The principle that a single large patch has higher value for biodiversity than several small patches of the same total area (SL > SS) is widely applied in conservation. We show this principle is incorrect, and that biodiversity conservation requires placing more emphasis on protection of large numbers of small patches (SS > SL). Analyzing 76 metacommunities (4401 species, 1190 patches) we demonstrate that, at equal habitat area, species richness accumulates more rapidly in large numbers of small patches twice as often as in few large patches (45.2% vs 19.9% of cases). This pattern is clear for plants, birds, mammals, and invertebrates, but reversed for herptiles. We therefore propose a new principle: for a given area, protecting the largest possible number of patches will maximize biodiversity for most taxa. Our findings indicate large untapped potential for biodiversity conservation worldwide, highlighting the need for a paradigm shift in conservation policy.

In biodiversity conservation, the “SL > SS principle” that a single (or few) large habitat patches (SL) conserve more species than several small patches (SS) is used to prioritize protection of large patches while down-weighting small ones. However, empirical support for this principle is lacking; most studies find SS > SL. We propose a research agenda to resolve this dilemma by asking, “are there consistent, empirically-demonstrated conditions leading to SL > SS?” We develop a hypothesis to answer this question, the “SLOSS cube hypothesis,” which predicts SL > SS only when all three of the following are true: between-patch movement is low, population dynamics are not influenced by spreading-of-risk, and large-scale across-habitat heterogeneity is low. We then propose methods to test this prediction. Many tests are needed, comparing gamma diversity across multiple landscapes varying in number and sizes of patches. If the prediction is not generally supported across tests, then either the mechanisms leading to SL > SS are extremely rare in nature, or they are outweighed by countervailing mechanisms leading to SS > SL (e.g. lower competition or higher immigration in SS), or both. In that case, the SL > SS principle should be abandoned.