Roles of predator-prey interactions and environmental drivers in sandy beach macrofauna populations

Sandy beaches are harsh environments where local physical conditions strongly drive ecological patterns. Ecological interactions are often overlooked in population dynamics studies, although the seemingly homogeneous nature of ocean beaches might support predation as a relevant ecological force. This study uses Lotka-Volterra models (LVMs) to assess whether populations of surface-active predators and prey on sandy beaches in southeastern Brazil are regulated by consumer-resource dynamics. Four air breathing species were monitored over 18 months across three beaches, including two predators (ghost crab Ocypode quadrata and tiger beetle Cylindera nivea) and two prey (sandhopper Atlantorchestoidea brasiliensis and golden ladybug Phaleria testacea). Bayesian stable isotope mixed models (δ¹³C and δ¹⁵N) indicated trophic assimilation between predators and prey and relatively narrow trophic spectrum. Environmental variables usually accounted for less than 16 % of the variation in predator and prey abundance over time according to Generalized Mixed-Effects Models (GMEMs). Importantly, LVMs demonstrated significant reciprocal or top-down effects between predators and prey across all beaches. The smaller the influence of environmental variables, the stronger biotic control. These findings underscore the potential of predation as major driver of population dynamics on sandy beaches, opening new avenues for research grounded in holistic, interaction-based approaches to sandy beach ecology.