As recognized in the 2020 Strategic Plan for the Conservation of Biological Diversity, the global marine protected area (MPA) network is far from comprehensive with no more
than 1% of the sea surface is currently included within MPAs and with a much smaller proportion (probably <0.1%) included within the fully protected ‘no-take’ subset. It is often the case that regardless of the stated goals of MPAs, the focus Is often on fishing activities and potential resource enhancement benefits while ignoring assessment of broader biodiversity conservation benefits. Our understanding of MPA benefits for biodiversity conservation is also greatly hampered by a limited set of MPA studies with basis of field observations and the small subset of those where outcomes have generality. Many MPAs have failed to achieve biodiversity goals because of 1) illegal harvest, 2) Legal harvest above levels consistent with biodiversity goals, 3) Hysteresis in patterns of ecological change, 4) Excessive spillover, 5) Insufficient time for MPA effects to manifest and 6) poor location. While models and prior hypotheses about the nature of marine reserves have been put forward, they need to be tested with data to improve fundamental understanding on the effects of MPAs on biodiversity.