Local negative frequency dependence can decrease global coexistence in fragmented populations

Most biological populations are rich in diversity, and negative frequency-dependent (NFD) selection is a well-known mechanism thought to underlie this stable coexistence of multiple variants. Recent studies confirm its widespread presence at local spatial scales, however it remains unclear whether these local-scale dynamics are sufficient to maintain diversity across larger, landscape-level scales. While prior theoretical work has found that local NFD selection can indeed promote global coexistence, these studies only analyzed contiguous landscapes. In contrast, many ecosystems are not contiguous, but rather spatially fragmented or exhibit spatial variation in the local carrying capacity. Using a population-genetic model based on the classic island framework, we show that, in fragmented populations, NFD selection can paradoxically reduce coexistence and shorten fixation times, relative to neutrality. Fragmentation also generates a nonmonotonic relationship between fixation time and population size, with diversity minimized at intermediate scales, contrasting with the classical species-area relationship. Extending the model to a multispecies context reveals that these effects also shape species-level diversity. This allows us to develop a test for the influence of NFD selection in fragmented systems, based on the predicted nonmonotonic species-area relationship. Applying this test to an avian diversity dataset from the Ryukyu Islands, we find a signature suggesting that NFD selection and habitat fragmentation may interact to suppress biodiversity in this system. Together, our findings suggest that fragmentation can undermine the stabilizing effects of NFD selection, calling into question its generality as a mechanism for maintaining biodiversity in heterogeneous landscapes.