Testing classical hypotheses on sandy beach ecology with a “goldilocks” indicator species: Study case with tiger beetles (Insecta: Cicindelidae)

Sandy beach tiger beetles (Insecta: Cicindelidae) depend on interconnected beach habitats for larval development and feeding. This study explored the effect of local and landscape factors on adult tiger beetle abundance across 30 sandy beach sectors in Southeastern Brazil. We tested the Cumulative Harshness Hypothesis, which predicts that human disturbances amplify the impact of natural beach harshness. This implies that the negative effect of urbanization (proxy of human disturbance) is more pronounced in morphodynamic types where the species is naturally less abundant because of physical constraints. We sampled 10 beach sectors from each morphodynamic type (dissipative, intermediate and reflective), conducting standardized visual censuses of adult tiger beetles along the upper swash zone boundary during neap tides. Concurrently, we collected sediment from the retention zone and supralittoral for granulometric analysis, measured beach slope, captured potential prey, and georeferenced the coastline to acquire data on urbanization level, vegetation cover, distance from rivers, and erosion rates through satellite imagery. Multiple regression analysis showed that higher tiger beetle abundance was associated with greater distance from river mouth, lower urbanization, finer sand grains, and steeper slope of the berm. The impact of urbanization was more pronounced on dissipative and intermediate beach sectors. It is suggested that steeper berms on reflective beaches reduce the risk of supralittoral flooding, potentially making the population more resilient to human disturbances compared to dissipative and intermediate beaches. This is a possible rationale for classifying tiger beetles as "goldilocks" indicator species due to its specificity of habitat requirements, thriving only when conditions are precisely optimal. Our results are useful for monitoring coastal impacts, guiding conservation and restoration planning, and offer a better understanding of the ecology of beach tiger beetles, which are neglected indicator species. •The Cumulative Harshness Hypothesis was tested with tiger beetles across 30 beaches.•Beaches further from rivers and less urbanized had more tiger beetles.•Urbanization impacted tiger beetle mainly on dissipative and reflective beaches.•Urbanization effect on tiger beetle was higher on beaches closer to rivers.•Tiger beetles is proposed as "goldilocks" indicator species.