Abstract.  1. Dispersal plays an integral role in determining spatial population structure and, consequently, the long‐term survival of many species. Theoretical studies indicate that dispersal increases with population density and decreasing habitat stability. In the case of monophagous insect herbivores, the stability of host‐plant populations may influence their spatial population structure.2. The tallgrass prairie in Iowa, U.S.A. is highly fragmented and most prairie insects face a landscape with fewer habitat patches and smaller host‐plant populations than 150 years ago, potentially making dispersal between patches difficult. Some herbivores, however, use native plant species with weedy characteristics that have increased in abundance because of disturbances.3. Mark–recapture data and presence–absence surveys were used to examine dispersal and spatial population structure of two monophagous beetles with host plants that exhibit different population stability and have responded differently to fragmentation of tallgrass prairie.4. Chrysochus auratus Fabricius exhibits a patchy population structure and has relatively large dispersal distances and frequencies. Its host plant is variable locally in time and space, but is more abundant than 150 years ago. The other species, Anomoea laticlavia Forster, exhibits a metapopulation or non‐equilibrium population structure and has relatively small dispersal distances and frequencies. Its host‐plant populations are stable in time and space.5. The results indicate that dispersal ability of monophagous beetles reflects the life‐history dynamics of their host plants, but the spatial population structure exhibited today is strongly influenced by how the host plants have responded to the fragmentation process over both time and space.

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