee effects – where individual fitness declines at low population densities or sizes – are pivotal in shaping the reproductive ecology of broadcast-spawning corals. In these species, successful external fertilisation requires that gametes remain at sufficiently high concentrations for long enough periods before turbulent water movement disperses them below fertilisable levels. As reefs degrade and coral populations fragment, this inverse density dependence can severely limit reproductive success, undermining reef resilience and recovery. This review explores Allee effects in corals during fertilisation, highlighting how spatial configuration, colony size, and hydrodynamics influence gamete encounters. We synthesise empirical and modelling studies that quantify fertilisation under varying densities and spatial arrangements, revealing the threshold conditions essential for reproductive viability. We also evaluate the roles of water flow, turbulence, and spawning synchrony in shaping fertilisation rates, with direct implications for coral recruitment. Methodological challenges, particularly in situ sampling biases and complex hydrodynamic conditions, are discussed, underscoring the difficulty of accurately measuring fertilisation success. We address how Allee effects inform coral restoration strategies, particularly outplanting design for fertilisation and genetic diversity, and identify equivalent implications for harvest management frameworks where selective removal of large, aggregated colonies may erode reproductive viability. Understanding these processes is crucial for predicting coral reproductive outcomes under changing environmental conditions and guiding effective conservation of degraded reef systems.