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Abstract: Ants play a central role in understanding the effects of habitat loss and fragmentation on communities and ecosystems because of their diversity, abundance, and functional roles in ecosystems. Species interactions involving ants are widespread and include other insects, plants, and vertebrates. Ants often do not show strong relationships between species richness and habitat area, but shifts in ant species composition are a more general pattern with an average 75% turnover in species composition among habitat fragments. Shifts in ant species composition and relative abundance due to habitat fragmentation have direct and indirect effects on species interactions of ants, including sap-feeding insects, seed dispersal, and vertebrate mutualisms. The loss of some ant species from small habitat fragments may have widespread effects in ecosystems because of their function roles as keystone mutualists or in soil modification. Boundary dynamics of ants across habitat edges and the surrounding land uses are particularly important to understanding the effects of habitat fragmentation because steep abiotic and biotic gradients may facilitate invasive ant species and cause sharp changes in the abundance and species interactions of native ant species.