Invasive species pose a major threat to global diversity, and once they are wellestablished their eradication typically becomes unfeasible. However, certain natural mecha-nisms can increase the resistance of native communities to invaders and can be used to guideeffective management policies. Both competition and herbivory have been identified as poten-tial biotic resistance mechanisms that can limit plant invasiveness, but it is still under debate towhat extent they might be effective against well-established invaders. Surprisingly, whereas bio-tic mechanisms are known to interact strongly, most studies to date have examined single bioticmechanisms separately, which likely influences our understanding of the strength and effective-ness of biotic resistance against invaders. Here we use long-term field data, benthic assemblagesampling, and exclusion experiments to assess the effect of native assemblage complexity andherbivory on the invasion dynamics of a successful invasive species, the alga Caulerpa cylin-dracea. A higher complexity of the native algal assemblage limited C. cylindracea invasion,probably through competition by canopy-forming and erect algae. Additionally, high her-bivory pressure by the fish Sarpa salpa reduced C. cylindracea abundance by more than fourtimes. However, long-term data of the invasion reflects that biotic resistance strength can varyacross the invasion process and it is only where high assemblage complexity is concomitantwith high herbivory pressure, that the most significant limitation is observed (synergisticeffect). Overall, the findings reported in this study highlight that neglecting the interactionsbetween biotic mechanisms during invasive processes and restricting the studied time scalesmay lead to underestimations of the true capacity of native assemblages to develop resistanceto invaders.